Tail Rotor Problems [Archive]

eden

14th Feb 2001, 02:19

On paper ...when you start to read this it may come across rather fussy or complicated.

But - in practice the Loss of the TR Drive is fairly final it's going to require a bloody swift reaction, to be able to engine off from the hover in order to have a chance of being in a attitude to achieve a decent cushioned landing. In forward flight - well if you manage to contain the yaw you might be able to postion (using low power or suitable PWR/IAS combo) the helo to a safe area for an EOL or you'll just have to enter auto, shutdown the engines and accept an interest EOL at the bottom. All very challenging and something I would rather not try unless I was in a sim.....that said if I was on the NOTAR I suppose I wouldn't have the problem ....probably something else to think about though ....I'm sure.

Now - If you have a Tail Rotor control failure

There's quite alot you can achieve. If you take your time:

1. Define the problem - is it AFCS, Hydraulics, a jam, cable snap or somebody's bag rat wedged behind the pedal. It could be as simple as that - somebody's FOD.

2. Check fuel - if you've got loads we can take all the time in the world to get it right, if not then you'll have to do the same assessments but with a sensible level of alacrity.

3. You need to consider what configuration the jam or failure has left for you to deal with. Is it either:

1. low power controlled flight (pedal Positon)

or

2. High Powered controlled flight (pedal pos'n)

I make this broad assessment criteria based on the position of the applied pedal at the time of the problem first coming to light.
The pedal applied will vary as to type and MR blade rotation.

First of all you need to do a handling check:
which should take the following form:

1. Maintain Wings level
2. Fly at Min power speed (usually 60-70kts)
3. Using collective apply a power setting that will bring the 'ball' in to the centre.
Don't concern yourself with collective and power settings too much.
Then check you RCDI and note the following

A. LOW POWER pedal situation -
for example: you find you have Wings level, Ball in middle and a ROD of 1000'/ min

This means that you could return to a large grass field in a skidded helo or a large runway in wheeled helo and carry out a pre-briefed auto or low powered descent to the ground touchng down at up to approx 60 knots and smoothly shutting down the engine(s) as you enjoy the rutted camel ride from hell across the airfield or you can differentially break if you have them fitted - Luxury. Now a 60kt touch down is very saucy but, in many aircraft a, doable get out of jail type manoeuvre. Obviously - a high wind day of 30 knots straight down your landing area is going to help immensely.

This is good news - you can now decide whether you want to do another handling check to see if you can still maintain your Wings level ball in middle at a slower speed and lower rate of descent. Who knows .....but if you have the fuel keep trying.

B. HIGH POWER PEDAL situation -

Lets do the handling check again -

WINGS LEVEL and put the BALL IN THE CENTRE with collective and find out where the power ends up. Lets say it provides the worse case scenario of a power higher than that required to hover.

We already know that the if we bring the IAS back towards a very low IAS that the PWR REQ'D will increase. As we decel we continue to try to maintain - WINGS LEVEL,BALL IN MIDDLE now by pulling collective to maintian our condition.

We see that we are climbing slowly - so what can we vary ?

1. Weight - it's reducing as we burn fuel ..so it ain't helping us
2. Even less IAS almost or even hovering .....but once we reach this we're at a point where the power req'd is now a MAX value.....but we are still climbing slowly.
3. Nr - if we vary Nr by way of a carefully controlled brief to co-pilot or crew member or self brief by the single pilot we can retard the throttle or SSL's to start a controlled reduction of Nr thus reducing Total Rotor Thrust - but maintaining useful power on Nr values. You should practice this before attemting the approach. If the Nr should decay too much too early use lever t regain Nr until you have had time to apply a little more throttle. Try to aim for a ROD around 200-300'/min

Your approach config should be practiced at altitude above your intended LS and your approach planned so that you start on an extraordinary long final (terrain dependant of course)to achieve a short final by approx 300 - 500' where you are continuing to reduce speed progressively and controlling your reduced Nr to give you a ROD around 200-300'per min at this stage on the approach the aircraft will not be pointing straight or wings level until you reach your IAS you assessed earlier. However, as you approach the hover you will have ground effect to contend with and will have to further reduce the Nr (retarding throttles and SSL's even further. As you approach the Hover or low speed you should aim to be satisfied with your heading, attitude and sink rate ...if so then smoothly retard the levers/lever or twist off the remaining power nice'n'gently and the helo will descend through the GE and you can .....shutdown and go back to the bar for several beers.

During both of these procedures you can always throw it away if you need to...... accepting the awkward manner of flight that you will achieve (out of balance wing low etc) Climb up, line yourself up again and practice it all over again - provided you haven't had to do the Engine OFf bit that is.

I have taught these techniques as have many other instructors - I'm sure, but the problems above are worth practising. If you've ever had a birdstrike that was big and it came through the perspex and wrapped it's wings and colon around your pedals ...you will want to have tried this at some point.

One important point tho' is don't get too wrapped up inside the cockpit it is possible to fly reasonably normally during the preparation and positioning. But at the end if it looks right outside the cockpit, the chances are everything will be alright so keep looking out and flying the beast.!!!

Sorry if it's long winded or if it's stuff you already know......I'm probably the only mug trying to attempt to paint the picture on paper..... back to my Guiness byeeee

[This message has been edited by eden (edited 13 February 2001).]

[This message has been edited by eden (edited 13 February 2001).]

ShyTorque

14th Feb 2001, 02:43

ImIanphere,

(Sorry if I've got your ID wrong as I can't tell if they are Is or ls!)

Although your T/R drive loss technique may work on an R-22 or other small single piston-engined aircraft it might not be applicable to turbine engined ones, if only for the reason that they often don't have throttles that are within reach of a pilot with his hands full of sticks. Co-ordination then becomes a problem if a second crew member is not available (or present)to assist.

I have flown a lot of time in one aircraft type that is very likely go out of control if autorotation is not entered immediately and both throttles subsequently chopped before flaring to land; the only sure speed / power combination being nil power and 75 to 80 kts in autorotation. I did a lot of time carrying out simulator training on this type [both in the hot seat and operating the sim from down the rear] and the experiences of those unlucky enough to try it for real confirm what we found. Simulators are not always accurate because they depend on good data being programmed into them. If that's not available then an "off model" best guess is put into the software programme by the designer.

The AFM usually gives some guidance but can't be relied on as gospel 'cos manufacturers don't often practice it deliberately for obvious reasons.
-----------------------------------------

In the case of a cable jam /partial loss of control of T/R pitch where a powered approach is being considered:

As one who has flies American and European aircraft (different directions of main rotor rotation), to help prevent any confusion with phrases such as "Lucky Left" or "Right is Right" (depending which type I am flying) I now simply remember which side the retreating blade is on (I tend to remember this from the start-up)! This side is then the good side to have the nose "cocked off" towards on the approach. As collective is increased to cushion the (powered)touchdown, the nose will come round, from increased Tq reaction, to point straight ahead. The retreating blade side is also the good side to have the crosswind coming from.

Hope this helps the discussion.

ShyT.

IHL

14th Feb 2001, 03:12

Hey Offshore: Do you know why the F/O in the Maldives accident would jettison the door while still airborne. I've always been taught to wait till you were on the water and the motion has stopped.

SPS

14th Feb 2001, 03:42

Helimutt,

I might not have put over my point too well
(on the thread this disccusion began).

I wanted to start it the right place, which is to take a look at the often misused term
'TR failure' but it appears to be getting done now.

I have to say that the difference between a
true TR failure and a TR drive failure would be completely unoticeable and agree with you on that point, as the result would still be zero TR thrust to balance torque whether the blades are still on the Heli. or not.

But I think the difference between the above and jammed pedals/controls or control disconnection WOULD be clear because the Heli would not yaw right at anything like the initial rate, even if its pitch reduced to minimum. Easy on paper though, quite different in the heat of the moment and if that is where you were going then I agree with that too.

But all good training begins with fully understanding what is going on first, and that is where I began.

A good thread which I will read fully and contribute to when I have finished work.

The last time I renewed by Inst. rating in UK We did TRUE TR failure in the hover and it was surprisingly easy to master. Intersting for me because I have witnessed a
loss of TR control on an R22 in UK but more of that later.

Lovely sunny day, slingloading is today's order and I'm off to help a very able student take a heap of tyres for a flying lesson of their own! (Yes, using an R22, and within MAUW and C of G!) :)

offshoreigor

14th Feb 2001, 03:56

IHL:

The CoJo was a long time friend of mine and his background was Mil. The only thing that has come out of the inquiry is that he had been trained to jettison the door prior to water contact in an imminent ditching situation. Knowing his background, I can't figure out who would advocate the door being jettisoned above translational.

From what has been published by TSB in Canada, the aircraft appeared to be returning to shore, when it went down. The initial report points to the door jettison as the cause of the loss of T/R. The door went throught the mast and then through the T/R.

As to why it was jettisoned, I guess we'll never know. I guess the best question to ask in this forum is, When should you jettison the doors. My opinion is only when you have a controlled ditching situation and only at a reduced airspeed i.e. less than 35 KIAS.

As for the responses to this topic, I think we've found a winner. No shortage of expertise here. I think this will be a real benefit to all pilots as the techniques we use on mediums and heavies are not always taught to those flying SE Light Helis. Although the same procedures will generaly work on all types.

One procedure I haven't heard yet is for a jammed pedal situation. On the 76 we teach our pilots to 'Beep Up' for a stuck right pedal and to 'Beep Down' for a stuck left pedal. The logic being, more power to reduce for the RPF (i.e. nose swings left) and more power available for the LPF (i.e. nose swings right). This technique can be equally useful for a 206, 500, 355 etc. Remember, the engine will still help you in the recovery whether it is a power reduction or power increase.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

eden

14th Feb 2001, 04:26

The procedure I have tried to put across will work for jammed pedals, problems arising from cabling and negative force gradient spring residual problems, stuck or jammed pitch setings on the TR - in short any CONTROL you might get. We all obviously pray that we don't get the extremes of left or right - but that could be the way the cookey crumbleth.

If you run back to looking outside and seeing what the different combinations of airspeed, wings level and ball in middle as a result of lever position(thus power being used) you can determine the type of approach you need to consider.

I obviously didn't explain it very well ....never was good at this writing thing! I'll have to stick to the frantic egg whisk flying I reckon!

Thomas coupling

14th Feb 2001, 13:38

Offshore: Good on you to start this one. It would be great to hear from a 'novice' about their views on such a sinister malfunction. We always seem to hear from the same 'pro's'....
Eden: Guess who's been thru beefers school then!! Brilliant contribution..if only it was so predictable in real life!!
SPS: How on earth can you 'practice' real TR failures in the hover. Do you have a disconnect coupling in the drive train??

Thing to remember, in addition to the effects of surviving the initial bang....if the TR decides to take off, you're left with a major C of G problem [as per the story of the vietnam helo pilot who got his tail blown off with a SAM 7].
Different a/c types have their pros and cons too. Fenestrons offer advantages w.r.t. some TR problems. Can someone out there offer observations on the 902 series with TR problems (equivalent).

What goes round...comes round.

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Thermal runaway.

ShyTorque

14th Feb 2001, 15:40

With regard to jettison before contacting the water. Some aircraft have flotation gear which may prevent a door being jettisonned once inflated and supporting the aircraft's weight. Not a good idea at speed for obvious reasons!

rotorque

14th Feb 2001, 16:19

Shytorque,

Glad you pointed that one out. We use to use a B222 on marine pilot transfers and the norm was to lose the door on short final due to the floats. We now use a BK117, which seems to be a little more forgiving when it comes to the doors. Although the rear sliding doors do come into contact with the floats whilst jettisoning, it dosn't stop the pax from getting out.

On the single engine stuff, I am under the impression that you will most likely need a small amount of power during the auto and flare if you have a 'basic' drive failure. All got to do with drag etc from the transmission.

One other thing to nut over. Does the tail rotor go into auto itself if you boot in right pedal during a drive failure (American heli's)? Maybe it could give an additional, albeit small, amount of stability to an unstable situation. (could you overspeed a T/R)

Food for thought if nothing else.

Robbo Jock

14th Feb 2001, 16:51

As a low timer, my only experience of TR problems is my CPL instructor's boot on the pedals (I never practiced or had this demonstrated at PPL). All I can say is that I would NOT like to do it for real! Things were stressful enough and wild enough close to the ground in the practice situation.
Eden's post is great, and I'm printing it off for future reference. One thing, though, I was briefed to aim for a runway if poss, as the concrete is somewhat more forgiving if the touchdown is made pointing more sideways than might otherwise be prudent. Having practiced to grass, I was hoping concrete would be slightly easier.

I've decided, reading this thread, to grab my old instructor and have him jam his boot on the pedal again and see how I get on.

SPS

14th Feb 2001, 17:12

Thomas Coupling -

I wrote 'The last time I renewed by Inst. rating in UK We did TRUE TR failure in the hover'

I used 'true' in preference to 'real' as I thought the latter unsuitable.

I can see your point, we can only simulate having no TR thrust but cannot disconnect the TR so we have to get as close as possible
by using full right pedal.

Isn't this actually a very accurate simulation though? The TR has to be capable of making thrust in both left and right directions, one of the main reasons for that requirement being that friction effect in autorotation (nose yaws left) must be balanced by TR thrust in the opposite direction to powered flight.

(The neutral position for TR thrust on the ground in an R22 is with the right pedal a good deal forward, not with the pedals level with each other. The TR is rigged to give more control range to the left for powered flight)

Somewhere inbetween full left pedal and full right pedal there is a completely neutral TR pitch setting which is the same as having no TR thrust at all. If you use full right pedal you are actually PAST the zero TR thrust to the right value and into the (narrower)thrust to the left range. That must be ADDING to the speed of right yaw that would result from having no TR thrust due to a true (true making the distinction between a complete loss of TR thrust and other problems such as control failure or jamming) TR failure.

So it seems to me that full right pedal in the hover is a very realistic way of simulating a complete loss of TR thrust in the hover. It gives a greater yaw rate than a complete loss of TR thrust would do in the same profile.

I agree that it would be good to hear from any low hours pilots reading on the sideline, I think that the forum would benefit if more of that were to happen.

Moving on, The control failure in the hover (UK) that I saw was caused by a TR swashplate bearing siezure. The swashplate then wanted to rotate with the TR driveshaft which broke the horn control connection and left the TR in a fixed pitch state. The Instructor got it down fine although it was yawing slowly, I cant remember in which direction but guess that it must have been to the left as the lever was lowered to land.

It is important to note that this was a machine left outside in all UK weathers and had done around 1200 hrs. The bearing is lifed for 2000 hrs but leaving it to be ravaged by those conditions can do it no good at all.

hoverbover

14th Feb 2001, 18:34

To Offshore
Great Subject, lots of experienced pilots "experiences" for novices to learn something from, however putting this into practice real time for us novices is another matter.

To Thomas Coupling

I think I fit in to the novice category very well (PPLH gained 10 months and some 200 hours ago in the UK)So here goes on my experiences/thoughts on the prospect of a TR failure.
When doing my PPL H experience of Simulated TR failure was limited to approximately 10 minutes of demonstration by my instructor, at this point the following thoughts sprung to mind.
1. Sh**
2. I hope that never happens to me.
3. How on earth can you Practic TR failure in its various forms.
4. I now know enough to make me think that if it happened to me I had NO chance.

Luckily after gaining my PPL H (which after all is only a licence to learn) I had to go to the US to do a conversion course onto a NOTAR helicopter(more of that later) but during the course we covered TR (fan) failures in all their various forms Fan failure and or fixed left pedal in the hover being my particular favourite. These failures proved very realistic (I guess) and only required flying the heli with your instructor working the pedals (the factory test pilots who have actually had the failures for real)This then gave me the following thoughts.
1.The PPL H course in the UK should either not cover it at all(ignorance is bliss!)or do it properely(maybe this was just my instructors and not the course ?)
2.I still have a lot to learn.
3.OK while it was only simulated, I now feel more comfortable with TR failures.
4.I would react better to the situation.
5.If it does happen I won't just be waiting for the crash , I will be trying to do something about it when I crash !!!!!!
6.BUT no one told life/situations to follow what it says in the book!

To Eden
You mentined the NOTAR system in one of your posts, and the fact it may be better with a failure. In my experience if the fan fails its jsut like any loss of TR, except for maybe above 35 Knots IAS when the steerable verticals are in effect, but then if you have a fixed pedal scenario they make it even worse in certain cases. The drive to the fan can fail just the same as a TR, the big bonus is that the fan is less likely to strike anything to cause the failurein the first place! Wether in flight or in the hover.

To All Novices like Me
Remember this in your quest for flying knowledge:
"In the life that is helicopter flying we start off with 2 Bags, One bag of LUCK which is FULL, the other bag is for EXPERIENCE and is EMPTY, Make sure you FILL the bag of EXPERIENCE BEFORE your bag of LUCK runs OUT"

Cant remember who told me this/wrote it but they are very wise.

Regards
hoverbover

offshoreigor

14th Feb 2001, 18:56

I think the big thing to remember here is that with the exception of a total drive failure or actual loss of complete T/R assembly, the one thing you will have lots of, is time.

Don't be in a mad panic to get down. Take your time and experiment with power and airspeed combinations that will give the best profile for landing.

On wheel equipped A/C, you will be able to run the A/C on at a considerably higher than published speed i.e. S76 60-70 Kts! Skid equipped A/C, if recovered at a runway, can also run on at a fairly higher than normal speed (obviously not great for the skids but at least you get down in one piece).

Someone asked about the T/R being 'Auto'd'. The SH/UH60 has the tail rotor mounted on an angle (I forget the exact amount). If a total loss of T/R drive is experienced it is recommended that the aircraft be banked 20-30 degrees (I think to the left). This causes the T/R to autorotate to some extent returning, I believe about 50% tailrotor control.

Well keep it coming as I do believe this may turn into a very educational topic for all.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

eden

15th Feb 2001, 00:35

hoverbover:

Thanks for that info' I was kind of alluding to the fact that I wasn't really aware or sure of what happened with these beasts, having never flown one. I felt that they might present a different set of problems and was kind of hoping that someone would jump on my ignorance a sort me out - so I am grateful for your advice.

Out of interest: for those who may have had a TR CONTROL problem (as opposed t failure) the techiques I describe work - having had a cockpit floor full of Seagull and derision wedged firmly amongst the pedals of a Gazelle during a low level sortie. Luckily -the problem it gave me enabled me to return for a slow speed running landing which was all very benign .....the greatest shock is a facefull of feathers, a facewash of Seagull blood but the remains made for good burgers at lunch!

lmlanphere

15th Feb 2001, 01:21

I mentioned this on a different topic, but Rotorque mentioned throttle control in autorotation on the previous page and so I thought I'd mention it again. In singles with throttle on the collective, reducing pitch but leaving the throttle alone and Nr in the normal range may produce just the right amount of torque to balance the transmission bearing drag at normal auto airspeed. However, during the collective application at the end, throttle will have to be held at idle to prevent the correlator from sending the power back up, and in the case of TR failures, this will leave a healthy yaw to contend with as the transmission bearing drag becomes unopposed. I suppose that a near zero speed landing would be something to consider in this case, if possible.

Thomas coupling

15th Feb 2001, 01:41

I've had a tail rotor control jam on me, the wires jumped the control run guide pulleys and the pitch setting luckily was jammed to a +ive setting. Luckily we were shore side at the time otherwise I would have had to recover to 3000 tons of moving deck the size of a car parking slot!!!

One of two things will happen to you if it is going to happen:

1. There will be an almighty bang accompanied with the inherent yaw in oppostion to the direction of the main rotors. If you are in fwd cruise then the rate will be in proportion to your speed. [In the hover, you will rotate very very quickly, so much so that you will become very disorientated and if not strapped in properly, will be flung outboard of the rotation. Should this be away from the throttle(s) then you won't be able to chop it/them]. The simple solution here is dump the lever and land, accept the very hard landing!!
In fwd flight you will also get a fwd pitch of the a/c dependant on the moment arm of the TR x weight (of chunk)lost! If a/c is still flyable: YOU MUST ENTER AUTO AND CHOP THOSE THROTTLES ASAP. At the bottom, the flare will assist with yaw correction but not fully, you will be committed to a zero spd landing if you don't want to flip head over heels. Keep the Nr up at max throughout, this too helps with yaw correction.

2. You will experience an undemanded yaw input proportional to collective input and possibly slight mechanical noises. This baby should give you time to change your underwear and hand over to the commander!!!
This scenario has been superbly described by others earlier. Time is on your side.
Concrete is better for landing because the skids/undercarriage can scrape more easily over concrete. Don't land on grass if possible. Do as many approaches as necessary before the real Mcoy, as long as you don't go below min pwr speed on each approach you won't lose heading control...Get rid of fuel,if possible. If you can limp to a major airport, get them to foam a strip of runway.

Go thru what you would do, every month or so, get a feel for your craft, play with stuck pedal/speed/collective/wind...this is what makes a professional stand out from a joy rider. Last but not least: DO A GOOD WALKROUND EVERY TIME.

http://www.gograph.com/Images-7298/AnimatedGif/redstar.gif
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Thermal runaway.

[This message has been edited by Thomas coupling (edited 14 February 2001).]

Doc Cameron

15th Feb 2001, 03:13

Chaps,
Loads of very interesting, well formulated hypothesis and real experiences alike. For my ha'peth worth there is no substitute for simulation. Most see the merits of a half decent sim for practicing those failures that we dare not try in reality (bearing in mind for the bean-counters present; we wreck more aircraft in training than in Operations!). This was certainly true of my experience with FSI's 212 sim at Fort Worth. Among the many any other failures, we underwent simulated T/R drive-shaft failures. The Company policy was 'find big space - enter auto - live happily ever after! The FSI instructors however had a different idea - they advocated and trained the 'find some sort of flight attitude with forward speed, run on to runway with power, gently rolling off throttle and lowering collective, to a gentle if graunching stop.' First try, I managed several pirouettes culminating in a nice little roll (no 10/10 from the judges there then!). After a couple of attempts I could get down in one piece but it still frightened me to death. My conclusion was that if confronted with the reality, I would tow the Company line - I truly felt that I would have a far better chance of walking away as autos are far more regularly practiced. I would would maybe reconsider if sim training was the 'norm' and I got to practice the FSI way on a yearly basis.

rotorfan

15th Feb 2001, 10:00

To all who responded to my request for a discussion on TR failure, a great thanks. Lots of food for thought here. Thanks particularly to Eden for his book on the subject. I am more sure than ever that at 75 hrs of heli time, I do NOT want to face a TR problem. But, like autos, I will try to practice it regularly. The repetition won't necessarily help, but, at least, there will likely be less of a feeling of panic if it occurs. Keep the thread going, please!

SPS

15th Feb 2001, 15:30

Little finger rule-

When your left hand is curled around the throttle stretch out your little fimger (LH!)

As you decrease throttle (the nose would yaw
more left) your finger will point left. Apply the reverse to increasing throttle.
The nose will yaw right as your finger points right.

Good for trimming up for a run- on from jammed controls (depending on the nature of the problem).

hoverbover

15th Feb 2001, 15:52

To Eden

Sorry if my response to you sounded as if I was telling you it wasn't intended that way at all. Its just that most of my flying has been on the NOTAR system, and most people have never had the chance.(I'll send you an email)As said before its very similar to a tail rotor but with some particular differences (some good some bad). Again sorry if my response sonded like I was setting you straight "I'm here to learn not preach"

Reagrds

hoverbover

The Nr Fairy

15th Feb 2001, 17:20

SPS :

That's only good for anti-clockwise rotating helicopters, though, no ?

Lu Zuckerman

15th Feb 2001, 18:03

I have posted this on several other threads and on another forum. Although there was no actual tailrotor failure the pilot perceived that the tailrotor gearbox had suffered a failure and as a result took corrective action which only made things worse. This could also be posted on the wire strike thread.

I was flight mechanic on a Bell HTL-1 (early model 47) and we were returning to base in Traverse City, Michigan from a stint on an Ice Breaker on Lake Superior. We had a leaky tail rotor gearbox so every fifty miles or so we would land and I would fill up the gearbox with of all things, fish oil. Very smelly stuff. My pilot, Dave Gershowitz, who incidentally was the first helicopter pilot to hit 1000 hours, was easily freaked out.
While flying in the left seat I was holding the oil can between my legs and reading a map. We were about twenty minutes out when Dave saw a bear. He very excitedly told me to look. When I leaned over, my legs spread and the can hit the deck making a loud noise. Dave thought there was something wrong with the tail rotor gearbox and he made a quick turn looking for a spot to land. In the process of making his maneuver he really made a hard over. On that particular helicopter type when there was an excessive side load on the mast it would cause the planetary gears to really growl.
Upon hearing that, he really wanted to get on the ground. He picked out a landing spot in front of what we later found out was a veterinarians office. To get from where we were to that landing spot we had to pass over a turkey farm and in the process we made the turkeys stampede and over 100 birds were killed in the pileup at the fence surrounding the turkey farm.

Oh yes, in the process of landing, we took out the vets' telephone line
------------------

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The Cat

eden

15th Feb 2001, 18:32

Hoverbover: no bad feeling taken - I was being very genuine about my ignorance and I am grateful for your info' - I look forward to your email and hope you can provide me with some more detail as to how the thing works.

I am going to have a look at a Notar machine in the next week or so and would appreciate as much background info on tail rotor snags that you can provide about it.

I too am here to learn and I hope never to preach too hard - cheers

eden - energy driven eccentric nirvana

RW-1

15th Feb 2001, 19:38

NOTAR itself really has moved the tail "rotor" into the body of the machine itself. A fan unit run off the trannie sends high pressure air into the boom, the air is then forced out thru two slots on the side, and due to coranda effect helps to offset torque, the rest of the air is coming out of the variable nozzle at the aft end, in which the amount can be adjusted by the pilot.
The vertical vins are also controlled by the bilot and the SAS.

A basic description. But as hoverbover was saying, since the fan is now inside the fuse you could sit all day with the boom in a tree (McD loves pics of this) with no problems :) Yopu are less likely to have things hit the fan too.

Boy what a great thread! Lot's of stuff to absorb. My hope is that if I suffer a tail issue, it's a control one with time to sort it out, if not them I'm into auto and hope for the best.

SPS: Like your 'lil pinkie trick!

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Marc

hoverbover

15th Feb 2001, 19:49

To Eden

Hope you got the email, so that everyone can understand(hopefully I might even understand it myself one day)I am posting my thoughts about the NOTAR here(I dont wish to divert the thread,but I dont think it warrants its own one and is kind of related)

The NOTAR system basically works from air pressure created by a fan at the front of the Tail Boom (Tube)This fan is driven by the turbine in the same way as any TR drive.
The fan is very similar to A FENESTRON in looks but with very short blades (about 60mm in length)The fans pitch is increased and decreased/coupled to the pedal inputs(Strangely enough it increases pitch in both directions) and consequently greater pedal input creates greater pressure in the tube.
at the end of the Tail Boom there is (for want of abetter expression) a rotating dustbin which allows the air pressure in the tube (from memory its around 3psi)to be directed for pedal turns in the hover.

There are also two long slots on one side of the boom (and a diffuser on the otherside)which are configured so that air continually comes out of the slots in a downward direction,so that in the hover the downwash from the Main Rotor Flows over the boom and using the principal of there then being a low pressure one side and highpressure the otherside of the boom (Coanda effect I think)This also creates some of the anti torque in the hover (think its 20 %)

Obviously as you move into forward flight the downwash is not flowing over the boom and the air pressure from the bucket is less effective so the 2 Rear verticals are also connected to the pedals (either by cable or in some configurations by linear motor)so in forward flight you have rudders!

And there you have it simple,a bit like me.

Flying it is very similar to a normal TR but the following are what I find to be the differences.
1.Firstly in the hover the pedals need a lot of movement,but in forward flight they are VERY sensitive (unless AC is YSAS equipped)
2.When its windy the pedal movements are quite big in the hover,but still plenty of power.
3.As the tail goes thru the wind there is no snap like a TR, everything seems far smoother
4.When you are coming into the hover from forward flight you actully have to apply more right pedal (as the downwash sticks to the boom)before a big boot of left.
5.In flight the vibrations/resonance of a tailrotor is non existant so its very smooth
With these in mind its just like any helicopter it has quirks,but I guess you learn to live with them and being a low hour pilot and having spent 50/50 in TRs and Notars I havent built up any great aversion to them. But when you land in a forest or in fact anywhere (even an airfield) not having to worry about people walking into(airfields) or things hitting the tail rotor does allow some comfort(I know the LZ is always checked and scrutinized but this thread would be here if accidents didnt sometimes happen,and its amazing how a running TR looks like a bicycle wheel to our canine friends who shouldnt be off alead but sometimes appear from nowhere.)
Autos:
These feel very similar to a normal TR.
TR Failures:
Again very similar but if the fan fails in forward flight you can actually land (at 40 knots ) using the rudders, having tried this to the concrete and to the grass it is a wild ride! (ever tried slowing down on skids on concrete with no TR authority form 40 Knots?and yes using the throttle/finger pointing method helps but you have to be quick to make it stay on the runway and not go pirouetting onto the grass)
As said before stuck pedals can be alot worse than normal TR because of the effect of the rear verticals
Problems:
Havent really found any yet, some people say that in heavy rain the fan has problems but having flown in heavy(maybe not american heavy)rain I have not found any problems.

Just make sure no one has thrown a bag of bolts in the fan intake on the walk round (can you believe this has actually happened to one, and it still flew,not as quietly though and there were a few red faces)

This is how I understand it,and have tried to make it as understandable as possible with goinginto the real tech side of it (that would tax my brain too much)and Im sure other NOTAR pilots will hopefully have more to add,with differing opinions I should think
Unfortunately some LOVE it some HATE it,me I just enjoy flying it.

Regards
hoverbover

hoverbover

15th Feb 2001, 20:01

To R W1
You posted while I was typing mine to EDEN sorry mine took so long to say what yours did.
However great the pictures with the NOTAR tail in the tree may look great, ask the guy who stuck his tail into a bush changed pedal input/rotated dustbin caught branches in hole that closed due to bin roating to let air out other side.
Result:cat caught by tail syndrome,and change of underwear for pilot.
Luckily he got away with it but the notar isn't infallable.!!!!!!!!!!!
Regards
hoverbover

RW-1

15th Feb 2001, 22:26

Hover,

Heard about that one myself ..... :)

Nothing perfect hehe ....

Liked your description, I had left out the variable aspect of the fan for simplicity, but you are right. Mine was a Q&D response.

The slots and high pres air add energy to the boundry layer of the airflow going around that side of the boom delaying separation, thus as I understand it intensifying the coanda effect.

I think you meant Florida heavy rain hehehe ...

Now if they could make it so that the heli doesn't look like it has a hige cigar stuck into it's A$$ .......

[This message has been edited by RW-1 (edited 15 February 2001).]

lmlanphere

15th Feb 2001, 23:08

what keeps the fan of the notar system running when the engine fails? or is it even necessary?

hoverbover

16th Feb 2001, 12:43

To Imlanphere

The drive to the fan is coupled to the main transmission in the same way as any TR drive,so providing ONLY engine failure and you enter auto its driven in just the same way as normal.
You do actually require the fan to work because after all it is just like losing an normal TR.(NOT GOOD)
Hope this clears it up.
Any other questions on the NOTAR fire away
and I'll try and answer them.

Regards
hoverbover

SPS

17th Feb 2001, 05:08

Nr fairy -

Hadn't even considered it to be abs. frank, don't fly them often. Good point and we'll leave it to the forum to see what they think (code for I don't know and I have to dash right now!)

RW -

Glad you like the little finger rule - It is just amazing what you can do with one!
:)

SPS

18th Feb 2001, 02:45

Rotorque -

Your thoughts on the tail rotor and the possibility of it entering autorotation are very interesting. I was similarly intrigued when making a study of the TR and I concluded with this –

<i>(Basic facts are established for the benefit of sideline readers and not to patronize anyone)</i>

‘Autorotation’ is a term used for MR harnessing of an incoming airflow to maintian RPM in much the same way as a windmill sails would (except that a windmill is static and the incoming airflow moves to it as wind but the Helicopter moves to the incoming airflow).

In this autorotative state the MR is not driven (rotated) by any power applied through its driveshaft from an engine but by the effect of the airflow, i.e. it is driven in reverse, driving the gearbox and anything connected to it.

The TR never enters autorotation in forward flight as it continues to be driven by the MR through the gearbox. It always receives shaft power for rotation. Whether that power is developed and delivered by an engine or autorotative force from the MR makes no difference at all. The TR is always driven by shaft power transmitted from a power source.

If this is taken one step further it is apparent that the TR is always making some thrust in one direction or another (unless TR blade angles of attack are truly zero or RPM are insufficient, etc.).

A Helicopter (of conventional design) in autorotation experiences ‘friction effect’ on the fuselage which tends to twist it in the opposite direction to torque, i.e. yaw to the left. This means that the TR must be capable of making thrust to either the left or right of the tail. It must balance torque during powered flight (by producing thrust to the right) and friction effect during autorotation (by producing thrust to the left).

So it’s a busy little thing eh?!

SPS

SPS

18th Feb 2001, 08:17

http://www.helicopterpilotsguide.com/Throttle_steering.GIF

First experiment.....Thanks to all who posted advice for graphic image placement.

Hope it is of use.

(Hope it works !!)

:) SPS

If you can see no light at the end of the tunnel then get down there and light it yourself.

Whirlybird

18th Feb 2001, 17:13

Just caught up on three pages of this thread after a week away, and off for another week in a couple of hours so will have to leave it for now. But one thing now really really bothers me. In company with someone else a page or so back, I never practised anything concerning tail rotor failure for my PPL, or while practising emergency procedures with an instructor sometime afterwards. I once mentioned stuck pedals to an instructor at some point, and got a quick little demo, but that was all. The fact that I'd read about what to do seemed to keep everyone happy. Doesn't keep me happy, now I think about it. Think I might find another instructor, or another school, and learn about this.

What are other PPLs' experiences on this? And many thanks to all who've contributed to this most interesting thread.

------------------
Whirly

To fly is human, to hover, divine.

Thud_and_Blunder

18th Feb 2001, 19:07

Sorry, been away - excellent thread.

Going back to a comment on page 1 by ShyT concerning throttle position in turbine helis, I'm pleased to say that the layout in the Bell 205 (throttle on the collective) saved an oppo of mine in Oman in 1983. He was in the hover over an underslung load at Saiq (6,300 ft amsl, OAT 20 deg...) when the tail rotor decided it didn't want to play any more. He rolled the throttle shut, kept the aircraft level and cushioned the touchdown with no damage to aircraft, aircrew, ground hook-up party or even the load. Top stuff - proves that what's been written in this forum really works.

RW-1

18th Feb 2001, 22:55

SPS:

Agreed, you wouldn't ever see the tail rotor in an autorotative state itself for the reasons you gave.

I have to admit though I haven't heard of friction effect on the fuse as the reason for left thrust, I always associated it with the tranny drag.
(Quoting from the basic heli manual: "The capability for tail rotors to produce thrust to the left (negative pitch angle) is necessary because, during autorotation, the drag of the transmission tends to yaw the nose to the left in the same direction that the main rotor is turning.")

Can you elaborate further?

------------------
Marc

Thomas coupling

18th Feb 2001, 23:27

When the helo is in auto, there is no torque applied to the main rotor! Therefore the airframe cannot yaw in the opposite direction to the applied torque. In the event of a total tail rotor failure, (a) the tail rotor will either have gone! or (b)have stopped, TR auto doesn't exist! There is consequently no friction effect.
When selecting auto after a tail rotor drive failure the a/c will recover some of the yaw induced by the malfunction(i.e. the yaw in the opp direction to the rotation of the MR). Then, when the engine(s) is/are stopped, this corrective yaw will increase (in your favour), though probably not sufficiently to allow you to be pointing in the direction you wish to go!. At the bottom of the engine off manouevre, when you flare like ****, the corrective yaw will again increase, substantially this time as the rotors speed up and the airframe (thru friction effect) yaws in the SAME direction as the main rotors. It is this rapidly developing change in yaw that one should strive to take advantage of to offset as much yaw as possible prior to cushioning the touchdown.. In reality because of lack of experience/judgement, 99% of all helo drivers will be unable to align the a/c fore and aft, and must therefore attempt to wash off ALL fwd speed (zero spd landing)prior to touchdown. Otherwise you will cart-wheel!!

http://www.gograph.com/Images-7298/AnimatedGif/redstar.gif

------------------
Thermal runaway.

SPS

19th Feb 2001, 00:08

Whirly,

I had the same concern when I got my PPL and a very good instructor showed me how much I didn't know.

The plain fact is (now I've seen it all from the other side) is that 45 hrs is barely (and often not) enough to get basic competence in most of the exercises (don't forget that bloggs may also have to learn all the other subjects from scratch at the same time)and it is almost dictated that this sort of 'extra curricla' subject remains just that. It is not in the PPL H syllabus JAR (or NZ) and there is not enough time for what is.

It is only the same as getting a basic drivers licence in many ways, no one teaches you skid recovery or similar unless you specifically ask fo it and even then they'll say 'Lets get the licence first' because they want you to succeed in that without too much blurring your focus.

Happily, more and more PPL's have a good, healthy attitude towards their flying and future training (like you have) and they
are doing what the rules don't. But let's not blame the rules either, we would complain like crazy if the PPL time where doubled (as it should be IMHO)and remember the govt, state or whatever cannot wrap us in cotton wool.

If it did we'd likely suffocate anyway!

To all PPL's I firmly recommend you go and do TR problem prcedures after gaining the licence, together with regular autos, EOL hover, etc. etc. and get to the stage where YOU have confidence in your ability.

If you want to know where you can get the best possible training for TR problems then mail me (NO financial interests in UK)

Fly safely all!

SPS

19th Feb 2001, 05:18

Sorry Thomas, beg to differ.

I agree that there isn't any torque applied to (or exterted on) the MR in auto. There isn't any in powered flight come to that. Torque is only applied to the fuselage in powered flight, the rotor is moved by power. Torque is a reaction to power.

When in auto friction effect does indeed make the nose yaw left and that is why we have to use far more right pedal than is required only to remove the left pedal input that was necessary to provide anti torque thrust to the right (at the tail) before power was lost.

You may prove this easily using one of those
model helis that are launched from a hand held spindle, spun up by pulling a string.
It has no TR and it will 'fly' with its fuselage yawing in the same direction as its MR blades. This is 'friction effect', a term I use as you cannot keep using the same ones. It causes unneccesary confusion (eg 'Friction torque'is a common and in MHO bad term) So, the above model has no TR but still has friction effect otherwise its fuselage would not yaw in the same direction as MR rotation.

The same is demonstrated by a model gyroscope (I still have mine and treasure it!) As it 'runs down' in rotor RPM the RPM of the outer gimbal will increase to almost parity as this happens, and it does so in sympathy with the rotor's rotation (if that's English!)

It is the same with full size helis in that friction in the drivetrain attempts to stop the MR and in so doing exerts a force upon the fuselage sympathetic to MR rotation. The desire is for them to get to equilibruim.
They will if allowed to.

So the TR has to make thrust to the RIGHT to stop this....and it is only driven by the MR through the gearbox.

Notice also that I said yest. that the TR does not enter auto 'in forward flight.'
(Just for interest). It CAN enter auto but to do it you must fly with 90 of starboard (or port) roll on and then descend almost vertically to get an upward incoming airflow going through its disc. I hope this is going to remain just theory and don't try this at home!

It brings up some interesting thoughts on addtion to TR induced flow though....

AH! and by the way....

French helis.... throttle same orientation,
more torque makes nose go LEFT, (right pedal being 'power' pedal'), close throttle little finger goes LEFT, nose goes right....NO!
LF rule is no good for the french. Sorry Jaques!

SPS

Never beleive what is in a box until it is open.

SPS

19th Feb 2001, 05:27

For the French it could literally be a 'rule of thumb'... :)

rotorque

19th Feb 2001, 15:19

Hi guys,

Just reading the last few threads and I thought that I should probably clarify my question (point) on T/R auto. My last post was in response to a tail rotor drive failure, ie: no drive at all. Sure, if there is any pitch on the tail rotor itself then there is no chance for an autorotative force. BUT - what if you could boot in some right peddal and zero the pitch ........ I have always been under the impression that there has been flapback on the tailrotor in forward flight, most of which is cancelled by the delta hinge but there is always a slight angle of incidence compared to the relative airflow. Nut over that one!!

As to the friction effect, (I always use transmission drag to explain it) You must use a little bit of engine power in the descent and landing when confronted with T/R drive failure. Firstly, in the descent not only do you have trany drag but most 'American' heli's, bar the B47, have a vertical stabilizer that produces lift to starboard (nose to port) in forward flight that must be accounted for with a little power. As the speed washes off when we flare we have to lose a little bit of power but still account for trany drag.

Lest to say - throttle control will still play a pivotal role in the successfull outcome of a tail rotor drive failure.

Here's a little side line. If any of you fly a B47, you probably found yourself using the throttle to keep you nice and straight when landing in gusty conditions without even noticing it. Hopefully the same will happen if/when the back end plays silly buggers.

Hey, don't worry - just don't forget.

cheers

SPS

19th Feb 2001, 15:40

Agree on the TR flapback - It does happen and thus it has translational lift too. It is also subject to a constant 'flare' effect which helps to reduce TR power requirment in forward flight (one reason we put in more right pedal as speed increases).

But it all means nothing if the thing does not rotate or its blades fell off!

And you are right, a little throttle tweak here and there helped the old girl to stay stable.....

hoverbover

20th Feb 2001, 01:44

To ALL

With regard to"throttle steering", I know this sounds stupid but what happens when "throttle" is not on the collective (109, Gazelle, etc)????
Having not flown this type of machine (yet) the question has been in my head.
I guess you run out of hands!!!!!!!!But seriously what is the answer(please forgive my ignorance)

To RW 1

I like the rain in Florida(its the only place I'd like to live other than UK),but when I was in Phoenix at MD they had more rain in 24 hours than I've seen in the UK for a month, but it felt just like home!!!!!!!!!!(they say the sun always shines on the righteous,guess thats why I'm always wet!!)

Regards
hoverbover

Lu Zuckerman

20th Feb 2001, 07:06

To: Rotorque and SPS

You stated,” I have always been under the impression that there has been flapback on the tailrotor in forward flight, most of which is cancelled by the delta hinge but there is always a slight angle of incidence compared to the relative airflow. Nut over that one!!”.

If you are addressing a two blade tail rotor such as that used on the Bell or the Robinson you can use the term “flapback” if you wish but what is happening is that the advancing blade has a greater degree of lift than the retreating blade. The advancing blade will flap inward and in doing so the delta hinge removes pitch and at the same time the retreating blade because it is mechanically attached will flap outward and the delta hinge will add pitch. In performing this flapping the delta hinge effect will equalize the lift across the tail rotor disc and this keeps the tail rotor from flying off due to the fatigue on the tail rotor gear box quill shaft. You get the same effect on a two-blade main rotor. If the advancing blade is caused to lift then the retreating blade will drop. The delta hinge effect will remove pitch from the advancing blade and increase the pitch in the retreating blade thus restoring the symmetry of lift across the disc. If the perturbing force continues “blowback”(flap back) will result.

On a multi blade tail rotor where the blades a free to individually flap only the advancing blade will flap and the delta hinge effect will reduce the pitch on that blade. The remaining blades will maintain their position in the disc track and only move when those blades become the advancing blade. On very large tail rotors where the individual blades are free to flap those blades are attached to the tail rotor head by a composite hinge that allows leading and lagging as well as flapping.

------------------
The Cat

SPS

20th Feb 2001, 13:32

LZ,

I do use flapback as my chosen label and it is similar enough to yours to be reliable.

I agree on the effects of the Delta three hinge and encapsulate it by saying that
"the Delta three hinge reduces flapping amplitude". But it still does flap whether two blade (which is where we were) or multi
blade for that matter. Thanks for the input, it looks like this thread will run and run until we have done all that could be done on the TR and that is actually no bad thing as it is too often ignored in P of F.

What about TR vortex next? Any takers?

And the Gazzelle/109 question - NO practical knowledge at all but I assume that the Pilot
would set the collective for correct (desired) ROD and move that hand to the stick and operate the throttle with the right or just operate the throttle with the left depending on position of the throttle.
But as I say, NO knowledge and I am just guessing.

offshoreigor

20th Feb 2001, 17:56

This has certainly turned out to be a very informative discussion. Four pages and everyones heads seem to be intact!

Thanks to all who have contributed, keep it coming but please let's keep it on topic and not too technical, it makes my brain hurt!

I know there is a temptation to get down to the last nut or bolt but if you can't affect it from the cockpit, then don't bother with it.

There's nothing wrong with having an in depth knowledge of all systems, in fact your engineers will love you if you do, but what I was after here were your thoughts on dealing with various tail rotor problems.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

Lu Zuckerman

20th Feb 2001, 20:04

To: SPS

You Stated,"the Delta three hinge reduces flapping amplitude". In fact, the delta hinge allows the tail rotor to, using another UK term, “flap to equality”. It is the positioning of the pitch links relative to the movement allowed by the Delta Hinge that either increase or decrease the pitch on the blades as they rotate. The amount of flap is a result of the aerodynamic forces acting on the advancing blade and the centrifugal forces acting on the blade system. A combination of these two forces will determine the degree or amplitude of the flapping. The Delta Hinge allows the flapping but it does not limit the flapping. Under extreme conditions the Delta hinge will allow the blades to hit their mechanical stops

------------------
The Cat

lmlanphere

20th Feb 2001, 23:01

to Thomas Coupling:

regarding your post on page 3, I don't follow your reasoning for the nose to continue yawing opposite the MR blade rotation in an auto once the engine has been removed from the equation. In an auto, once throttle is reduced, there is an immediate requirement for right pedal (american helo) which would lead me to believe that if the TR had failed, the nose would be yawing in the direction of MR blade rotation (left in american helos). Perhaps you could clarify your post, or correct mine... thanks

SPS

20th Feb 2001, 23:54

LZ

OK, let's look at it another way.

If a TR were produced and fitted without Delta 3 hinges (of course, the pitch links would still have to have control over blade pitch) would you expect it to flap more (its amplitude increased) than its Delta 3 equipped relative on another Heli.?

I think you would.

It could still flap to equality as does any MR, but would do so more slowly than if D3 equipped.

I accept that a TR with Delta 3 is still capable of hitting its stops but it would do so LESS readily (if at all) than a similar TR
without D3.

Flapping is indeed caused by dissymetry of lift which is countered by flapping to equality but the difference with D3 is that the process begins 45 degrees or so EARLIER
in the cycle and thus -

"The Delta 3 hinge reduces flapping amplitude."

Now let's not have a blood and guts, OK?

ShyTorque

21st Feb 2001, 00:40

A timely reminder here chaps! Some of this advice is becoming very type specific and needs to be heeded with great caution. It must be borne in mind that each aircraft has its own foibles and characteristics. In particular, the throttle response from a piston engine is very different from a turbine, mainly in the response and lag departments. Similarly, some aircraft are more directionally stable in yaw than others.

ALL the single rotor turbine powered helicopters I can think of require an engine off landing for a tail rotor DRIVE failure (possible exception is where the throttle/s is/are on the collective, not too many of these types about nowadays and I can't comment as I'm not type qualified on any of them). There is a very great danger involved in removing hand from collective in this situation to attempt to adjust power on a remote throttle /ECL except to shut down the engine/s, with the collective lever already on its bottom stop. Trying to adjust power to keep straight with a remote throttle in powered flight is highly likely to result in disaster, especially on a twin where you are manipulating two ECLs. Some turbine aircraft do not even have a "manual" facility anyway. Exceeding the upper or lower Nr limits is one easily possible result. Also, if you get it wrong, once the aircraft begins rotating you are very likely to have lost it completely. In my experience it is far better to accept that an engine off is the final outcome because you can be more sure of the aircraft handling. Some are forgetting that the aircraft's "weathercock" effect can be used to great advantage in autorotation, using what would normally be quite extreme sideslip angles to fly the aircraft towards a landing area.

Having experimented, practised and taught tail rotor drive failures many times, albeit in simulators, thank goodness (in both directions of main rotor travel) I think the most important advice anyone can receive is that of the manufacturer of the aircraft.

Read the Flight Manual, guys.

eden

21st Feb 2001, 03:22

Shytorque: (my response assumes you have alluded to my post on the 1st page) I'm suprised you feel that way about the power manipulation ..... I agree if poorly or excitedly used then you could very easily end up using too much or too litle power and have a resultant limit bust on the NR - max or min values. (if it's transient - who really cares - you have a bigger fish to fry)

IT IS IMPORTANT to STRESS that any manipulation of the POWER may only be necessary if you have a TR CONTROL problem or a TR FAILURE (in FWD FLT - and need to power on position to avoid the conurb or oggin below). In addition to this it is also important to ensure that either your crew or yourself as a single pilot fully appreciate the problem by EXECUTING A THOROUGH BRIEFING (if possible exercising some Throttle manipulation - in order to gauge it's effect prior to making the approach and using the NR variation in anger)

Hey - the problem is SEVERE and it requires skill, discipline and sound judgement, ALL within the capabilities of most HELI pilots (otherwise most of us wouldn't be playing the game).

It is essential that every opportunity is given for junior and senior pilots to practice such problems - it highlights the difficulties but more IMPORTANTLY - if considerately and sensibly practiced will give everybody more confidence in understanding that it might not be the end of the world.

By way of qualfication: I too have taught the TR problems in the SIM and in twin and Single engine MIL & CIV helos. I must agree to differ with you about avoiding using the aircrafts performance envelope.

But - support your concern if it points towards those that might hastily respond to the problem without considering their actions or those who try to use the techniques without proper and prior instruction.

Hoverbover: Interested what you were getting at in your throttle steering query ....could you expand on what you're not quite sure about?

eden

Thomas coupling

21st Feb 2001, 05:34

Imlanphere: In normal fwd flight, the airframe will try and rotate in the opp direction to the MRB, correct? If one increased collective(power), this reaction would try to increase(obviously we prevent it from doing so by applying corrective yaw pedal input). If one was to reduce the collective to almost a min pitch setting (auto) then the opposite would occur, correct?
Now, when the TR lets rip (!), then the airframe will immediately yaw in the opp direction to the MRB, correct? If you lower the lever fully (auto) then this dramatic yaw swing will be reduced (but not fully), correct? BUT, because your engine(s) are still switched on! the drive train will continue to transmit some power (torque) thru to the head thus the residual yaw will STILL be in the OPP direction to the rotation of the MRB. Now, when you switch the engines off....there is no REACTION thru to the MRB from the running engines, other, now than plain old friction, and it is this force which causes the airframe to attempt to yaw in the direction of the MRB's. This results in a further reduction of your yaw problem but will not completely remove it unless you are travelling with sufficient fwd speed to act as a wind vane also!!!

TR failure (fwd flight)will always provide you with a fighting chance if the engines are switched off. Leave them on (singles or twins) and you are dicing with death, literally!!

Throttle manipulation in modern helos (post twist grip) is NOT an option...

http://www.gograph.com/Images-7298/AnimatedGif/redstar.gif

------------------
Thermal runaway.

Vfrpilotpb

21st Feb 2001, 20:15

Tail Rotor failures,
There must be some of you Pro/Mil types out there who have actually hador suffered a T/R failure, it would help low hour jocks like me to read what your reactions and cures were at the time of impending doom, please tell!
My Regards 180Hr R22/R44/B206

------------------
Peter B

lmlanphere

21st Feb 2001, 23:04

thomas coupling: in the example above, are you stating that the nose will be yawing toward the advancing side, or toward the retreating side after engine(s) are switched off? (assuming some weathervane from normal autorotation airspeed)

ShyTorque

22nd Feb 2001, 02:22

To all,

Please note: I was & still am talking ONLY about the tail DRIVE failure here. The actions for tail rotor CONTROL failure are quite different.

Eden,

No, I was not specifically alluding to your post but I am becoming concerned that general advice on this thread is apparently heading towards favouring experimenting with power on, rather than entering autorotaion in what is probably the most critical emergency a rotary pilot will ever have to deal with, i.e. tail rotor DRIVE failure. Hopefully most of us will never have to deal with it for real. Get it wrong on the day and in many (if not all) aircraft you're possibly in a loss of control situation. I have seen many an occasion (in simulators only - agreed and thank goodness! - but in at least three different ones, two American and one French) where a pilot has elected not shut down the engines following a tail rotor DRIVE failure and has completely lost control well before hitting the ground. I lost control myself on a number of occasions before I had got my own skills up to scratch.

There is an infamous piece of film where a Bell 47 (throttle on collective, please note) on a photographic sortie over Sydney harbour, back in the 1960s, lost its tail rotor drive. The (relatively highly experienced) pilot tried to fly out of it under power and completely lost control. For its finale the aircraft descended in a very rapid flat spiral, finally tumbling over. The aircraft crashed hard onto a factory, killing all on board. I was first shown the film back in the 70s by a military QHI, himself now long since retired, who was a personal friend of, and an ex-colleague of, the pilot involved. The mainstream thinking even back then on the '47 was that the correct action in the event of tail drive failure was to autorotate and shut down the engine, which puts the aircraft in a more controllable and predictable flight regime. If he had done that he might have got wet but still been enjoying his retirement today. Whatever, this unfortunate pilot didn't survive to debrief it.

The yaw from transmission drag in autorotation is much less than from the torque reaction in normal powered flight and is quite controllable by use of opposite cylic, in other words cross-controlled. Many tail rotors have considerably less "negative pitch" range than "positive pitch" for this reason.

I cannot see how varying engine power on throttle will do much else more than cause Nr variations and further problems, especially bearing in mind that on something like the R-22, for example, that Nr can be absolutely critical. The main transmission power to keep the aircraft in balance following loss of tail rotor drive would only be that required to offset transmission drag plus any aerodynamic yawing effects from the tail and insufficient to sustain more than a descent.

I accept that it MAY be possible in a controlled situation for an experienced, well trained and pre-briefed crew to do something different and make a safe landing in some cases (hence my comment on reading the flight manual) BUT often, tail rotor drives fail at critical moments, at low speeds close to the ground and sometimes without much warning. A delay of a second or two before pilot action can make a big difference to the outcome. We used to get pilots to realise this in the sim by feeding in vibration as a pre-warning that something was amiss, then fail the tail drive. After a little coaching, usually the guys would get it right, they offloaded the rotor by entering autorotation and usually could fly the aircraft on cyclic, accepting the out of balance situation that resulted. Having done that, it was relatively straight forward to make a controlled landing PROVIDED that the engines were completely shut down before flaring at the bottom for the EOL. If the engines were left running even at ground idle the residual power caused loss of yaw control during the landing.

However, when we later didn't pre-brief them to expect it and gave no warning to the pilot by missing out the vibration, they tended to be slower with the collective lever and quite often lost control because with power applied the aircraft yawed rapidly and then rolled out of control. These guys were no dummies, believe me.

I do know of one notable exception to the above. A military Super Puma operated in the middle east in the early 1990s had a tail rotor drive failure at a high cruise speed (140 kts). The pilot found that each time he tried to gently slow down the aircraft yawed very badly. He admitted afterwards that he didn't attempt to enter autorotation, possibly as he was not too confident of his ability to carry out a good EOL. He elected instead to find a long runway, made like a fixed wing, and ran the aircraft on at 140 kts! He managed to keep the aircraft upright, but he used an awful lot of tarmac to stop! I would not personally have risked that technique but then I was fairly good at engine offs, having had lots of training and given lots of instruction in them on that type (including in the dark and on instruments to ground level just to make it a bit of a challenge if you like!).

I think following drive failure the tail rotor auto-rotating stuff is a red herring. If you react to yaw with pedal you will rapidly slow and probably stop the tail rotor. An auto-pilot heading hold will do the same. I don't think we should kid ourselves that it is going to start up again!

Sorry for the long drawn out post but I think it's very important for people with not much training on tail rotor malfunctions to get it right in their minds before it catches them out for real. Good topic!

eden

22nd Feb 2001, 02:43

Shytorque: Yeah - agreed .......I misunderstood where you were coming from.

CONCUR: TR failure - forget throttle playing!

Nr variation with a CONTROL problem can make all the difference and is something to be practised and thought about. Again ONLY - with someone who knows how.

Thanks for the reply - I too saw the same video it is very scary stuff.

I assume you were on PUMA sim?

rotorque

22nd Feb 2001, 07:37

Shytorque - good post.

To have access to a sim would be a god send to us all, I could only dream of getting my hands on one.

Just to clarify - the tail rotor auto thingy was just a scenario. A thought provoking question really. There was no intention of trying to use it to help in a T/R problem. I know for a fact that I would instictively try to boot in opposite peddal if I lost the T/R drive - that alone would be the end of the conversation of T/R auto.

I did throw in a comment on whether you could overspeed a tail rotor by having it in the 'auto' pitch setting (zero pitch) with a drive failure. Could there be a risk of exacerbating the problem if you threw off a T/R blade without even knowing it was going to happen?. Once again just a tongue in cheek thought. No need to do the tests.

I am going to have to back peddal a little now cos' on a previous post I talked about using a small amount of engine power on descent and landing when confronted with a T/R drive failure. As mentioned previously it is very important to know how each aircraft responds to various problems. I wrote the response with the B47 in mind. It was not my intention to use it as a generic answer to tail rotor problems although I guess it was centred on collective mounted, throttled helicopters. I was 'lucky' enough to work with a bloke who had a short shaft failure in a B47 whilst mustering. To this day he does not know how he flew away but I presume he had just enough residual forward speed and a hell of a lot of crossed controls. Anyway he was able to fly back to the station which had a gravel strip. When he rolled off the throttle completely to enter auto he found that the aircraft yawed to the left by a considerable amount. In the end he had the engine ever so slightly past the idle all the way to the ground and as he leveled he wound the throttle off completely and landed. All of this to the benefit of us young blokes.

To this date I have taken the example as gospel but always wondered whether the tail rotor had some residual thrust or if the position of the free wheel unit in the transmission played a part in the outcome. I don't know.

Reading the posts I have come to the conclusion that I could be in varying degrees of trouble if I use the same thought process for different aircraft. I am therefore following the thread with renewed interest and an open mind.

Shytorque's experience in the sim may be invaluable. Please note, as mentioned earlier there really does need to be a distinction between collective mounted throttles and power levers. The rules do change.

keep it coming.

noaltitude

22nd Feb 2001, 14:08

I have been holding back as I thought this may have been mentioned before but so far I haven't seen mention of this so I thought I might add my 2 cents. I have been shown a recovery from a low speed high power JAMMED pedal event. As the aircraft starts to rapidly rotate cyclic can be aplied toward a constant heading ie "stir" the cyclic in the opposite direction of aircraft rotation. It allows you to gain airspeed in a constant direction and lower power post translation to a point where the aircraft will once again fly in something close to balance.

Interestingly enough the instructor who demonstrated this tecnique subsequently experienced a loss of tail rotor and tail rotor gearbox in the hover and landed without rolling into a ball.

212man

23rd Feb 2001, 01:29

I agree that simulator training is a very valuable way to experiment with handling TR problems but would add the proviso that the simulation may be innacurate.

Having flown the FSI 212 sim at DFW several times it is easy to come away with the impression that a TR drive failure can be controlled fairly well. In an extreme example which was basically a bit of 'wazzing' I took off from the simulated aircraft carrier, had a loss of TR drive at about 65 KTS, flew the thing horribly cross controlled to about 1000' abeam the ship then autorotated back down to the deck and survived. I don't believe that to be a true representation of the a/c's behaviour. The SAS sim at Stockholm, on the other hand, is a newer generation. TR drive failures can be truly aerobatic events, generally resulting in the sim freezing with the 'world' inverted, to protect itself.

Having spoken to pilots that have had TR drive failures in two separate AS332 accidents, one happened in a relatively high power setting in level flight and even entering autorotation did not stop the rolling/pitching/yawing; the PNF had to retard the speed select levers before things settled down. The handling pilot compared the manouver to a lomcevak! (negative flick roll going vertical, in an aeroplane). They executed a text book ditching and were all saved. The other one happened in a descent at 70 kts after the pilot had diagnosed the probability of impending TR drive failure and was a far less dramatic event, again resulting in a perfect ditching with no casualties.

Yet another accident with a 330J in the cruise resulted in the a/c tumbling all the way down to the sea.

So, being aware of the variables and trying it out in a sim is great, but come the day there is a very good chance things may not go to plan.

I suppose that is one thing in favour of fenestron a/c; they should fly happily in a straight line at about 100 kts with no drive.

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Another day in paradise

Flypro

23rd Feb 2001, 02:50

Having experienced a tail rotor failure in a rather large Wastelands product from a 40ft hover over the sea, I can assure you all that the previous theory lessons, good though they are, go straight out of the window!!.
In my case there was no warning, the helo just started to turn. Both myself and the other pilot automatically applied full opposite boot, thus we were both aware that tail rotor control had been lost before the end of the first 360 degrees. The rate of turn was rapid and disorienting but lowering the collective immediately significantly reduced the rate of turn - albeit in mid plummet.
We hit hard and bounced out of the sea into hover No2 where the a/c started to turn again before droppping into the 'oggin and rolling over. All escaped with no injuries.
Two lessons learnt from this. 1.In a hover you must lower the lever quickly, hack the engine if you can and cushion the touchdown with collective. 2.If you dont have your shoulder harness locked in every hover you are a prat!

eden

23rd Feb 2001, 03:13

Flypro - are you the "Certsmith"? who transitioned from Dark to Light? - Sorry If I have mis id'd you and if I have -I'm sure you have no idea what I'm talking about.

GOOD POST THO'

ShyTorque

23rd Feb 2001, 03:42

212 Man and Flypro,

Glad we all agree then, on the simulator thing about possible inaccuracies. (Please read my post of 13th Feb, 22:43 hrs if you haven't already; where I mentioned "off model" programming by the sim manufacturer). My experiences on the 332 sim carrying out T/R failures of all types were interesting to say the least.

Some (very experienced) pilots were quite alarmed by the response of the "aircraft" (sim). However, they unanimously agreed that they had a far better chance of a survivable outcome following their sim training. If nothing else, a discussion of the possible different scenarios is extremely useful. The proof of the pudding came a few years after we began training RAF pilots; a military 330J (no floats!)was successfully ditched following a tail rotor spider failure. The pilot said afterwards that had he not done his sim training he knew the outcome would have been different.

Personally, I would rather have a total drive failure than a runaway to FULL maximum or minimum tail rotor pitch, at least on the Super Puma. Either of those was in our experience, uncontrollable in most flight regimes. If the tail rotor ran away to maximum positive pitch on that type we found that even pulling the collective up as far as it would physically go (clunk) would not stop the right yaw caused by this failure. You then end up in a rapid spiral climb at full power and rapidly run out of ideas (throw baggage into the tail rotor to fail tha drive? It was suggested by one pilot!).

Similarly, if the pitch ran to fully "negative", even shutting down the engines did not stop the left yaw. This results in a spiral descent, very nasty too.

This may well be so on many other types, which is probably why many are fitted with some sort of centering device for the hydraulic servo for the cable break / malfunction situation.

One thing is for certain these days. I always check the tail rotor components very thoroughly when it is my turn for the check 'A'! I have already discovered one newly re-conditioned (!) and faulty tail rotor control spider.

ShyT.

Flypro

23rd Feb 2001, 21:10

Eden.
Sorry, I aint he. Not a clue what you are talking about,and I certainly did not transfer to light blue - there is a code you know!! http://www.pprune.org/ubb/NonCGI/eek.gif

Shytorque (now where have I seen a bar with a very similar name?)

The RN and RAF SeaKing Simulators are both very good for practising tail rotor problems and certainly in the case of tail rotors stuck at high or low pitch they fly very much as the real aircraft does. For a tail rotor failure at height they rely on data supplied by Wastelands as nobody has had one for real yet (I think).
In the Sim, to retain any control whatsoever the a/c must first be dropped into autorotation (and quickly at that). Once established, if possible go for an engine off landing, if not you can try to reapply power and sometimes manage to arrest the rate of descent for long enough to position the a/c over more suitable terrain for the subsequent engine off.
Another unofficial technique for tail rotor failure that works well in theory and in the Sim is to shut one engine down and set the other one at low torque, just enough to counter friction within the transmission. The a/c then handles almost exactly as it would in a normal auto.
I think the large keel area of a SeaKing prevents it from immediately spinning after losing the tail rotor in forward flight and give one time to lower the lever.
Hope this helps!

Finge Et fuge (Bodge it and Scarper)

[This message has been edited by Flypro (edited 23 February 2001).]

ShyTorque

24th Feb 2001, 00:01

Eden,

Intrigued but I don't know where your bar is.

I thought of my user name after spending a few years dodging large hawks that circle eastern parts looking for - Shyte! Torque is appropriate for someone in our line of work. I also now circle looking for - Shyte!

Someone else on this forum has copied it but he's not as subtle as me!

lmlanphere

24th Feb 2001, 01:02

it seems that a full right or left stuck pedal would be a highly improbable situation - simply because they aren't used to the extremes all that often - unless there's some reason why the pitch would continue to increase or decrease to its full stop position, I guess.... seems that most "stuck" pedal situations would occur during flight regimes where the amount of stuck pitch was appropriate....any thoughts?

212man

24th Feb 2001, 02:57

Does it not depend where the centre of pressure acts relative to the feathering axis and hence how the centripetal turning moment moves the pitch?

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Another day in paradise

Flypro

24th Feb 2001, 03:17

Shytorque.

I was thinking of *****hawk (though I reckon someone else posts under hat name)

Imlanphere.

A hydraulic hardover that you can't switch out would do the trick ie full range deflection.

212man.

Ask LZ - he's the theoretitian!!

ShyTorque

24th Feb 2001, 04:02

The RAF Puma flew for 20 years plus in a condition likely to give a hardover in yaw due to the lack of a centre-ing device in / on the servo mechanism. It would happen if you lost one of the 2 pull cables to the tail rotor hydraulic servo valve. On this type there is no manual reversion and no way of isolating the hydraulics.

We looked at this type of failure in the sim, and discovered it was probably irrecoverable, as I intimated earlier. I did some personal research in the mid 1980's and discovered that the Norwegian aircraft had a centre-ing device fitted. Myself and other QHIs afterwards bleated long and hard about this and I think we eventually got through to the powers-that-were, as all the RAF aircraft are, I believe now modded with a centre-ing device. If not, they Bl**dy should be!

Even with such a device fitted, a broken pitch control spider can still give a hardover. Check the UK AAIB site for the report on the S.Wales police helicopter that landed on the roof of a house last year.

It's worth all heli pilots training for this in a sim. It doesn't matter which type as long as the main blades go round in the same direction as your helicopter (otherwise the yaw directions can get a bit confusing). Many heli pilots believe that closing down the engines is the answer to all tail rotor malfunctions. It ain't!

ShyT.

[This message has been edited by ShyTorque (edited 24 February 2001).]

lmlanphere

24th Feb 2001, 21:30

I believe I read somewhere that if the pitch control mechanism were to be severed somehow the tail rotor blades would work themselves back to zero pitch automatically, but maybe thats type dependent

Ed Winchester

25th Feb 2001, 00:15

ShyTorque,

If the centering device you refer to is the tail rotor spring bias unit, then I think we are still waiting for it to be fitted (along with a torque meter/anticipators/Makila engines!). As you have said, the simulator is an invaluable tool to get you thinking about tail rotor malfunctions. I'm pretty sure that if it happens for real however, the 'What the f...' factor will precede the 'THROTTLES!' call by some distance!

Lama Driver

25th Feb 2001, 00:21

Been following this thread with some interest. A couple of points/questions. I had a TR drive failure in a 204 over trees during longline ops (just powering up for a lift). I found the most surprising element of the failure was the severe nose tuck almost immediately drive to the TR was lost. It required almost full aft left cyclic input to maintain a wings level attitude during the early part of the rotation. I was lucky in that I identified the failure early and shut the throttle after 180 degrees of rotation which immediately stopped the rotation and levelled the aircraft. The trip down thru the trees was "intersting" but both myself and my co are here to tell the story today.
I have never done any sim time but I am intereted to know whether the 212 sim replicates similar tendencies to what I experienced. Also over a number of years of flying I have never heard any instructor discuss this point when briefing for TR failure. Any thoughts?
PS
Loss of drive to the TR was caused by the failure of the grease coupling on the TR driveshaft immediately aft of the TR output quill on the Trans. As a point of note never trust the Zinc chromate paint lines on the couplings to indicate high coupling temps. The Bell temp indicating strips (like they use on the R22) are the only way to go!

Out of Balance

25th Feb 2001, 02:43

This will probably be common knowledge to most of you, however, it was certainly news to me at the time.

Flying the 412 sim at DFW I was given a tail rotor drive shaft failure in forward flight. The drill is to enter autorotation immediately but my instructor suggested that I continue flying (80kts) and see what happend.

Horrible right yaw and nose tuck ensued, but it was certainly controllable, so we decided to return to the airfield for an autorotation. As the airfield was in our 4 0'clock position I began a gentle turn to the right (into the yaw). Bad move. We very quickly lost all airspeed and began to lose control of the beast.

If you need to turn, then turn away from the yaw as it is much easier to maintain airspeed.

Thomas coupling

25th Feb 2001, 19:05

Lama Driver: your input must be worth its weight in gold...thanks for sharing your experience, hope some of it is remembered by others. http://www.pprune.org/ubb/NonCGI/eek.gif

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Thermal runaway.

Lu Zuckerman

26th Feb 2001, 00:35

To:212 Man

“Does it not depend where the centre of pressure acts relative to the feathering axis and hence how the centripetal turning moment moves the pitch”?

Hopefully this does not start another international incident due to semantics but I believe you are talking about “Centrifugal twisting moment”. The centrifugal twisting moment coupled with the aerodynamic forces are what you feel when you move the pedals during flight, or in other words, you experience the associated feed back. Propellers on aircraft have the same condition. If say on a Hydromatic propeller you lose oil pressure the centrifugal twisting moment will return the blades to low pitch and if you don’t catch it quick enough you can have a runaway engine.

To: Imlanphere

“I believe I read somewhere that if the pitch control mechanism were to be severed somehow the tail rotor blades would work themselves back to zero pitch automatically, but maybe that’s type dependent.”

You are correct in stating that the return to low pitch is type dependent. Some helicopters that do not incorporate compensating weight would return to flat pitch if the control were severed. Many helicopters incorporate dynamic or static counterweights. The static weights are incorporated as a part of the blade on the Bell 407. Helicopters like the Sikorsky S-58 incorporate dynamic counter weights that move when the tail rotor pitch is changed. The purpose of these weights is to counter the centrifugal twisting moment so that the pilot only experiences the aerodynamic forces if he has to fly with the tail rotor boost shut off. On this type of tail rotor the dynamic or static weights will under ideal conditions maintain the blade pitch in the last commanded position when control is severed.

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The Cat

Thomas coupling

26th Feb 2001, 02:25

Lu: calm down, we are do'ers not thinkers...who the hell cares anyway if one has suffered a TR failure the god damn thing has stopped!!!!Feathering or no bloody feathering!!!

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Thermal runaway.

offshoreigor

27th Feb 2001, 20:05

Thanks to ALL for the excellent posts.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

DeltaTango

30th Jun 2001, 17:11

A question to you rotorheads:
We had an Apache lose it's tail rotor a few weeks back, which then went on to land safely by maintaining a high speed so the tail would act as a stabilizer.

Anyone care to explain this in a bit more depth?

Cheers
DT

Ancient Pelican

30th Jun 2001, 20:08

DT,
can you explain in more detail what you mean by "lost" tail rotor. Did he/she lose the tail rotor gear box and blades, lose tail rotor control, or lose tail rotor drive.
The exact emergency procedure is different for all of them.
However, the vertical stablizer has a function to act as a air foil and produce thrust to help maintain streamlining of the fuselage. This unloads loads the tail rotor, the horse power that would have been used to drive the tailrotor is now avialable for the main rotor to use for lift and thrust. The streamlining is most efficent at some airspeed, probably in the neighborhood of 60kts, that the designers have determined. The streamlining should allow the helicopter to be landed with out tail rotor contol, or drive with a smaller pucker factor margin. The exact procedure is individual to the type of heliocpter, so this is a generic statement of the function of the vert stab in tail rotor failures.

Nick Lappos

30th Jun 2001, 20:39

Loss of tail rotor is a big problem in any helicopter, especially since the vertical fin (which now must provide all your anti-torque) is often reduced in size to reduce blockage of the tail rotor to help increase tail thrust for better crosswind control.

The previous post that discusses the different problems depending on how much hardware is shed is correct, but only in the additional problems that a CG shift bring about, where the tail is immediately a lot lighter at the same time that the tail rotor thrust is entirely gone. For the rest of this discussion, let's assume that the tail rotor thrust is now zero, and no other problem is compounding.

The vertical fin can help in recovery from a loss of tail rotor thrust by providing some right thrust, but it will do almost no good until speeds of about 1.5 Vy, typically about 110 knots in a high speed helo (S-76, Black Hawk), maybe 80 knots is a slow Bell type machine, and maybe 60 knots in a light piston.

This is because the fin area is usually small, and it takes some forward speed for it to generate enough lift (side force) to be much help.

If you try to fly home at higher speed, the increased power you need means that you also need more anti-torque, so you might not make a powered flight situation work.

How do you know? I suggest that if you experience loss of tail thrust in cruise flight, (sharp right yaw in an American helo, left yaw in a French or Russian machine -- I will use the American convention below) get control of the yaw situation first. Lower the collective immediately enough to stop all yaw and allow a steady state descent. Carefully guard your airspeed, try to stay at a high speed autorotative glide speed (about 1.5 Vy in most helos). If all is going poorly, go to full auto and take your chances on the landing. If the glide looks good and yaw is well behaved, you MIGHT try to gently increase power to reduce the rate of descent, while maintaining speed. The aircraft will start to yaw right as power is applied, so you must cross control by applying left bank to keep the heading stabilized. You will probably have to go more nose down because you have much more drag, and you must watch for two big danger signs - excessive yaw like a flat spin, and loss of airspeed indications, where the pitot probe might give up working in the big sideslip. At the first sign of either danger sign, lower the collective pitch and the nose to get the torque down and the speed up.

For most helos, you will not be able to get to level flight because to torque you need will produce an excessive sideslip or even a flat spin (the ultimate problem in all this test pilot heroic stuff). Settle for a reduced rate of descent, since it really helps you extend your choice of landing areas. If you can get to as little as 750 feet per minute descent, and you are doing 100 knots, you can travel 3 miles from 1500 feet, almost 7 miles from 3000.

At the bottom, transition to a reduced collective flare, and make a part auto-part running landing. Don't increase the collective at touchdown unless you have decoupled the engines, as the sudden torque rise will create a big spin, and a real crack-up.

The skills needed to fly out of an anti-torque failure are not trivial, and most pilots are best served by thinking and performing an autorotation, period.

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Flight Safety

1st Jul 2001, 05:56

Nick, I'd like to ask you a rather odd question, given your test pilot background. Would a small drogue chute help with a loss of tail rotor problem?

My thinking is that the drogue could be released from the extreme end of the tail boom (below the tail rotor), and the chute would also need a jetison mechanism. I was wondering if you've ever heard of such an idea.

My thinking is that following the loss of tail rotor thrust, and once the yaw has been stabilized and a descent rate with good forward airspeed established, then the drogue could be released. It seems to me this could help to add additional stability to the yaw so more power and collective could be re-applied. Of course good forward airspeed would have to be maintained for the chute to remain affective in contolling the yaw.

If the chute were effective enough at controlling the yaw so that a small climb rate could be established, then you'd have your choice of landing areas. A run on landing might be easier to accomplish with the chute, and at the very least perhaps you could choose the spot to perform an autorotative landing, which is much safer to perform if you can be selective about the landing area.

Does this idea make any sense?

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Safe flying to you...

[This message has been edited by Flight Safety (edited 01 July 2001).]

GulfPLt

1st Jul 2001, 06:25

I have had two T/R malfunctions. The last one was in an A-Star while on final to an offshore platform. Prior training had taught me how to land after experiencing a T/R malfunction, but not how to get out of the spin. I increased the collective, which initially increased the spin, but I had to avoid the crane on the platform. Started feeding in forward cyclic and eventually got it under control. Not easy on any day, but a little more trying with two passengers screaming like girls. Flew back to the beach with a slight slideslip. Above 40 kts. the aircraft will streamline. This goes for a 206 also. Did a running landing in the grass. Slowed down to 40 kts. on final and slowed it up some more. This initially increases the yaw rate. At the bottom, pulled on power, the nose aligned, and ran it own. I wouldn't do an auto unless you have loss of components. Why give yourself another emergency to deal with.

Flight Safety

1st Jul 2001, 07:34

GulfPLt, you're experience reminds me of an idea I've often had regarding loss of T/R.

I saw a video about a year ago of an ANG Blackhawk performing a rescue of an injured climber in a small but deep rocky ravine (if I remember this correctly). The ravine was too small to land the helo in, and the rocky sides were so narrow they would have caused rotor contact if a landing were attempted. Surrounding this small ravine on 3 sides was fairly flat terrain.

In the video, the Blackhawk had just gotten into position to lower the winch cable, when suddenly it lost tail rotor thrust. The Blackhawk began to spin, and for some reason that was not clear to me, it went straight down into the ravine in a slow flat spin, destoying the main rotor in the process as it contacted the rocky walls. The airframe landed right side up at the bottom and all the crew survived (some with injuries), and another Blackhawk had to arrive on the scene to winch all persons out of the ravine.

What puzzled me was why the pilot didn't apply more collective and climb, so he could straighten it out with a little altitude and attempt a landing on the surrounding flat terrain. I know this would have increased the spin rate, but as long as the airframe structure can tolerate the increased spin, this would have been far better than destroying the helo in the rocky ravine.

So that's my idea, why can't pilots do what "GulfPLt" did, and just climb if needed while enduring a higher spin rate, if that's what it takes to get out of trouble following the loss of T/R, especially at low altitude above an uninviting landing area?

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Safe flying to you...

Puch

1st Jul 2001, 09:11

Regarding the T /R loss. Two cases are to take into account: loss of the T/R itself or loss of the control ( linkage between pedals and rotor).

The first case is worst because, there is no more thrust anymore and the stability of the fuselage is ensured only by the fin, if the helicopter is equipped with such device. In such a case, if the failure occurs in hover, there is no chance to recover the helicopter as the torque, at the occurrence is too high. If the failure occurs in flight, the helo is flyable but the only solution to rejoin the ground is autorotation.

In case of loss of control, the case is less dramatic as there is a remaining thrust. I remind my instructors teaching me the high side loss or low side loss, with specifics procedures according to the case. Modern helicopters have sometimes a damper located on the T/R system which set the T/R blade pitch angle at a standard angle in order to provides a permanent thrust. In such a case, running landing can be attempted with good chances of success.

Take a Dauphin. With its huge fin, loss of T/R control in flight could be unnoticced. It is possible, but not advised, to land such helo in hover while experiencing a T/R loss of control if the helo is light and there is a 15+ knots of wind set exactly on the RH side of the helo during a low flat approcah to land.

Cheers

DeltaTango

1st Jul 2001, 12:32

Ancient Pelican-he lost the rotor completely, it detached itself from the tail.
The crew got a reading saying it was stuck, but saw it fall/spin away.

you all keep saying that you can still fly the helo. At high speeds I can understand this, but how can you keep it straight during landing?
The only landing I can see is a running one-yes?

Thanx for the replies-very interesting.
DT

212man

1st Jul 2001, 15:00

I think you will get a dozen different answers to the same question with this one, depending on individual's experience.

Touch wood, I have not experienced a tail rotor problem though I have spoken to several pilots who have and I also include stuck pedals occasionally on Proficiency checks.

What is evident, with a stuck pedal, is that the transition from seemingly controlled flight to loss of control can be very rapid and with little warning. In the event, disciplining your actions against your instincts can be dificult.

The yaw rate that could develop with total loss of thrust must be huge and extremely disorientating. I suspect that if you were to exacerbate it by pulling collective to climb, you could get yourself really 'lost in space' and the rate of yaw may not beacome recoverable despite any actions. Throw in loss of componants and the ensuing C of G shift and things get really interesting. I could imagine that with a teetering head there would be a real risk of blade strike to trying to dive on speed while rotating like a Derbisher.

I know, personally, 4 pilots who have experienced loss of tail rotor componants and thrust in 4 different Puma and Super Pumas. In every case they were in the cruise or a shallow descent, with good speed. In every case they rotated rapidly about three axes and in two case crashed/ditched out of control. The other two were recovered to autorotation only after closing the speed select levers (collective insufficient on its own) and successful ditchings followed.

I also read an article in the Navy's flight safety magazine, written by the crew of the EH101 that crashed following a tail rotor control failure at 10,000. The 4 crew bailed out after all attempts to regain control were lost. The description of the rotational g forces and attendant escape problems was particularly interesting!

So, basically, I think it would be true to say that a tail roror failure may not be the end of the world and there are many cases of successful recoveries from such problems (eg Gulfplt). However, I think it is niaive to think that it is a straight forward problem that can be recovered from using a standard technique. Glib remarks like "I don't understand why they didn't JUST pull power and gain height" seem to show a lack of understanding of the nature and seriousness of the problem.

BTW, I don't know if you are at the FSI Bell facility,FSI, but if so I do think the simulation of tail rotor problem in the 212/412 sim is a touch optimistic. You come away (or could do) thinking it would be no problem at all.

Like the idea of the drogue shoot, FW have used them for years for spin testing.

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Another day in paradise

The Nr Fairy

1st Jul 2001, 15:00

See these recent threads which have appeared on Rotorheads :

Tail Rotor Problems (http://www.pprune.org/ubb/NonCGI/Forum11/HTML/000765.html)

Tailrotor Fun $#@! (http://www.pprune.org/ubb/NonCGI/Forum11/HTML/000527.html) - Harry, did you get any responses and have you had a chance to collate them ?

As I've never had a t/r failure for real, I don't suppose there's someone out there who has access to the right people and the right toys to play with this sort of stuff in the sim, is there ? Perhaps a bottle of whisky for the best answer ? :)

[This message has been edited by The Nr Fairy (edited 01 July 2001).]

Heliport

1st Jul 2001, 16:11

Nick Lappos
Thanks for the post. Hope to read more contributions from you from time to time.

http://www2.gdi.net/~nlappos/jobsite.jpg

Nick Lappos is Sikorsky's Assistant Chief Test Pilot. A former U.S. Army Warrant Officer and a combat veteran of Vietnam, he flew Cobra attack helicopters in D Troop, 1st Squadron, 1st Cavalry. An Aerospace Engineering graduate of Georgia Tech, he came to Sikorsky Aircraft in 1973. Since then, he has flown in the YUH-60 Black Hawk program, flown the first flight and structural shakedown of the S-76, the "Shadow" cockpit research aircraft, the adaptive fuel control experimental aircraft, and the "Fantail" fan-in-fin demonstrator. He is now assigned to the RAH-66 Comanche project, where he is part of the pilot team flying the shakedown and envelope expansion.

In 1988, the Society of Experimental Test Pilots awarded Nick their Tenhoff Award for the most outstanding paper at the Annual Symposium. The American Helicopter Society awarded Nick their Feinberg award as most outstanding pilot for his contribution to the understanding of maneuverability in 1989 and again in 1998 (as part of the Comanche Team), and in 1994 he was named a Fellow of the Society. He was a member of the American Helicopter Society's Handling Qualities committee and AGARD Working Group #19 (Functional Agility), and is currently a member of the NASA/FAA Air Traffic Management R&D Executive Steering Committee.

Nick has written a number of technical papers, and articles for magazines such as "Rotor and Wing," "Interavia," and "Defense Helicopter." He holds 15 U.S. and a number of foreign patents on flight and engine controls and cockpit displays.

http://www2.gdi.net/~nlappos/gearup2.jpg Photo of Apache during initial flight tests.

baranfin

1st Jul 2001, 22:11

this is a video clip of a t/r failure at hover. Its pretty sobering.

http://www.helis.com/movies/taiwan.rm

212man

1st Jul 2001, 22:34

Ties in with what I was trying to say. "why didn't they just pull pitch and fly away?" Yeah, right!

In fact the initial problem wasn't a tail rotor one. The a/c had a P2 leak due to a maintenance error, when it tried to come to the hover the power loss meant it started to sink and the Nr drooped. The pilot tried to arrest the descent by pulling more collective and they settled on to the water, at which point the tail rotor drive shaft failed as the fenestron started sucking water rather than air. The newly released power to the main transmission was sufficient for the a/c to get airborne again, with the pilot still pulling, at which point the a/c started rotating freely. Sadly, one of the pilots drowned.

Now you can see why check lists for a failure in the hover often say; "lower the collective immediately and accept the yaw". If you don't, it's going to get a lot worse very quickly.

PS The Taiwan 365 info came directly from Eurocopter in response to a question about fenestron power consumption in the hover (I'm writing from Marignane)

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Another day in paradise

widgeon

2nd Jul 2001, 02:18

Hm that Apache looks at lot like a commanche. I guess is hard for us paleface to tell them injuns apart though. Nice pics.

Flight Safety

2nd Jul 2001, 05:27

212man, where did I EVER suggest a modification to SOPs, WHERE?

My ideas run along the line that I'm convinced that more can be done in helo design to make loss of T/R more survivable (both for pilots and airframes). Obviously this puts the ideas in the realm of experimental flight test, where they belong. Clearly ANY new procedure has to be thoroughly understood through analysis and flight test before incorporating them into ANY SOP or emergency procedure. You're not foolish enough to violate this fundamental wisdom of flight, and neither am I. Enough said.

Anyway, FWIW, I was already having second thoughts about the drogue chute idea, as I see 2 problems already. One, a tether of any length could allow the chute to get caught in either the tail rotor or the main rotor with bad results following. Second, in forward flight with any airspeed, the chute could get caught in the rotor downwash causing an interesting pitch problem.

A third idea is that it might be helpful to design the tailboom as a structure that's both fairly narrow with broader flatter sides. This might help in 2 areas. One, it could add more keel area to the rear of the airframe, thus augmenting the vertical stabilizer in yaw control in forward flight after T/R loss occures. Second, if you added collective to climb out of a bad situation, the broad flat sides could help control the spin rate by offering significant drag in the spin (all to be tested and validated in fligh test of course). The tailboom and remaining airframe structure would have to be stressed for the spin rate produced by a slow climb after loss of T/R, if such a capability were to be designed into an airframe.

The main subject of this thread was landing after a loss of T/R. I think its fair to discuss both ways to accomplish this with current SOPs, and what could possibly be done in the future to improve this particular situation in helo airframes.

All I know is that climbing out with increased spin rate after T/R loss is a reasonable idea with some merit in certain circumstances, as it surely kept GulfPLt from getting tangled up with a crane. His passengers may have been screaming like girls, but at least they were able to safely clean out their drawers after he landed.

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Safe flying to you...

[This message has been edited by Flight Safety (edited 02 July 2001).]

Heliport

2nd Jul 2001, 09:57

Army grounds Apache fleet for safety checks

WASHINGTON (Reuters) - The U.S. Army has grounded its entire fleet of Apache attack helicopters to inspect the tail rotors on the aircraft, an Army spokesman said Thursday.

The inspection was ordered as a precautionary measure following a June 11 incident involving a faulty tail rotor on an Israel Apache, spokesman Thomas Collins said.

"This action is a precautionary result of an Israeli AH-64A aircraft incident where the tail rotor head assembly separated from the aircraft in flight,'' Collins said.

Collins said each inspection would take about 30 minutes and would determine whether the tail rotor needed to be replaced. He could not say when the inspections began or how long they would take to complete.

"We're trying to accomplish the inspection as quickly as possible, but we also have to be as thorough as possible because the safety of our soldiers is of paramount importance,'' he said.

Collins said the tail rotor on the Israeli helicopter came apart during a routine flight. The aircraft landed safely at an Israeli air base, he said, adding that an investigation was under way to determine the cause of that incident.

The AH-64 Apache attack helicopters, built by Boeing Co (BA.N), have been grounded twice before, both times to check potentially dangerous tail rotor problems. The tail rotor controls the aircraft's ability to turn left or right.

The fleet was grounded in December 2000 after the discovery of a faulty tail rotor "swashplate'' assembly and in November 1999 after the Army found suspect rotor bearings and transmission problems following crashes.

Six Army Apaches crashed in 1999. Army officials said investigators found that a heating process used by Boeing to make the bearing assembly extra hard led to stress corrosion fractures in the bearings. ^ REUTERS

Flight Safety

2nd Jul 2001, 10:32

I wanted to try and summarize these weird ideas on loss of T/R I've posted, and relate them to what I think is the current state of dealing with the loss of T/R in the helo industry. My opinion is that dealing with the loss of T/R thrust is not nearly as well developed as it can be, and this poses some safety problems.

In the Penny Farthing helo configuration, we all know that the loss of T/R thrust is a significant failure that no pilot wants to face, but all pilots should be prepared for. For this discussion I'll focus mainly on total loss of T/R thrust and exclude jammed pedals, stuck cables, etc, and those failures with CG changes.

There are 3 basic methods of responding to this failure, which deals with the loss of the anti-torque required for directional control of a Penny Farthing helo.

Method 1 Reduce the Main Rotor torque to zero. This is the "reduce the collective, chop the throttle and autorotate to a landing" method. It is the method of choice for failure in the hover, and for inexperienced pilots. This is taught by all flight instructors to new pilots, and all new pilots learn autorotative techniques in training.

Pros: Stops the airframe from experiencing severe yawing. Relies on the autorotative ablility of the airframe for a "safe" landing. All pilots have a degree of training in autorotation. Most simple recovery technique.

Cons:Technique varies by airframe type. Not all airframes are created equal in autorotative landing ability. The choice of landing area is severely restricted.

Method 2 Low to mid range Main Rotor torque method. This is the run-on landing method where forward flight with a controlled descent rate at sufficient airspeed provides enough anti-torque for directional control, to allow a run-on landing in either grass or on concrete.

There seems to be a lot a gray area in this method. Not all airframes are well suited to this method. Not all pilots are trained in this method. This method also generally requires a lot of skill from the pilot (Nick Lappos spoke eloquently on this in his earlier post). This method is generally not well developed (with certain exceptions), in either airframe capability or in pilot training.

Pros: The pilot has much greater choice of landing areas. The airframe usually suffers far less damage upon landing.

Cons: Requires a lot of pilot skill to accomplish. Can only be used when some altitude is present at failure. Not all helos are well suited to this method. Not all pilots are capable of this method. Maintaining altitude (or a slight climb) may only be possible with significant forward airspeed, and only in certain airframes.

Method 3 High Main Rotor torque method. This is the climb out from either low speed or a hover method. This method is used to climb out of a bad landing area at low altitude, so a transition to method 2 can be made. This is the method GulfPLt used in the event of his earlier post.

This method is not well understood at all, and presents considerable dangers. Anecdotal evidence suggests that this method does work at times, but accident investigations show that it often does not work.

There is usually not enough alternative anti-torque available with this method to prevent spinning of the airframe, as spinning of the airframe is considered quite dangerous in a helo.

Pros: Can prevent a forced landing in an inhospitable area during a low altitude failure.

Cons: Requires great skill from the pilot. Requires an airframe, avionics, engines, fuel system, etc, that can tolerate a high torque induced flat spin. Persons can be thrown out of the airframe while spinning if not belted in. (Added in edit) A pilot can become disoriented in a spin. Structural failure of the airframe could occur in a spin. Collision with an obstacle might occur in the spin.

Conclusion

I guess my final conclusion is that not enough has been done to help helo pilots deal with the issue of loss of T/R. Most new or low time pilots are at a loss as to what do if a loss of T/R event occures except to use Method 1 discribed above, and the results of this method are not always satisfactory. Method 2 is highly useful in certain circumstances, but many pilots are not trained for it, and some airframes are not built to use this method without making great demands on the skills of the pilots.

Method 3 does have some merits in my opinion, but the demands on both pilot and airframe are even greater than in method 2. When I posted some of the ideas previously, it was an attempt to think of ways to better improve the ability of all pilots (through airframe improvements) to perfrom a satisfactory recovery from a loss of T/R event. I also don't understand several things related to method 3. Why does a spinning airframe HAVE to be dangerous (apart from not being belted in, and poor directional control near obstacles). FW pilots practice spin recovery all the time. But as GulfPLt pointed out in his post, he had no training in how to recover a helo from a spin. Why are pilots not trained for this? Are there methods to prevent (or at least limit) the pilot disorientation of a spin? Is there any aero-medical research on this?

Reading the other thread referred to by "The Nr Fairy" demonstrates just how daunting this event is to nearly all helo pilots, and it's my opinion that more can done to improve both helo airframes and a pilot's training to help a pilot get through a loss of T/R event safely.

To vfrpilotpb, I took the word "some" out of the text, as you are correct.

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Safe flying to you...

[This message has been edited by Flight Safety (edited 02 July 2001).]

Vfrpilotpb

2nd Jul 2001, 12:39

Flight Safety,

I am a reasonably low time Heli pilot covering the range from R22,R44 and B206, here in the UK in order for you to get your Licence you must be able to demonstrate to the Flight Examiner that you are fully capable of enterings and controling autorotaion and to land from that Auto,as opposed to your comment that some knowledge of Auto is taught, however it seems that because of the complexity of simulating T/R failure the only thing that is covered during civillian training is the Cfi jamming the control's so you get some feel of what is going to be the problem of directional control as speed decays, you soon realise that engine speed is the solution to keeping straight, but in the main we civvie pilots are not able to do much more than ask, read and talk to very experianced pilots about the problem of tail rotor failure.
Being interested in this from a selfish , safety of my backside point of view, I would ask that "Ancient Pelican or Nick Lappos" PLEASE give us the benifit of their huge experience in this area (T/R Failure) so some of us less experienced pilots could benifit from their knowledge , possible AP has more than anyone , as this thread has ben told of the total loss of his TR, and he is still posting here, that indicates his ability to comment with authority.
My Regards

rotorque

2nd Jul 2001, 14:47

Flight Safety,

Your goal is an admirable one, but one critical reaction of a spinning helicoper is the force of airframe drag. When a helicopter spins, in particular, the tail boom and vertical fin (but also the cabin area) act to roll the helicopter over. This is due to the fact the boom and fin are positioned below the level of the rotor head producing a coupling that is detrimental to the control of the helicopter. In nearly every case the helicopter will bank over, as it spins, to the point where either blade / boom contact is made or the disk is tilted to near vertical where it cannot support the weight of the aircraft.

One other point is that in most cases you could not posibly translate from your method 3 to method 2 in a developed spin. The ability of a human to make the control inputs necessary to keep a helicopter upright and accelerating at the same time whilst spinning is basically out of our reach. Not only that, but the rate at which our pilot will be required to go from cyclic stop to cyclic stop will be faster than the rate the control servos will allow, he will now be fighting the hydraulics as well.

The last point may not be as relevent on our modern machines now with hingless or rigid rotor systems etc, but more often than not you will come to grief if you try it in a 206 or 47 or similar.

Enough said, my point really is to highlight the fact that the helicopter will most likely NOT stay spinning in one plane, if power is applied during a tail rotor failure in the hover or at slow speed.

Cheers

212man

2nd Jul 2001, 21:16

Flight Safety,
I don’t recall mentioning SOPs in relation to your comments, however I did feel your remark below was unrealistic:

“What puzzled me was why the pilot didn't apply more collective and climb, so he could straighten it out with a little altitude and attempt a landing on the surrounding flat terrain. I know this would have increased the spin rate, but as long as the airframe structure can tolerate the increased spin, this would have been far better than destroying the helo in the rocky ravine.

So that's my idea, why can't pilots do what "GulfPLt" did, and just climb if needed while enduring a higher spin rate, if that's what it takes to get out of trouble following the loss of T/R, especially at low altitude above an uninviting landing area? “

The following extract from the Uk’s AAIB on an AS355 last year makes interesting reading on this subject:

“…..diverted to a second task which involved hovering at 500 to 600 feet agl over a residential area near the M4 motorway. The visibility was good with a last reported surface wind of 200°/12 kt. Sunset was at 1910 hours and daylight was fading, but the pilot was still able to fly by visual reference.
The helicopter had been hovering in the area for about 10 to 15 minutes, facing in a south-westerly direction, when it suddenly made an uncommanded yaw to the left through some 180 degrees. The pilot immediately applied full right yaw pedal to counter this yaw. However, although the helicopter stabilised for a moment, it then yawed more rapidly to the left. At this time he called out to the two observers on board to warn them of a problem with the helicopter. He partially lowered the collective lever in an attempt to regain control and applied some forward cyclic to gain forward motion and airspeed, but the helicopter then entered a steeply spiralling/yawing descent to the left. The pilot realised that he would not be able to recover full control of the helicopter and abandoned his attempt to fly out of the situation. He concentrated on keeping the helicopter as level as possible whilst looking out through the right side window for visual reference, since he found the forward view too confusing due to the rapid yawing motion. He adjusted collective to achieve what he judged to be the best combination of rate of descent against yaw, and when he caught sight of the surface in his peripheral vision he pulled the collective lever fully up to cushion the impact. “

(Full report at; http://www.aaib.detr.gov.uk/bulletin/jan01/gsaew.htm)

So, I was not intending to cause offence to anyone, merely to add some words of caution to those on this thread who profess to be low time and in search of wisdom. Mind you, most of us are low time when it comes to TR failures, and long may that continue!

As has been said ( I think I may have done too, but obviously in invisible ink) in a teetering head design the resulting fueselage movements that might ensue following a high rate of yaw, could and probably would result in main rotor contact and break up.

The advice of Test pilots is invaluable without doubt, but remember also that any deliberate examination of handling characteristics in this regime will be done in a controlled and systematic approach following extensive briefings and simulations. They are of well above average skill and in current practice with handling extreme situations. Your average line pilot who may not have done even an autorotation for many months or years, and at the end of a hard days flying will probably have a very tough time indeed dealing with a loss of TR drive, in any flight regime, unless in the cruise in a Dauphin perhaps.

You may get away from a TR failure in one piece, on the other hand you may not; it depends on many factors not the least of which will be LUCK!

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Another day in paradise

The Nr Fairy

3rd Jul 2001, 00:19

Also culled from the same report 212man got his bit from :

"Prior to joining the company he had flown helicopters in the Royal Navy for a number of years. In 1993 he was involved in an accident to a Westland Sea King helicopter in which the tail rotor drive shaft had failed. He had witnessed, on that occasion, a change in engine sound or mechanical noise associated with the loss of control, whereas on this occasion he had not heard any such changes."

Prior experience counted for him when he had his second problem !!

Flight Safety

3rd Jul 2001, 02:28

Thanks 212man. I apologize that I reacted as strongly as I did, and I understand your desire that no one be misinformed, especially the new and low time pilots. I'm glad we're still friends. :) :)

I agree with your comments and I understand the decision made by the pilot in the accident report.

One think that still troubles me is why loss of anti-torque spins in helos are so daunting to most pilots. In FW flying, aircraft have to meet design certification requirements for spins, and pilots have to be trained in spin recovery techniques. Why is it that airframe design for spins and pilot training for spins is such a relatively "unexplored" area in helicopter flight (other than "lower the collective, chop the throttle and autorotate")? Is it because the "egg-beater" can be sat down anywhere (which is only a partial truth because in reality it can't be sat down just "anywhere") that not much effort has been put into "flying" spin recovery techniques?

I also want to look at the FARs (when I get some time) for helo certification, to see exactly what the certification requirements are for alternative anti-torque and spin stability, and to see what requirements actually exist.

For what it's worth, I think more work needs to done on method 2 listed above (the low/mid torque, run-on landing method) so that this loss of T/R recovery method is not nearly so daunting to most pilots.

I guess after seeing the ANG Blackhawk (loss of T/R) crash video last year mentioned above, I just started thinking that other alternatives have to be available other than just entering into a flat spin, and accepting the fact that you have to go down in a totally inhospitable area, that for those pilots and crew just destroyed their helo and seriously injured some of them. What was weird is that while the spin before descent was brief, it didn't really look that unstable. The helo just hovered for a moment, in a nice (not too rapid) flat spin before it started descending, and the descent was pretty flat as well.

It's the characteristics of these flat spins that I don't think are very well understood, and a "flying" recovery from these spins certainly isn't well understood. The only recovery method that is fairly well understood is the "autorotate, you're going down" recovery method.

It's what guys like GulfPLt were able to accomplish, that make me think that loss of anti-torque spins don't have to be as daunting as we perceive then to be now. I'm just hoping that research and investigation can provide some better options in the future, as the options we have now for loss of T/R recovery are pretty limited in my view.

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Safe flying to you...

[This message has been edited by Flight Safety (edited 02 July 2001).]

HeloTeacher

3rd Jul 2001, 19:39

I'll start by saying I think from Gulfplt's description that it was a partial loss of T/R control. Not so?

My experience has all been simulated thus far, in the simulators at Bell/FlightSafety (FS) in Fort Worth and with the Canadain Military (CF) in Gagetown, N.B.

It was mentioned before that FS's assessment of vertical stab effectiveness was optimistic. The CF agreed and adjusted their aerodynamic programming accordingly. In the sim the response was pretty basic, lower collective, roll off throttles if you have time to think about it, and cushion at the bottom. Like most of these items, practice allows you to land something that was extremely difficult the first couple times.

I have also simulated countless stuck pedals of varying degrees in the hover, climb, cruise, descent, even on the ground prior to T/O. This is in the RH22, RH44, and BH06. I have seen several pilots try to pull pitch and climb away when the right yaw got away from them and they wanted to go around and try it again. I make a point of maintaining the failure, just so they can see why this is such a lousy idea.
Invariably the right yaw accelerates beyond their ability to focus on the horizon and they lose the ability to maintain a level aircraft, any wind/airspeed and associated weathervaining only exacerbate the situation as the tail whips through 90-180ish yaw. I rarely let it go more than one full rotation and never yet has the pilot thought that I took it too early. The reaction I teach in this situation is to stop the a/c (cyclic flare to zero groundspeed) and perform the failure as per the hover. They are already close to ground now so it's a short hover chop to the ground.

I would love the opportunity to get in a light helicopter sim and compare rotation rates with what we get for varying aounts of stuck right pedal in the hover. What I generally use is a stuck right pitch sufficient to maintain a right rotation in the hover despite immediate closing of the throttle.

Now, to respond to soem comments above:

Spins: A fixed wing spin and a rapid yaw are two completely unrelated beasts. The autorotation is the closest rotary equivalent to a plank spin, and is handled quite well. Control in the spin gets quickly out of hand because you are sitting out in front of the center of rotation, trying to look at a world that is moving too quickly to focus on. The immediate (normal) reaction is to focus in tighter to the a/c to pick up details, taking the horizon out of the picture, and pitch control gets ugly. Additionally, if the a/c is rotating at, lets say, 60 RPM, easily achievable, then the RRPM is effectively 530 - 60 = 470 RPM (this is for the R22) or 89%. For the R44 it is 85% and the 206 it is even less. Even more pitch is now needed to effect that climb, meaning more power, a higher rotation rate, and a lower effective RPM. Overtorque or overpitch follow and game over.

Running Landings: Nice if you have a place to do it I guess. Here we rarely fly in anything close to hospitable terrain and always teach a spot landing using cyclic flare and landing as per failure in the hover. When I was on mediums in the IFR world the running landing was standard but there is a lot more machine to toss around.

Of course, these are my opinions, they won't cover every situation, and feel free to disagree and tell me so.

RW-1

3rd Jul 2001, 20:58

I have to say I'm with Nick's last sentance.

I've been conditioned to treat a anti-torque failure as an immeadite auto condition, as at this pint I've not the skills (I may feel differently later on as I gain experience) to try to continue flight with the condition. Also, once established, adding small amounts of power as he stated withthe "glide extention" could be done.

But I love reading the varied respnses here on this thread !

Nick,

Tell us more about the Commanche!

Am I correct in that it is single stick for cyclic and you rotate it for tail inputs?
(this I have heard and cannot confirm)

It seems to be a joy to fly ..... I missed gettin gto see it when it was brought down to Ft. Laud beach.

I'd love to fly up to Vero to get a once over, but alas ..... http://www.pprune.org/ubb/NonCGI/frown.gif

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Marc

[This message has been edited by RW-1 (edited 03 July 2001).]

212man

3rd Jul 2001, 21:36

Helo teacher,
exactly! The FSI sim at DFW (not the level D one, I can't comment on that) is far to easy to control. I even carried out landings on the aircraft carrier at night in it (for fun) after taking off and losing the TR gearbox at 60 kts on climb out. Very ego boosting but not realistic.

On the other hand, the level C+ SAS 212/412 sim at Stockholm is great; you are upside down and wirling like a Pitts special in a flick roll if you don't dump the collective AND roll off the throttles (good lesson for those who friction too tightly).

Also as you say (and I did too), the transition from seemingly controlled flight to total loss of control when dealing with stuck pedals, is very rapid and needs to be well understood (before hand) as you are then in a really bad situation.

Must go, I'll have think about the FW spins but I don't have time now. I any case the aerodynamic loads will be considerably less.

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Another day in paradise

RW-1

4th Jul 2001, 01:36

I also forgot to add that at least in the cicare simulator (a full size flying sim itself that volar has, uses a rotax to power it) I can experience either a T/R control failure, or a full blown loss of tail rotor failure without worry of killing myself, this is one of the good things about that simulator, which is really a one person heli in a "frame", which I described some time ago.

So while I may not try to fly out of a full blown tail rotor loss, I might at least not be panicked when it happens having experienced the sudden quickly developing yaw.

For a pic of it and description, go to:

http://orbita.starmedia.com/~cicarehelicopteros/

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Marc

4dogs

4th Jul 2001, 17:56

Flight Safety,

For my sins, I am a helicopter and aeroplane instructor and have demonstated and taught low level aerobatics in both aircraft categories.

In a stiff wing spin, depending on the aeroplane, the pitch/roll/yaw rate is actually relatively slow. However, the pilot is unable to control the attitude during the spin and the aeroplane generally consumes sh*tloads of altitude while you sort it out. After gross anti-spin control (remember that all controls are still working) defeats the spin, one then recovers from the steep nose-down unusual attitude and flies away. Alternatively, you eject/parachute/strike the ground.

In a helicopter, life is considerably different. If one loses T/R authority in the cruise, there is a reasonable chance of keeping it all together. If it happens in the hover, the onset of rotation is rapid and eye-watering. Unlike our aeroplane, the helo pilot must make control inputs to maintain some semblance of a survivable attitude - the machine has no inherent ability to keep all of its bits in the intended location.

Here is the scene: the helicopter has started to spin rapidly at the torque currently set and the options are:

a. reduce the torque and slow the spin in order to bring the reaction time for survival manoeuvres closer to your capabilities, or

b. increase the torque substantially to generate vertical movement, knowing that one will dramatically increase the yaw rate and the demand for control inputs that are required simply to keep the rotating bits between you and the sun. In most cases, this requires a conscious move to take the reaction time for survival manoeuvres further from your capabilities.

Now I reckon 'b' is a poor option unless, as I have found myself, falling into the mouth of an active volcano is the only alternative. In that case, sangfroid is but a dream.

I used to teach loss of T/R authority in the high hover, simply because the almost automatic entry to autorotation created its own, very real, problems. Thus, the height took all the fun out of a relatively straightforward recovery. Suffice it to say, the control motions required to keep the sunny side up while trying to gain sufficient forward speed to alleviate the rotation and permit either a running landing or some sort of fly-away manoeuvre were always illuminating, both for me and the victim.

And one could never ignore the gut feeling that both Mr Bell amd Mr Robinson's mast stop margins were always threatened to a point that neither could have contemplated.

Bottom line - the only similarity between F/W and R/W spins is that a rotation is involved - that is all.

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Stay Alive,

[email protected]

lmlanphere

5th Jul 2001, 23:55

just wanted to comment on the chute idea mentioned a ways back - a possible problem with this idea is the overall increase in parasite drag which will slow the machine down (which isn't preferable in TR failure situations) to maintain speed then requires an increase in pitch/power which in turn increases torque - and your yaw problem.

Flight Safety

6th Jul 2001, 01:20

I've been busy the last couple of days, and been wanting to add some final comments about this. I want to add how I think the run-on landing method (method 2 listed above) can be improved for any helo, and how a possibly safe way of conducting a high torque climb after loss of T/R could be accomplished (method 3 above). After this post, I think I'll be finished with the subject.

Regarding the possible future improvement of the run-on landing after loss of T/R, Nick made the following opening statement in his very excellent post:

<font face="Verdana, Arial, Helvetica" size="2">Loss of tail rotor is a big problem in any helicopter, especially since the vertical fin (which now must provide all your anti-torque) is often reduced in size to reduce blockage of the tail rotor to help increase tail thrust for better crosswind control.</font>

He then followed with an excellent discription of how to enter into this form of recovery. My focus however will be on his opening statement.

We all know that he's very correct in his statement about the size (or area) of the vertical stab being limited, but this raises the question of whether other possible means of increasing the aerodynamic anti-torque of the airframe might be available aside from the vertical stab. One possible solution would be to use something like FW delta fins mounted in rougly the same position as they are on FW aircraft, that is under the tail boom near the rear.

Here are the reasons why I think delta fins might work to improve aerodynamic anti-torque on a helo. If they were mounted at an angle that was more vertical (down) than sidways, then they could serve as additional sources of vertical stability (anti-torque) on the helo in forward flight. These could then replace the vertical area lost to the restrictions placed on the vertical stab, since being mounted under the tail boom would place them out of the airflow path for the tail rotor. Size (area), placement, mounting angles, and distance from the end of the tail boom (the moment arm) could all be determined by computer simulation, wind tunnel testing, and flight test.

Delta fins could also solve a couple of other problems. Being mounted under the tail boom, they would not pick up much rotor downwash like the horizontal stab on a helo sometimes does, and therefore would not contribute much to a pitch problem in hover. Delta fins would offer very little drag in forward flight, thus the normal aerodymanics of the airframe in forward flight would be little effected by them. There's also the possibilty that the range of the helo could be extended slightly, if the anti-torque provided by the delta fins is more efficient than that provided by the tail rotor in forward flight. If so, then power diverted to the tail rotor for anti-torque could be reduced in foward flight, thus adding a little to the range.

One possible problem with delta fins is what effect they might have in crosswinds, since the area of the side of the tail boom would effectively be increased by them. The test methods mentioned above could determine this possible effect.

Increased vertical area provided by delta fins could help a lot with providing alternative anti-torque in the event of loss of T/R, and might make performing a run-on landing much easier to accomplish.

Now to go on to the issue of trying to make a high torque climb safer following loss of T/R.

Several facts were pointed out in these posts that are very good points. Someone pointed out that drogue chutes are really useful for spin recovery in FW aircraft during flight test. Nick and I exchanged a brief e-mail where he pointed out to me the problems of using a drogue chute, especially as a "fly home" device because of the excessive drag. He also pointed out that the increased drag only adds to the anti-torque problem. He did however point out that a drogue might be useful in spin recovery.

Someone else pointed out that the vertical stab on a helo can impart a roll to the helo airframe if it's in a main rotor torque spin, since side air force on the stab would provide a lever arm rolling the tail boom from top to bottom.

So here's how I think a high torque climb following loss of T/R might be made safer.

The delta fins mentioned previously could offer an additional advantage in this scenario. They could offset the area of the vertical stab in a main rotor torque spin, by offering air resistance BELOW the tail boom roll center, thus offsetting the lever arm of the vertical stab. This issue could be addressed at the same time when designing delta fins for the foward flight anit-torque problem discussed above. It should be possible to design the delta fins so that the moment arms of the fins and the vertical stab cancel each other out in a main rotor torque spin. Adjustments in the area of the vertical stab and delta fins could be made so that there's no roll component at all in a main rotor torque spin.

Besides delta fins, the next item I think is needed to make a main rotor torque spin safer is a drogue chute. This could work by using the chute for what it's historically been good at, spin recovery.

Someone pointed out the disorientation of the pilot in a spin, especially the faster it gets. Someone else pointed out that there would be a loss of main rotor RPM in a spin because the speed governor's RPM reference is to the airframe, and if the airframe is spinning, the governor would slow the main rotor RPM in response.

The droque chute could be the solution here. It would have to be mounted at the end of the tail boom (to maximize the lever arm) and at the roll center so it wouldn't produce its own roll component in a spin. Poping a drogue chute could significantly slow down the rate of a torque spin because of the anti-torque that it would provide. Thus it might prevent a significant main rotor RPM loss, and prevent pilot disorientation.

The size of the chute would have to be determined by the test methods listed above, and the tail boom might have to be strengthened to handle the shock of the chute's deployment (it's already been designed to handle the standard anti-torque loads).

With delta fins and a drogue chute, I see a future hypothetical high torque climb working something like this:

You're hovering at low altitude over a forest doing logging work, when suddenly the T/R losses all thrust. You could autorotate into the tops of the trees, or you could climb out and try to fly it to a run-on landing.

You quickly decide to climb out. The spin must be at least partially established so the chute will fill upon deployment and not get foiled. You won't have to to wait long, as the spin will already be established by the time you decide to climb out. So you hold the collective where it was previously in the hover, hold the cyclic in the center, and pop the chute.

The spin stabilizes quickly and you apply small collective inputs, both to compensate for the slightly lowered main rotor RPM and to start a slow climb, as a slow climb lowers the anti-torque requirement and produces a slower spin rate. You also continue to hold the cyclic in the center to keep the rotor disk flat. Forget about directional control as you won't have any, you'll just drift with the wind during the climb. If you start drifting close to an obstacle, you might have to think about increasing the climb rate to avoid it, but only if absolutely necessary. The only instruments you can count on in the spin are the baro altimeter and perhaps the VS indicator to monitor the climb.

When sufficient altitude has been reached to transition to forward flight, it's time to terminate the climb and exit the spin. To do this, you lower the collective to start a brief autorotation. With no main rotor torque the drogue will quickly slow the spin and as the spin slows to a stop, you apply forward cyclic to start forward airspeed. This should be done before the spin stops completely as again you want to keep the chute filled so it won't foil. The chute should help to keep the pointy end forward as airspeed builds up. When airspeed is sufficient so that aerodynamic anit-torque provided by the vertical stab and delta fins can take over, then you can jettison the chute and fly to a proper location to perform a run-on landing. (My instincts tell me however that this spin recovery would be a little more complicated than presented here, due to the nose down moment caused by the chute when the collective is lowered).

I know this is very forward thinking, and if I had an R/C model helicopter (which I don't), I'd modify it to at least test the basic aerodynamics of these ideas (the human physiology aspects aside). It would need a channel to neutralize the tail rotor pitch (to simulate the loss of T/R), a channel to pop the chute and a channel to jettison it.

Gee I wrote a book, sorry about that guys.

------------------
Safe flying to you...

[This message has been edited by Flight Safety (edited 06 July 2001).]

212man

6th Jul 2001, 19:43

By chance, I'm at the stage in my 155 course where we covered TR problems today. It has to be said that the Fin on a fenestron a/c makes the whole process infinatley easier than a conventional type, though it does require a reasonable amount of space.

I was pleasantly suprised by how controllable the final touchdown was, and the benefit of right roll to reduce the speed still further before the nose aligns with the runway.

Definately no need for an auto with this one.

------------------
Another day in paradise

RW-1

6th Jul 2001, 19:52

Some fun from my model bretheren (I have 2 heli's myself)

Hehe, these guys decided to stop the rotor while doing an auto, and figured they would still have enough negative collective pitch (which models do have) to recover. I had stated that they would lose the bird, not from exceeding crit AOA on the blades, but because they would suffer blowback as the model then accellerated with stopped blades.

Well I'll let you see the video:

http://ronlund.com/hhsmall.avi

Blades departed shortly after stopping :)

Heli: totalled., a bummer, but made a nice video.

------------------
Marc

4dogs

7th Jul 2001, 09:35

212man,

Presumably you are referring to in-cruise failure? Or did you deal with fenestron failure in the high hover?

------------------
Stay Alive,

[email protected]

212man

7th Jul 2001, 19:04

4dogs,
yes, in the cruise followed by run on landing. Obviously in a the hover you'd be pretty poorly placed and would need to lower the lever and acept the yaw (can't "chope throttles in this one").

Roofus

8th Jul 2001, 02:13

Hi Peeps.

Sorry to join this a bit late! (I've been on me hols!)
Firstly....any tail rotor failure in a high hover is very bad news. Each aircraft has different handling capabilities. I've had two tail rotor 'problems' one was a drive shaft failure & one was a control failure.

The most significant difference was the change in noise. When the drive shaft failed the noise change was obvious, the control failure didn't noticeably alter the noise.

Some have talked of pulling power & climbing. People if you manage it my hat is off to you. Amazing courage. My drive shaft failed at 40' just coming in to land, the obvious solution was to just dump it on the ground & live to fight another day. I lost T/R control at some 550' in the hover.
Should I have climbed? You may think so...I was there & I can assure you it was never an option! The rate of spin may from the outside have looked sedate, but in the cockpit it felt horrendous! The centrifugal force was initially enough to lean me (& hence the cyclic) to the right. Climb & increase the rate of spin???? Forget it!
Some would say chop the throttles & take away the torque reaction! Ha! Forget that! An autorotation from a 550' hover in a As355F2!!! The word SPLAT springs to mind!! No people...I can assure you I had plans for my engines & they were staying right where they were! The best plan I could come up with was to fight the aircraft down as gently as possible! It appears that between Lady Luck & me we did OK. The AAIB described the impact as relatively benign! Oh & by the way to make it benign I used everything BOTH engines had to offer! Chop them??? No way!

Forward flight failures are a different kettle of fish, as are jams etc! The variables are countless!! What I would say is that my military & civil (thanks to the North Sea & a couple of fantastic sim instructors!) proved invaluable! There ia an old addage in aviation.....I won't teach you to suck eggs.....but we walked away.

By the way...with relation to the simulator training....it teaches you what to do in many of the scenarios.....but the sim can never prepare you for the violence of the rotation! Particularly in the high hover, in a heavy aircraft legendary for it's poor tail rotor authority!! :eek:

[ 07 July 2001: Message edited by: Roofus ]

Skycop

8th Jul 2001, 04:36

Thankfully I never had a real failure yet, but have lost tail rotor authority on a number of occasions (very slow flight, OGE, well out of wind, strong w/v. A little alarming but easily surviveable).

However, from previous experience as a sim instructor (including an RAF trial on tail rotor malfunctions on the Puma for general guidance of said type's pilots) and other experiences on French and American types (different directions of main rotor rotation), to avoid confusion in my own mind, here are a few rules that I now always bear in mind:

Rule 1: If the tail rotor driveshaft fails I expect an engine off landing either immediately or at some very imminent stage. Note that if this occurs, the (bad) yaw is towards the advancing blade of the main rotor (advancing side is therefore bad, read on). Fly accordingly and be aware!

Rule 2: If the tail rotor suffers another type of failure i.e. cable / control / stuck pitch, aim to keep the nose of the aircraft on the main rotor's RETREATING blade side and set the aircraft up for a running landing in that configuration. That way any increase in collective to cushion the touchdown will bring the nose towards 12 o'clock and with good judgement will give nil or minimum yaw on touchdown. If the nose goes through the 12 o'clock position towards the advancing blade side, increase airspeed and / or go around.

Also, with a tail rotor control problem, if possible keep the wind on the RETREATING blade side on final approach. This gives "weathercock effect" assistance in the correct direction. (Note that this helps offload the tail on ANY normal approach).

Personally, I have always found the American teaching "lucky left / rotten right" potentially confusing as I have alternated between American and French types.

Retreating blade side is ALWAYS lucky.

Hope this helps someone someday!

SC

UNCTUOUS

8th Jul 2001, 11:53

My first tail-rotor failure was actually a self-induced tail-rotor drive-shaft failure. The drive-shaft jumped out and flailed because the first hangar-bearing behind the jet exhaust (in an Iroquois) got overheated and failed due to a lubrication break-down. The reason the lubrication broke down was because I'd been hovering downwind winching trainee crewmen up and down for over 30 minutes. Not a significant downwind component either -just about 4 or 5kts. However it was sufficient to pool the hot exhaust gases around the No 1 hangar bearing and cause it to seize and jump out.

Of course you could always criticise anyone for hovering out of wind however given that the target was a rock in the middle of a creek and that the only hover reference was out of wind, it seemed a reasonable idea at the time. But full marks to the crewman instructor who cut the cable and dropped the two guys in the creek before we landed on the bank (luckily without spreading the skids). Naturally I got criticised, but up until that time nobody had ever mentioned that particular vulnerability of the tail-rotor drive-shaft. No doubt others have managed to do the same trick since - and others yet unborne will repeat it. It was interesting that I had a buzz of about 30 sec duration (only) through the T/R pedals before it broke lose with a loud bang.

RW-1

9th Jul 2001, 17:36

>>Rule 2: If the tail rotor suffers another type of failure i.e. cable / control / stuck pitch, aim to keep the nose of the aircraft on the main rotor's RETREATING blade side and set the aircraft up for a running landing in that configuration. That way any increase in collective to cushion the touchdown will bring the nose towards 12 o'clock and with good judgement will give nil or minimum yaw on touchdown. If the nose goes through the 12 o'clock position towards the advancing blade side, increase airspeed and / or go around.<<

Oh man, that is one rule I will try to incorporate into my memory! :D

Thanks SC!

The Nr Fairy

11th Dec 2003, 20:27

The CAA have published a report into tail rotor failures. It's accessible via their web site, here (http://www.caa.co.uk/docs/33/CAPAP2003_01.pdf).

It's 255 pages so I haven't yet read it, but I imagine it will cause some discussion. And I wonder if Roofus will start a company dedicated to training t/r failures . . . :D

Flingwing207

11th Dec 2003, 21:32

Wow - some very interesting reading. I only skimmed it - there's a lot to be absorbed from all that analysis.

Thanks for the link!

Hilico

12th Dec 2003, 06:07

I had a quick look at Section 8, Training. It makes a big thing of the use of simulators, preferably with 6 DOF (Degrees Of Freedom). Fine for the military with the Lynx, but do CHC or Bristow have anything so elaborate? And coming down the pecking order, I have yet to come across an R-22 sim with any DOF at all.

Page 155 names several schools that describe TRF at least in passing, but - for a report released in November 2003 - I thought it strange that SouthernAir were mentioned. Didn't they go the way of all good things a little while ago? Like, two years?

But fantastic spot, Nr. You also beat me to it with the December bulletins.

NickLappos

12th Dec 2003, 10:08

What a tome! A most impressive body of work, with many things to learn and more to ponder. The discussion by Steve O'Collard at the end is worth the price of admission!

Some concerns I have:

The failure rate appears all out of whack to me. I see the rate of approximately 20 per million hours on the tables (notably P. 40) but that seems awefully high, based on the failures I know of for the aircraft I follow. For example, at 20 per million, I would have expected about 50 TR failures on S-76 fleet, but I can only recall maybe 5 if I stretch. For Black hawk, that would have to be about 80 tail rotor failures, again a big number.

Introducing a new and potentially eye-watering system like a parachute that always deploys when the tail rotor fails and never ever deploys otherwise is an interesting concept. It is kind of like the guy who carries a bomb aboard all airliners that he flys on, because, "The odds of there being two bombs aboard is so very low, I am completely safe!"

The study does survey lots of options, and does call for more and better design attention for TR systems, certainly important if those pesky data are accurate. And the call for monitoring for failures is positively 21st century, as the S-92 has about 20 vibration monitors for the drive train alone, all reporting through a Level A software system.

Thanks for the steer, Nr. Good stuff.

IHL

12th Dec 2003, 22:15

Ya , Thanks for posting the link NR.

Dave_Jackson

16th Dec 2003, 06:09

Would two tail rotors be safer than one?
http://www.UniCopter.com/Temporary/TwinTailRotor.jpg

Would no tail rotor be safer than one?

:D :D :D

Flying Lawyer

20th Jan 2004, 07:40

Summary:

The report contains what the CAA describes as the "deliverable" from from the QinetiQ research project ‘Helicopter Tail Rotor
Failures’ carried out for the CAA under a contract awarded to the Defence Evaluation and Research Agency (DERA) in 1997. The project was co-funded by the Ministry of Defence.

The project studied tail rotor failures and their consequences and was carried out because of overwhelming evidence gathered by the UK Tail Rotor Action Committee that TRFs were occurring at rates much greater than the airworthiness design standards require. This was true for both tail rotor drive and control systems, and applied to both civil and military types.

The principal aims of the study were to analyse and quantify the nature and extent of the problem, and explore ways to reduce failure/accident rates and/or mitigate their effects in the future. In addition, existing training procedures and handling advice
were examined and means of improvement suggested to prepare aircrew better for the effects of TRFs.

The study looked into the nature and extent of helicopter tail rotor failures, techniques/technologies to reduce occurrence and/or mitigate consequences, existing emergency procedures and handling advice, current pilot training practice and simulators, existing airworthiness requirements. It included piloted simulation-based experimental work.

(If you're not on broadband, it may be easier to download the file first.)

Shawn Coyle

21st Jan 2004, 01:20

I've browsed through this weighty tome, and was impressed by the detailed work.
More specific comments will have to wait for a thorough review, but aside from generating more TLA (three letter acronyms) that will probably cause confusion, there is little to fault with the work done.
We do need to simplify things here so that we can help to train everyone
for example - for engine fuel control problems, the advice of 'high Rotor RPM, high engine bad (for multi-engine machines) and Low Rotoro RPM - low engine bad' helped to simplify a very complex subject.
Does anyone have any thoughts on this - like (for N. American direction of rotation main rotors) - yawing right, no sinking- loss of tail rotor, yawing left plus sinking= loss of engine (on the assumption that your tail rotor control system is unlikely to give you a hardover, but could perhaps jam)

ShyTorque

21st Jan 2004, 05:26

Shawn,

For pilots who fly both American and European types (opposite main rotor rotations) and I am one, there is a big danger of confusion during the initial stages of a T/R malfunction, which can jeopardise the recovery. At a simulator for the S-76 I attended, there was constant reference to the terms "Lucky Left & Rotten Right", I'm sure you will have heard this used, too. I don't like this term because for a european helicopter it is "rotten left & lucky right".

I thought myself through this during my RAF time carrying out instruction on the Puma Sim. I came up with the following maxim:

The LUCKY side, no matter what the direction of blade rotation, is the RETREATING side!

Much easier to remember, as most pilots do actually remember which way the rotors went on start up!

Heliport

22nd Jan 2004, 23:03

Merged threads.

TheFlyingSquirrel

13th Jan 2005, 13:22

All you experts out there - If T/R failure occours in foward flight and enough forward speed and height is available to enter autorotation after chopping the throttle - should the aircraft be flared before making a fast run on landing or not? I am very interested to see what advice is given here by ye old lags. Thanks.

407 too

13th Jan 2005, 15:32

the question some one will throw back, is, "what type of failure"

- loss of component (t/r departed)
- loss of t/r control (pedal inputs do nothing)
- fixed pitch failures (how much pedal at failure and which pedal)

type of failure dictates recovery, autorotation may not be necessary

:ooh: i guess that someone was me

MightyGem

13th Jan 2005, 16:32

As he's chopped the throttle I would think it's a drive failure. Can't recall anyone telling me not to flare. It would be useful to get rid of some excess speed. Of course there may well be yaw in the direction of the blade rotation, due to friction in the transmisssion.

TheFlyingSquirrel

13th Jan 2005, 16:33

I mean total T/R failure - component loss, broken gearbox, snapped drive shaft - aircraft still able to maintain main rotor thrust.

rotorboy

13th Jan 2005, 16:50

FS,
You still have some very differnent problems. Loss of component ie: GB or T/R departed , life becomes extreme very fast, due to cg change. You gonna spin , buck and it'll be a wild ride (more than likely) Do what ever you can to keep it flying , even in an auto. It aint gonna be pretty.

snapped driveshaft, or gb fails, use airspeed and or collective pitch adjustments to keep it flying if possible, to a spott where you can auto to or run it on without a scratch if your lucky. Lot of this depends on AC , gross wieght, other enviromental factors, etc.

Every one will be differnt....
like some of these salty old guys around here say. Thousands of hours of sheer boredom, moments of stark terror.

RB

407 too

13th Jan 2005, 17:07

lets say an auto is required --

i've been told (no personal experience), and it makes sense to me, that a flare just before touchdown, besides loosing forward speed, and held (mild flare, not standing it on it's end) so that the stinger makes contact with the ground will have the efect of straightening out the airframe before the skids touch.

having the heels of the skids touching first will also help keep you aligned

wouldn't want to touchdown full skid with a 45 deg nose angle:ouch:

MightyGem

13th Jan 2005, 17:26

If it's a Squirrel and a simple drive failure, then you should be able to maintain control, fly to a suitable field and carry out a powered run on landing. As one of my colleagues did a few years ago.

ShyTorque

13th Jan 2005, 19:11

There is no straightforward answer. The main variables are aircraft type and landing area condition. If the only place to touch down is full of obstacles (the pilot might have little choice) obviously one needs to reduce forward speed by flaring as much as possible. Shame to survive the impact but at high groundspeed and subsequently watch a stone gatepost get closer to the "between the legs" position.... :\

Some aircraft are best engines shutdown (most I have flown are), some are OK at engine ground idle, and some may be better placed with regard to a speed / power combination. Reading the Flight Manual is always good advice and simulator training helps too.

At night I would want to reduce g/s as much as possible, even at the expense of a higher rate of descent.

Droopy

14th Jan 2005, 01:46

Try this though it's hardly light reading CAA study (http://www.caa.co.uk/publications/publicationdetails.asp?id=1127) The one line version being:- know the right technique for your aircraft.

helimatt

14th Jan 2005, 02:12

407 Too,
Better to level the machine before putting it on the ground. Landing heels first with the disk tilted back runs a good chance of chopping of the tail boom.

Homer_Jay

14th Jan 2005, 07:17

Flying Squirrel,

In answer to your question -which, if I'm not mistaken is - "If I've entered autorotation after a T/R failure should I flare at the bottom?"

Yes under most circumstances. If you have rolled the throttle off to achieve aircraft control after T/R failure and the area where you are about to land is not a runway then flare. In a crash forward speed kills so reducing the forward speed to zero just prior to touch down will significantly improve your chances of survival. Directional control should not be too much of a problem through the flare if you have a fin. Expect some yaw in the direction of rotation as you intial the collective. This is due to you imparting a decelerative force on the blades when you initial. The torque reaction of the fuselage is to turn in the direction of blade rotation. The attitude now must be levelled prior to cushioning at touch down. During the touch down you will be rotating but much less wildly than if the throttle was open. Fly to touch down in a level attitude. Hopefully the aircraft will remain upright but even if it doesn't the lack of forward speed will hopefully ensure minimal injuries.

However, depending on the aircraft type an autorotation is not always the immediate solution to a T/R malfunction. The manufacturer's recommended solution is probably most appropriate.

In the case of a loss of T/R Drive or loss of control linkage it is probably advisable to fly the aircraft to a runway. Enter autorotation by lowering collective and only roll the throttle off after achieving a slow enough run on speed and lined up with the runway.

In the case of catastrophic failure and loss of components, the longitudinal CofG problem is now going to dictate whether the aircraft can be controlled and therefore whether the throttle should be rolled off straight away.

There's not enough room here to go through what to do for a jammed pedal.

Note these are general solutions and may vary from type to type and configuration to configuration. ie wheels vs skids, PCL vs twist grip etc

Your best bet is to sit down for a long chat with a crusty old instructor and satisfy yourself you can identify the type of T/R emergency and the best way to deal with it.

PS The suggestion that friction in the transmission is the cause of rotation after initial at the bottom of an autorotation is a myth. It belongs in Mr Lappos' thread about urban myths. The rotation is due to the decelerative force on the blades at initial.

ShyTorque

14th Jan 2005, 08:44

"However, depending on the aircraft type an autorotation is not always the immediate solution to a T/R malfunction. The manufacturer's recommended solution is probably most appropriate.

In the case of a loss of T/R Drive or loss of control linkage it is probably advisable to fly the aircraft to a runway. Enter autorotation by lowering collective and only roll the throttle off after achieving a slow enough run on speed and lined up with the runway."

First paragraph of that advice = :ok:
I agree with you there, but the second paragraph = :(

I have to strongly disagree with the second paragraph. It is VITAL to know your aircraft type because on some aircraft, after a loss of tail rotor drive, any attempt to "fly to a runway" other than in autorotation may result in loss of control. This technique may be a good one if the tail rotor is still producing thrust, i.e. control of the pitch angle has been lost.

We must be very careful with this subject. Trying to give general advice on how to handle a tailrotor malfunction may be inappropriate.

Homer_Jay

14th Jan 2005, 11:53

ShyTorque,

I see your point, no fin could equal disaster without the throttle off. I can only reiterate that a long discussion with an instructor about the type/s flown is the way to go. There is no simple "one size fits all" answer. Please see signature below.

Cheers.

jellycopter

14th Jan 2005, 19:56

To add my two-penneth regarding run-on speed and ROD; 1500fpm is about 15kts downward, 500fpm is about 5kts, 200fpm is about 2kts etc etc. Therefore, after the helicopter has broken its contract with you in such a major way, a relatively high RoD at touchdown is probably preferable to a high-speed run-on as the overall overall inertia, and therefore bone-breaking forces, will be less.

What about flaring to zero-speed? Not a recommendation; but a genuine question?

J

the coyote

15th Jan 2005, 05:55

jellycopter,

If I had to do an autorotation to maintain control and/or a powered run on landing was not possible, that's exactly what I would do, flare to zero touchdown speed.

It will yaw late in the flare, as you commence the collective application, but who cares? I would rather contact the ground with no speed and turning than roll it over at speed. The key is to make sure the power is off, lose all the speed completely and get it level prior to touchdown.

TheFlyingSquirrel,

As to your original question, what were you trained to do?

Thomas coupling

15th Jan 2005, 07:32

Silberfuchs: tell us what happened to you 10 yrs ago?

Staticdroop

15th Jan 2005, 09:06

I believe that one solution, as there are many ways to skin a cat, is to fly to an area where you will be happy to autorotate into, preferably an airfield as there is a runway and crash facilitys, otherwise best choice available. Let someone know, MAYDAY, dont delay to long there could be a lot of metal thrashing about back there and it could get worse shortly.
Enter Autotrotation close the throttle carry out Immediate Actions make sure the fire making stuff is shut off. As you approach the height specified in the a/c manual flare the a/c reducing speed to zero if possible and aim to come down the last 10 feet verticaly using the collective to cushion the impact. You will spin due to friction but that should be manageable, if you come down verticaly you will hopefully reduce the possibility of rollover associated with high speed run ons.
This may not be the perfect solution but it is the plan i have worked out for myself to give me something to work on come the day.
All comments welcome:ok:

Gaseous

24th Jan 2005, 21:26

Have a look in the POH. My Enstom POH details what to do for every concievable type of TR failure. The details may not be relevant to other types as Enstrom make a big issue of the fat tail boom helping get you out of trouble.

Arm out the window

25th Jan 2005, 00:43

A good start point for fixed-pitch type failures was taught in the RAAF for a long time, works well for helicopters with twist grip throttles like the B205, 206, and we had twist-grip mod squirrels.

In a nutshell, set up on a long low final for a good run-on area and slow the aircraft in 10 kt increments, stabilising level and unaccelerated to see what the yaw angle's doing.

If yawed left (rotor anticlockwise from above helicopters), keep slowing down in increments until you get 40 to 50 deg. left yaw. or 20 kt. Maintain the speed you're at until over the landing area, then smoothly slow to the hover. If it turns left, reduce your Nr in small increments with beep or throttle (yaw will initially speed up, then slow as the tail rotor slows down and you pull a bit of pitch to stay in the hover).
When the yaw's stopped, put it smoothly on the ground.

If yawed right, slow to a minimum of 20 kt or maximum 20 deg right yaw. Maintain that speed until over the run-on area, then take a 'bite' of throttle to swing the nose straight and conduct a fastish taxiing auto. Use any further throttle to 'steer' (the nose will go in the direction that the bottom of the throttle goes).

In the yawing left in the hover case above, if it starts to go right, again take a bite of throttle and do a hovering auto.

This set of techniques works really well, and has the advantage of being a bit 'scientific' and repeatable, ie gives you some procedures and limits to hang your hat on.

For more catastrophic things like loss of tail rotor components etc, I'm all for setting up for a zero-speed auto as some people above are advocating.

slgrossman

25th Jan 2005, 01:44

There are a few helicopters that, depending on the components lost and gross weight may achieve sufficient streamlining to continue powered flight to a more suitable landing area. Others can't be streamlined sufficiently no matter what the weight, and will spin until the torque is removed.

Still others fall somewhere in the middle and the outcome depends on airspeed and other factors.

In any case, while it may be possible to run the aircraft on, the better choice seems to me to be zero-airspeed autorotation, which greatly reduces the kinetic energy that needs to be dissipated at touchdown.

-Stan-

VeeAny

25th Jan 2005, 11:17

You guys may find this interesting reading, its a CAA paper on tail rotor failures, and the procedures highlighted in a number of flight manuals. A bit long, but interesting if you like that kind of thing.

http://www.caa.co.uk/docs/33/CAPAP2003_01.PDF

Cheers

V.

McHover

28th Jan 2005, 13:34

I was a pax on board an Alouette III recently and experienced loss of tail rotor command first hand. The solution to getting us to the ground safely seemed to be a combination of a few of the last posts.

Immediately the problem occured (just after take off from a clearing amongst some cocnut palms) the pilot elected to head back towards the nearest airfield (a virtually unmanned third world scrape in the jungle), a journey which took an agonising 40 minutes.

His approach was long and low, progressively easing off the speed as we got closer to the field, allowing him time to assess the degree of control. We had the benefit of about a 20kt head wind which meant that he was then able to bring us almost into the hover.

So as the yaw started to kick in and the outside world started to rotate he executed what one might term a "positive" landing. The Alouette was equipped with wheels but the pilot chose to land in the long grass off the runway which seemed to help in absorbing the impact of our landing. The outcome however was that we all emerged unscathed (and the a/c was easily repairable).

Hats off to our pilot, he handled it extremely calmly and I really hope that I get the chance to fly with him again. Unfortunately we had to abandon the heli and switch to fixed wing to complete our task. And the cause..., a failure of the t/r command cable.

McHover

28th Jan 2005, 18:57

It was a clean break of the control cable just forward of the helix drum around which it wraps in the tail rotor assembly.

Please bear in mind that I am not qualified in the engineering department however as far as I could see there seemed to be no signs of wear and tear, and the operator informed me that it was only six months old. The break was so clean that it looked like it had been cut.

I have a picture of it if you're interested.

McHover

JerryG

30th Jan 2005, 06:05

I’m grateful to this thread for sending me back to the notes I made the day after suffering a complete T/R drive shaft shear in a 109 some years back. (I caution when reading this that the 109 of course has a very big fin). Part of those notes read:……..

According to both the previous tail rotor failures on 109s the aircraft doesn’t yaw with a failure in forward flight above 120 kts since the tail rotor is completely unloaded and the fin is taking the entire anti-torque load. However in both these cases the entire tail rotor assembly had departed the aircraft. Mine was attached and sometimes windmilling, sometimes not. (I had a chase S76 join me to take a look).

There was a definite and violent yaw to starboard and there was no way it was ever going to fly in the direction it was pointing, or even within 40 degrees of the heading. Although that was uncomfortable it would happily fly at a good range of power settings and between 60 – 100 kts, leaning heavily left “wing” low. The envelope is undoubtedly much wider but I wasn’t brave enough to explore it!

I flew dummy approaches to two out of wind runways in the hope of gaining some help with the heading but the whole thing felt very unnatural and there was no conspicuous advantage to be gained.

I trained the camera man in the left seat (we were returning from a job together) to chop the overhead throttles on my shout. The into wind runway was chosen and I entered a 60 kt constant attitude auto with the engines running (and without intending to flare). The nose stayed 20 degrees to the right of the centreline. At about 20 feet we chopped the engines. It felt very late at the time but was actually a bit early as I was amazed how quickly the nose yawed to port whilst I was pulling in a bit of pitch. I was hard pressed to get onto the ground before the nose swung through the centreline.

In retrospect I would not do an auto next time. I think I would go for a flat approach at 60 kts and accept the increased yaw to starboard (probably as much as 45 degrees). I would then try to gently arrest the descent and then the speed down the centreline at about 5 –10 feet before chopping the throttles, giving more time to worry solely about timing the touchdown to coincide with runway heading. This assumes somebody available in the other seat to look after the throttles.

I found one of the hardest aspects was keeping my feet on the floor instead of constantly trying to pedal a control that wasn’t there.

…………Now don’t ask me for more details, it was a long time ago, but those notes were made the very next day. I’m certain they are very specific to the 109.

I do very clearly remember being astonished at how the brain speeded up with adrenalin and could remember comprehensive details of stuff I’d read or been told about T/R failures from long before. I couldn’t have remembered it all the day before and probably not the day after, but it was all in there somewhere when needed at the critical moment.

JerryG

TheFlyingSquirrel

30th Jan 2005, 10:11

http://img.photobucket.com/albums/v650/theflyingsquirrel/Tailrotorcommandcable1.jpg

http://img.photobucket.com/albums/v650/theflyingsquirrel/Tailrotorcommandcable2.jpg

topendtorque

20th Feb 2005, 19:06

Interesting notes, pitch controil fails or drive shaft fails. simple, play with your throttle, wind it off just before you touch down at very slow forward speed. don't stuff up the aproach.

If drive shaft fails in the hover, man that's scary, after the second rotation one's self preservation usually remembers training, wind the throttle off idiot, they don't stop, only slow down a bit.

What not to do is don't wind it off, check out the story from Sydney Australia many moons ago when this happened in a tele-news 47 after the old leightweight style T/R thru bolt let go, very messy and all on camera.

If the G/Box is gonna fall off, do your CofG, then work out how you might kiss your posteria goodbye!

But refer to the recent seaking crash on the after deck of travelling ship, many cowboys reckon that T/R fails, no-one noticed the rapid quick flex down and up of the M/R when it unloaded in a typical vortex ring state, of course the T/R parted company with the rear end of flight path when it also got fouled up air.
Maybe no one has ever seen a cowboy film when the wagon wheel goes into slow flicker motion when in synch with and at right angles to the camera?? T/R was still going ok when it went past the cam close up and also when it smashed with high power into the deck and disappeared to the port side of the vessel.

CareBear

24th Feb 2005, 17:15

A technique that works in the Huey Simulator (or AB205 etc) following a loss of drive to the tail rotor:

1. Enter auto and roll throttle off to reduce tendency to spin to a minimum
2. Establish fairly fast auto to get some benefit from the fin
3. During auto gently re apply throttle
4. during the flare as the speed reduces and you start to pitch pull, the aircraft will begin to yaw due to frictional torque produced - at that stage start rolling off the throttle and that will help to keep straight.

Don't know if this is a "works for all" technique but it works on the Huey so my guess is it might work on 206 and other Bells.

Any one have use of a 206 sim to give it a go? Please let us know the results.

SASless

24th Feb 2005, 18:21

While a very new US Army pilot...in a land far-a-way....bummed a ride to H-3 Heliport in Saigon...sat in the front seat. Normal flight to H-3 with one stop in Di An for fuel....then to H-3. Arrived at H-3 to the hover....brief discussion about where to park...still at a hover....I point left ...the pilot looked left...and the aircraft turned right. Me being quick witted assumed the guy was doing a clearing turn or something. As the nose passed what had been the original 12 o'clock position we continued to turn right but at an ever increasing rate or rotation. The first turn felt almost normal in rate but did speed up as it progressed. As the nose passed the 12 o'clock position for the second time....and the rate of rotation was becoming interesting....being really quick witted...I asked the pilot what he was doing. As the nose passed the 12 o'clock position for the third time in a blurring eyeballs against my Aviator Ray-Bans.....again being quick witted I ascertained the pedals were full left, the world was still spinning left, and that my now deaf and dumb pilot.....was not reacting in as correct a fashion as I desired.

I asked him politely, in my slow measured southern drawl, to reduce throttle.....sounded kinda like an excited Geordie SgtMajor calling drill commands....."Chopthethrottleforchrissakesyouidiotwhatareyoutryingtodokill uschopthethrottlechopthethrottlepleasechopthethrottleplease!

In desparation, fearing this guy was going to be like my Commanding Officer trying to decide what kind of marks to give me on my annual evaluation, and that I had other more pressing business to attend to.....I politely without saying anything to him...snapped the throttle off from my side. Not that I was aggressive with it...but the leather covering on the throttle came off in my hand as I recall with weird clarity.....and the rotation immediately slowed to an almost sedate pace....the world was only partially blurred. We landed with a good thump, sounds of paperwork being generated, and the aircraft took a decided list to port and settled by the stern. I opened my door, stepped up to the ground and tipped my hat to the pilot and headed for Saigon to visit the Christian Serviceman's Reading Center for a Kool Aid and cookies.

It is amazing how quickly the rotation slows or stops with the reduction of torque in a loss of thrust situation.

ppng

24th Feb 2005, 18:45

Anyone notice that the 'big guns' haven't pitched in so far? Probably it's because there is no single answer to this. React today in a certain way and you might get away with it - do exactly the same thing in the same machine tomorrow and we might all die (different weight, alt, temp, TAS, etc). Try it in a different machine (whichever way the blades go) and everone is toast every time. Trust the simulator if you're brave enough, but remember the way it responds is just a guess.
ANSWER (assuming control failure):
1. Don't panic.
2. Fly what you have left.
3. Don't panic.
4. Consider the wind direction for landing (having already thought about this on the ground one foggy day in advance!)
5. Don't panic.
6. Do everything slowly.
7. Don't panic.
8. Don't hit anything while not panicking!
9. Don't panic.
10. Good luck!

the coyote

24th Feb 2005, 19:47

I did one in a Robbo once as a demonstration for a student. I set the pedals to a pre determined position that gave neutral TR pitch (almost full right pedal). Rolled off the throttle and entered auto. At the bottom at the end of the flare it started to yaw left quite slowly and then more as the collective pitch pull came in. We touched down about 90 deg left of the original heading, so the key is to make sure you lose all ground speed. Either that or run it on at about max touchdown speed of 35 KT before it starts to yaw.

CareBear,

As per your scenario, I have used a very similar technique in the B206. Once established in the auto at speed, roll the throttle back on. Use only enough collective in the descent to prevent an Nr overspeed. During the flare and pitch pull, as it starts to yaw under torque, roll off the throttle to keep it straight and touchdown. Very controllable.

I think a genuine drive failure would be noticeably different to a simulated one though, due to the tail rotor still providing some stability even if it didn't have any pitch applied.

John Eacott

25th Feb 2005, 06:18

I guess the original question "in forward flight" has morphed into a general TR failure discussion. Whenever possible, practice, practice, and if you can, practice. Two I've had were both my fault, and instinctive reaction got us out in one (relative) piece on both occassions.

Low level filming in a 206L, I got distracted and hooked a wire under the toe of the left skid. The Mayday was instinctive, the decision to pull power to break the wire was the only solution (we were hanging nose down in our straps by then, with full aft cyclic), and I have no recollection of rolling off the throttle when the wire broke and I cut the tail off with the rotors. But it must have been instinctive, because we didn't spin :cool:

We were lucky enough to come down upright, but I snapped the collective trying for that last degree of blade pitch, and the back does still hurt now and then :rolleyes:

http://www.helicopterservice.com.au/photos/pprune/Wire%20strike%2010a.jpg

http://www.helicopterservice.com.au/photos/pprune/Wire%20strike%2011a.jpg

The other was in the BK117, at night, backed into a tree in a high hover. The decision to dump the collective was done without thinking, and got us out with a slightly spread set of crosstubes, rather than rolled up in a ball.

No obvious training is going to account for every eventuality, but lots of practise (or mental preparation) will go a long way to making the right move "in extremis", the correct move :ok:

NickLappos

25th Feb 2005, 11:35

ppng said, "Anyone notice that the 'big guns' haven't pitched in so far? Probably it's because there is no single answer to this. "

Right on!

The idea of a "failure" consumes paragraphs describing what tail rotor thrust is left - lots, too much, cruise power, almost none. full right?

Then we deal with individual models of helo, and then with varying flight conditions, and then with experience levels, whew!

I'd rather answer, "Honey, do I look fat in these slacks?"

Boiled down to a short, accurate helpful answer, I like ppng's checklist just fine!

Aser

11th Mar 2005, 01:24

I found one of the hardest aspects was keeping my feet on the floor instead of constantly trying to pedal a control that wasn’t there.
hehehe After a complete loss of T/R and having a place to autorate, I almost break the pedals in the 412 simulator when in ground run after the flare!
:}

NickLappos

11th Mar 2005, 11:08

Carebear and Aser,

Having helped build and qualify several training simulators, I can assure yu that they can teach you virtually nothing about extreme emergencies like tail rotor failure. I suggest that you do not be fooled by their seemingly perfect ability to mimic the real aircraft. Their behavioor for tail rotor failure, as well as all other flight beyond the normal envelope, has never been verified, and is based on assumptions that have no real flight data in them.

When you draw conclusions about the REAL helicopter's response to situations beyond the normal flight envelope based on flight in the simulator, you are probably wrong, but definately beyond the sim's ability to help.

212man

11th Mar 2005, 11:42

Nick, you have beat me to it! I, too, was going to say don't believe the simulator when it comes to TR failures. If you compare the FSI 212 sim with the SAS one the behaviour is at opposite ends of the spectrum. The FSI one displays pretty benign characteristics and is not a big deal (I even autorotated onto the carrier at night) the SAS one will tumble instantly and freeze to prevent damage to the jacks. The truth is probably somewhere in between but closer to the SAS one.

Even a brand new level D sim for the 155 does not accurately replicate the TR drive failure and requires a run on landing about 40 kts higher than experience has shown true.

At the end of the day, the sim can only reproduce chracteristics derived from test flight data and checked against the QTG; anything outside of that is extrapolation and guess work.

Aser

11th Mar 2005, 14:27

My first and only experience with a level D sim was in SAS (the 412) and men... it was really AMAZING!
I'm just a guy that flew only r-22 and b-206 before, so I don't have much idea about the reality in the flight model, but after a complete t/r faiulure in hover , 2000' hover , final, etc. and being able to put down the chopper without a "red screen" I was feeling very confident to react for the unexpected.
Maybe it isn't very real , but I thought it was a wonderful tool to develop "instinctive reactions based on the procedures" isn't it? :sad:

Obviusly I'm not going to check if the rolls and fall back from hover maneouvers that I saw in the sim can be done with the real aircraft. ;)

Regards.
Aser

skitzs

11th Mar 2005, 17:20

John Eacott amaizing pictures!! glad you are with us to tell the tail and share the experience................ CARRE BEAR...... is right about his method for TRF and yes it works very well and easy in the 206, its your only option when a zero speed landing is needed, no need to roll the throttle off though any collective applied must be late / last sec and snatch to avoid early torque rotation, use then the pwr as long as you can ( even a second) to help prolong rpm, letting the nose rotate to match ground travel, if any, B4 using throttle to maintain it, done right and with practise you it is very effective and in the 206 very easy (dont try it in an r22):D ... one point to make, the flare, apart from the obveouse is used to put the machine at best full right angle to ground travel if you do this then you got it won, also easier to get closer to the ground not worrying about the tail, anymore than a few Kts of wind and you will land it zero speed. And yes i show this to advanced students to show the "more ways to skin a cat"thoery and just to get them thinking, obveousley comes with practise and experience.

Dennis Kenyon

13th Feb 2013, 15:30

Just to add a few notes regarding the R44 failure and as mentioned already, many FIs in the UK offer specific training so pilots have a sporting chance of coping with the condition ... albeit being outside the EASA/CAA approved PPL syllabus. Yes, it is a fact that too many T/R failure cases result in a major accident involving fatalities but I'd like to think that given a reasonably amount of dedicated instruction and subsequent practice, plus a slice of luck at the moment of failure and a fairly flat piece of ground, most pilots should be able to land the helicopter safely.

So to add my pennorth can I continue. When in practice exercises, I have my pilot place both feet on the cabin floor clear of the yaw pedals. I then make a 'best guess' at the yaw angles that would be experienced by observing the power settings in combination with the airspeed being used. I accept that these may not be exact, but the exercise does provide, as near as I am able, the changing yaw angles and attitudes required to maintain control.

For the exercise, the major items to be considered are ... power in use at the moment of failure ... then the next three being airspeed, height and aircraft weight. Helicopters having a large keel surface such as the Big Bells and the Enstroms generally allow a better level of control relative to airspeed selected. Taking a standard current production American aircraft as our example, a partial or total failure of the T/R system does, as we all know, produce an immediate loss of directional control as Newton's third law of motion takes over and engine torque drives the nose rapidly RIGHT (left in most European makes).

To remove any mystery of the condition, I teach my pilots that they don't actually have a problem! Pause to think! What they do have is an engine that they can no longer control. So initially we just carry out an exercise we have practiced many times ... ie to get rid of the source of the problem by lowering the collective lever and closing the throttle to enter autorotation. With a reasonable airspeed (say 70 knots) and lowish power (say under 50%) the RIGHT yaw might extend out to the 'two o clock' position but whatever, the pilot's immediate action of power reduction prevents further yaw, while the lowered lever drops the nose either increasing or at least maintaining speed. Using cyclic the pilot's first task is to level the 'wings' and maintain at least 60 knots. With the RIGHT yaw stopped, transmission friction take over causing the nose to slowly commence a yaw to the LEFT but again highly dependent on the speed being maintained.

With the nose rotating toward the LEFT 'ten o clock' position, the pilot can now use engine to control left rotation and by raising the collective lever increasing power to yaw the nose once again RIGHT ... the plan being to discover the maximum power setting that can be used at a given airspeed to allow a controlled yaw to the original 'two o clock' position. It should be borne in mind that the resultant 30 degree yaw involves large ASI errors. Opposite use of cyclic assists control, ie with right yaw use left cyclic, the plan being to achieve straight & level flight if possible, or at least a reduced rate of descent ... much depending on aircraft weight.

IF control can be achieved with steady height and airspeed at the 'two o clock' position, the world becomes your oyster! Or at least allows more time to select a suitable emergency landing area. If the former, you may even have time to return to your base airfield or at least reach a better landing site, alert the emergency services and carry out safety emergency drills.

Once over the selected landing site, the helicopter should be positioned so that a shallow rate of descent can be made to the landing site. Lowering the collective lever allows the nose to yaw LEFT back to the 'ten o clock' position while maintaining speed and directional control with a combination of lever position (power) and cyclic (speed). Ideally the approach needs to be made at progressively reduced speed bearing in mind the ASI errors.

So far, the procedure hasn't been that difficult but if a non-damaging landing is to be achieved, the handling co-ordination requirement to control the final rate of descent, the airspeed and yaw is now at its highest. At this point the pilot is aiming to bring the helicopter to within a few feet of the landing surface with as much power as can be used to achieve the lowest rate of descent and minimum forward speed.

At around 20 to say 30 feet the helicopter is initially flared with aft cyclic to further reduce speed but then pushed forward to level the skids for the landing attitude. As the helicopter sinks collective lever is raised allowing the nose to yaw RIGHT back toward 'two o clock' but at around the 'one o clock' position the throttle is closed quite rapidly and as the nose yaws back toward the 'twelve o clock' position the cyclic is pushed forward to make rapid skid contact with the surface. With skids on the ground, collective lever and perhaps throttle is used to keep the helicopter straight allowing skid friction to stop forward movement.

It needs to be borne in mind that once the landing site is reached and descent commenced it will not usually be possible to make a 'go-around' and the pilot is committed to a landing.

I appreciate these suggested handling techniques are more applicable to a failure from a fairly benign configuration and may not apply to a worse-case take-off or landing or low height zero speed hovers where more advanced handling would be required.

I also hasten to add, that these notes are for guidance only and perhaps to promote discussion on such an important life-saving exercise, but I'd have to say, I've never been a supporter of the usual PFM recommendation to handle the T/R failure problem by ... "entering autorotation." For sure that technique will stop the rapid right yaw problem but will not allow the helicopter to be landed safely since the initial right yaw will, as outlined above, be followed immediately by a yaw to the left and in my view it matters not a lot whether the helicopter impacts the ground in a right or a left spin. And just to remind pilots that our European friends turn their rotors differently so where I say RIGHT .... use LEFT and vice-versa.

Just to close, much of the above has been learned from my four 'actual' T/R failures I've experienced in 14,000 plus hours over a forty-year career. Luck allowed me to survive the first in 1973 when a T/R shaft sheared on a sales demonstration, the second was a hover failure when a licenced engineer obligingly fitted the T/R blade assembly in a reversed 'trailing edge' first configuration which being an 'ace' pilot, I did't spot on the pre-flight inspection. With some experience under my belt, I completed a high speed run-on landing at Cranfield in 1986 and a later further occasion when I landed on a cricket field at one of Noel Edmonds 'Mr Blobby' events in Somerset. The final episode occurred at the 1999 Biggin Hill Air Fair when, with the hoped for 'slice of luck', I managed to get the Enstrom FX down without further damage. But Guys & Gals out there, I'm still waiting for my first engine failure!

Safe flying to all and I sincerely hope my words will help another pilot one day. DRK

nigelh

13th Feb 2013, 23:42

Thank you Dennis ...a very useful explanation , as always:D
Would it be fair to say that landing with the wind off the left side would also be helpful ? ( USA heli )

Dennis Kenyon

14th Feb 2013, 20:12

Hi Nigel ... interesting you should raise that point. For my Biggin Hill recovery, I made a point of landing in a 'Three o Clock' cross-wind making an initial touch down on the right skid so once I'd achieved ground contact the right side wind and right skid friction held the nose straight in the 'twelve o clock' position until the last few yards. With both skids down M/R friction took over and the machine completed a 100 degree left spin as it stopped, fortunately I was on a hard grass surface. IF I had been able to make more accurate use of collective lever/throttle, no doubt I could even have avoided that! Regards. DRK

Hughes500

14th Feb 2013, 21:00

Nigel
Very interesting, personally I think it is better to be straight into the wind so you keep precious airflow over your stabiliser to help keep it straight as one runs on. Remember it is accleration that kills us, so touching down at the slowest possible speed must be the best, thus landing into wind. An example in a 500, stick about 1.5 inches of right pedal forward you can do a zero zero landing without moving the pedals and keeping straight in 15 ish knots of wind ( 2 up half fuel) drop the wind to less than 10 kts and you cant do it without closing the throttle to remove the tq reaction as you slow down

topendtorque

14th Feb 2013, 22:40

The word benign was used which is easier to understand that what I usually say as - arrive at nothing with nothing - instead land in the most benign manner.Skating along close to the ground washing off even a little airspeed, you will end up pulling too much power and thus yaw, or too steep an approach the same applies.

With practice with wind from either the quarter or nose a zero zero touchdown can be achieved and allow the last little rotation to wear itself off as you hold off with the toe of right side skid down. That is with T/R fail simulated with 2/3 pedal in the '47 or R22.

As it finally falls you will see the cyclic all the way over toward the skid toe with lever fully up.

A good pre exercise is to fly along at slow airspeed and wind the throttle on-off-on-off a few times to learn the torque /rotation control and heading effect of the throttle.

Thomas coupling

15th Feb 2013, 12:29

Hughes500 - be very careful who you give advice to here...Dennis wrote the book and you are advising him on how it should be done:ugh:

Tail rotor failure - my pet subject. A VERY complicated phenomena, made much much more difficult to understand when the main aerodynamicists the world over, aren't fully comfortable with what happens and how best to handle it. We teach this malfunction on a daily basis to the best pilots in the world and the data we get from Westlands / AW / Sikorsky is a little scant, to say the least.

Very few helicopter pilots fully understand what to do when the tail rotor stops. 99% think they know and a tiny proportion prepare as best they can for the time it may happen to them. BUT like most aviation related skills it is a perishable skill and needs constant rehearsal.
Mosy military pilots allow for this in their training repetoire, but I suspect once a civvy qualifies and gets their ticket, they rarely if ever train in anger with this phenomenon.
There is a world of difference between tail rotor control failure and tail rotor failure. The former will allow most helicopters to arrive safey on terra firma in most instances (except with gross mis handling) The latter relies heavily on luck more than on judgement.
I won't drag on with more detailed explanations but what I will say is anyone who successfully lands a helo after a tail rotor Failure deserves atleast a Distinguished Warfare Medal , to say the least.
(Actually a DWM would be an insult - but I couldn't resist it after reading in the nes today that the yanks are awarding these for Drone operators :eek:)

What Dennis and TopendTq did / said is as close to copy book as one can get.

nigelh

15th Feb 2013, 14:06

Ok TC ...we will keep this one civil:ok::ok:
With all due respect to you superior training and experience ( actually not being sarcastic ) ...I would have thought that t/r malfunction , as in stuck , controls could be far worse . If we take it that a t/r with no drive produces little to no thrust , other than a small weather vain pull or push , you then have the ability to change heading by use of collective and throttle either way ...yes ?
In the event of the t/r sticking on a high power setting , say max take off ....ie fullish left pedal ( in yank machine ) then your options are vastly reduced . You can pull full power to get straight ....but then how do you lose height and make a landing .....if you reduce power you are going to spin . The only way I thought you could cope would be to do very tight turns using a lot of power but still managing to sink until close to ground . I would be interested to hear how full left pedal would be easier to cope with .
Ps. For these purposes we are not including c of g issues which you would have with the departure of blades , xmsn etc

[email protected]

15th Feb 2013, 17:15

Nigel - TC beasts us with these complex TR malfunctions on a regular basis and a stuck pedal in the manner you describe is a regular exercise - the answer is to reduce the Nr if you can because that will reduce the amount of TR thrust and make things easier to handle.

You still end up in a slow, high power approach but it is manageable, even with very high TR pitch settings.

If you mend your fences with TC he might even let you have a go:)

nigelh

15th Feb 2013, 18:09

Ok ...but can he supply the helicopter ??!! I recall I have done that exercise with drooping NR but I don't think with max left rudder . I shall do it with someone like JJ next LPC for sure . Certainly a good exercise to keep up to speed on ( we all spend so much time practicing engine failures when they are probably the very least likely bad scenario to happen . Hence DK with none .... Touch wood :ok:)

Thomas coupling

15th Feb 2013, 21:16

NigeH, We have an opportunity here to publicly discuss, probably the helicopter pilot's worse nightmare - Tail rotor failure. Excepting of course all those who fly tandem or contra rotating helicopters.
If you're happy let's start a separate thread on Tail rotor malfunctions and see what comes of it.:ok:

I suspect this has been discussed time and time again on Pprune but my search didn't throw much up :yuk:
It would actually bring together some of the best and worst views and opinions, experiences and suggestions. People like Dennis et al could tell us what actually happens.......
Let me know what you think?

topendtorque

15th Feb 2013, 22:18

the answer is to reduce the Nr if you can because that will reduce the amount of TR thrust and make things easier to handle.That is true and where wind from the quarter can come in handy.

However may I register a small divergence.

I always aim to enter with 100% RRPM as i look at the critical surviving factor as the highest remaining RRPM to retain flying control after throttle is wound off for heading control.
Torque variation gives heading control, therefore maximum torque is required under the prevailing benign power required condition
If RRPM is decayed then so too is torque and if you wind throttle off to straighten up starting from low RRPM it gets spooky real quick.

A separate thread could be a good idea.

Yeah I'll stick my mitt up for one those dang DWMs too although i got one of those company star and bar turnouts for surviving with no damage a T/R drive shaft pop out at thirty feet with zero airspeed.

Because of that we initiated T/R failure at the hover OGE (1500 feet min please for lighties) as well as the usual in the hover IGE.

[email protected]

16th Feb 2013, 06:30

TET, your point regarding Nr is certainly valid for horrid low inertia rotors like the R22 and R44 - I would want as much Nr as possible in one of those - read the R22 accident report from the latest UK AAIB, it makes for sobering reading about low inertia heads and how quickly low Nr can turn into NO Nr!

Ref torque though, as you slow the rotor down the torque goes up so your max Tq will be as you try to accelerate the low Nr back up to normal setting.

In the high TR pitch scenario we are talking about, you can use lower Nr to help manoeuvre and get a rate of descent - then, once you are near the ground at low speed (with a higher power demand) you can wind on the throttle a bit to help directional control.

Nigel, what TC can supply is a top-drawer simulator where such things can be practised over and over again.

Geoffersincornwall

16th Feb 2013, 14:49

Just before you get carried away just remember that unless your simulator has been programmed with actual flight data then it would have been programmed using a mathematical model and given the evidence I have seen thus far I would be a touch sceptical.

The major instinct you want the trainee to grasp is an instantaneous (or as near as) recognition of a TR malfunction and if it is one that involves total loss of directional control to then lower the collective fully whatever the phase of flight.

A few seconds later then you may have a chance to reassess the collective position according to your circumstances but I believe we can say with some confidence that the longer you delay your reaction the more problematic the outcome.

G.

diraf4410

16th Feb 2013, 15:47

hye...i'm new here...can anybody tell me why tail rotor is more sensitive when we hovering down wind...

sycamore

16th Feb 2013, 18:33

Selamat ,diraf, all helicopters have a measure of directional stability;that is,they are like a weathervane,pointing into the relative wind,in the normal flight case,going forwards,and or sideways; that is a generalisation,as some,those withe long streamlined fuselages,are better than those with `skinny` tails.However, hovering tail to wind/downwind alters the airflow pattern around the fuselage,tailcone,tail-rotor,horizontal stabiliser,and you are now in the regime of the weathervane,with a larger `side-area` being affected,and any slight yaw will attempt to make the aircraft want to turn back into the natural wind. So,it requires a lot more `work` on the pedals to keep the helo pointing `downwind`.Anyway, most helicopter pilots are very `light on their toes`,and the really good ones wear `dancing shoes` anyway...!!!!.

jellycopter

16th Feb 2013, 19:51

Geoffers stated: "just remember that unless your simulator has been programmed with actual flight data then it would have been programmed using a mathematical model and given the evidence I have seen thus far I would be a touch sceptical"
I'm with you 100% on that! They're good for teaching the initial recognition and reaction but I'm not convinced thereafter.

Hughes500 stated "I think it is better to be straight into the wind so you keep precious airflow over your stabiliser to help keep it straight"
I think you may well have a point in the H500 as the front of the skid tends to touchdown first (as does the R44). However, on the Squirrel (which is the type I've taught this on most often), with a slight nose-up on the skids as it runs on, it's posssible to touch the back of the skid down first which acts as an 'anchor' and straightens the aircraft up as it slides on. Therefore, a slight cross wind (from the right on the Squirrel but from the left with Yankee helis) helps this to happen. It takes practice but if the surface is smooth, it's possible to slow down significantly with only the aft of the skid in contact before gently completing the landing. Remebering not dump the lever during the slide-on as you breathe a sigh of relief.

Nigel H stated: "I shall do it with someone like JJ next LPC for sure"
Although I've got my FE(H) suspension revoked, I've not completed the renewal flight and seminar so I'm not in a position to do your next PC. I've had the wind knocked out of my sails wrt examining over the last few years and am not sure I'm ready to take on the responsibility again. I'll gladly spend an hour or two doing some revision of techniques though if you come over to Sleap (FOC of course).

My only addition to the discussion is that if it starts to get away from you at the bottom ofthe approach, it's almost always possible to go-around if done soon enough using gentle cyclic to build translational lift. Then re-organise your thoughts, learn from the mistakes of the last approach then have another go.
If you don't make that decision to go-around soon enough and the aircraft starts to spin, try to wash off all the speed, chop the throttle and cushion the touchdown. If done over a hard surface, the probable yaw at touchdown will be inconsequential if the aircraft is kept level.

JJ

Edited for typo

nigelh

17th Feb 2013, 11:46

Thanks JJ , i will def come and do that soon .:ok:
With ref to examining etc i dont think you have a choice .....you cannot let the lunatics run the asylum :ugh: We need you back in service making people better , safer pilots ....ASAP .
ps I still remember being a bit heavy handed in FIBS , i think doing a go around , and doing part of my climb out almost backwards :eek: I remember thinking ...hes got to take the controls now !! I thought you were bloody brave leaving me to sort it out ...but it gave me huge confidence that i could bring the nose back myself !! ( All this not quite so easy in a civvy 350 as against the Mil version with coll throttle ..)

Geoffersincornwall

17th Feb 2013, 15:01

........ is not an option when you have a FADEC and NO THROTTLES poking from the roof. No such thing a a manual reversion and no possibility to play with the RRPM. (You can tell I've just started my 189 course!)

G. :ok:

topendtorque

17th Feb 2013, 21:31

welcome also to diraf4410

`light on their toes`,and the really good ones wear `dancing shoes` anyway...!!!!. Possibly funny terminology there, maybe sycamore is funny fellow, but I don't think so. Most of us may prefer to be known as quick on our feet rather than have any pixie toed connotations attached.

I tend to fly around in socks only with boots kicked off after the first coffee pour time, because they give my feet cramps.

Ref tail feathers into wind; It's a bit like shooting an arrow out of a bow backwards. It will be very unstable and the pesky tail feathers will take over very quickly and reverse the order of flight.

cheers tet.

[email protected]

18th Feb 2013, 05:54

189 course Geoffers? Positioning for UK SAR perhaps???;)

Geoffersincornwall

18th Feb 2013, 06:10

UKSAR ?? Only if they want me to look after the zimmer frames. My day has been and gone I'm afraid, but I'll draw my pension this year and be happy to know that with 44 years in the flying game I can continue to embrace the latest technology and have teaching opportunities that are only available to a few.

The very first pilot 189 ground course is underway and AW are keen to have all in place by the time the aircraft enters service.

At first glance the step up to the 189 (electrically and avionically(?)) will be straightforward for 139 guys and gals, slightly more uphill for those with 92 and 225 and one giant leap for mankind for 61 drivers.

We live in exciting times.

G. :ok:

Hughes500

18th Feb 2013, 17:44

TC
Sorry havent replied earlier been in Peak District load lifting. Not trying to give advice to Dennis, just pointing out that on the type I examine a lot on that is what you can do. Do it regularly with all students as well as high power settings such as hover power, simulate by sticking pedal in and leaving it until one reaches best rate of climb speed, students really enjoy it.
Now should try it in a Notar, makes life much more interesting !!!!!!

Thomas coupling

18th Feb 2013, 18:03

Crab: stop hijacking the thread - get a life outside SAR :)

Hughes500 what type you examining on?

Hughes500

18th Feb 2013, 21:37

Hm

500's, 300's and 341/2

jellycopter

20th Feb 2013, 12:39

Sat in the office, a little bit bored, I decided to re-read Dennis K's original post about t/r problems.

What struck me is that the advice, although I am sure it is accurate, was also quite confusing. I've also found lots of pilots that are confused by t/r problems with many choosing to bury their heads in the sand and hope it never happens to them.

One of the causes of confusion is the widespread use of the words 'tail rotor' when referring to either drive-shaft failure or control failure/stuck pedal.
At my current place of work, the t/r problems are referred to as 'Tail Rotor Drive Failure' or 'Tail Rotor Control Failure'. Both problems are distinct and seperate and require different responses from the pilot, but in the heat of the moment, the drills can get muddled. I think that some of this may be due to the similar names given to the problems.

What I tried to suggest at work, but sadly there appears to be too much inertia in the system, was to rename the drills to avoid confusion; my suggestion fell on deaf ears. I'd like to see 'Tail Rotor Failure' referring to a loss of power to the rotor, ie Shaft failure. As for a control problem, I'd like to see it referred to as a 'Yaw Control Malfunction'. A distictly different name which should (may?) prompt distinctly different drills.

Thoughts........?

JJ

sycamore

20th Feb 2013, 13:26

JC/JJ,don`t forget that the big eye-popper is a loss of t/r and or gearbox leaving,and the resultant change in Cof G,depending on the loading may well take you outside your limits ,and leaving marginal f/a cyclic control. Another point is whether you have wheels or skids..

nigelh

20th Feb 2013, 15:18

One step at a time !! I agree with JJ that the two scenarios are totally different and require different reaction .....so why not make them toally different by calling them different things . Look at LTE and LTA etc etc
I think its fair to say that having a big chunk of your chopper fall off is a different thing again and will probably be very painful .

Hughes500

20th Feb 2013, 18:00

Think the problem really revolves what you can show a student when flying
Yes you can stick the pedals to try and represent a failure but you cant stop it turning ! As one of my customers who had a blade pitch horn failure you cant simulate the massive vibration, banging round and the smell of fear !!!!
Does puuting full right pedal ( US machine) in the same as having no tail rotor turning ? Would love to know what thrust the tail rotor actually gives for say every inch of pedal movement ie full right pedal is 10% thrust etc etc

nigelh

20th Feb 2013, 23:00

Surely there must be a way of finding out where zero pitch is on the t/r ??

Thomas coupling

20th Feb 2013, 23:31

Having talked offline to a number of contributors, the general consensus is that an attempt should be made to simplify the dynamics associated with tail rotor problems – the pilots nemesis.
I'm applying the mnemonic: KISS.[Keep it simple stupid!] on purpose so that all helicopter pilots might understand or relate to it. :)
There are aerodynamicists, test pilots, victims, individuals out there who know much much more than I and it is hoped they will contribute too. All I ask is that they remember the golden rule throughout: KISS. This is not meant to be a technical blow by blow description of vectors and forces. It is meant to shed some light on a poorly understood malfunction(s).

My main reference is the comprehensive CAA report:

http://www.caa.co.uk/docs/33/capap2003_01.pdf

It discusses many aspects on this subject. Some are fascinating, some are complex...all are thought provoking. Aviators can never stop learning and if it helps one guy out on the day of the race....job done.
Warning: This is not, in any way to be seen as subject matter expert advice. It is background chatter; something to bolster your confidence if and when the day comes. Enjoy.

I invite as many people out there to spread the word and accept the following:

Tail Rotor Failure [TRF]: Mechanical failure resulting in the TR ceasing to act as an anti torque device, either by stopping rotating or breaking up or by departing flight.

Tail Rotor Control Failure [TRCF]: Mechanically sound, still rotating, restricted movement.

Catastrophic TRF: If the tail gearbox departs flight, the associated change of CofG will almost certainly cause the aircraft to pitch out of control due to the (weight x moment arm) change caused by the removal of “x” Kg of metal at the end of that moment arm. ENDEX.....

Contained TRF: If the tai rotor stops rotating due to bearing failure; gearbox seizure, drive shaft failure or (closest to my heart): thomas coupling failure! Its raison d'etre ceases.
In the hover: A/c will yaw and pitch immediately and uncontrollably. Time permitting, chop the throttle(s) to reduce (not stop) the yaw. LAND ASAP. Hope for a safe touchdown. Another reason why you should not hover too high. ENDEX......
In flight: There will be an undemanded yaw and pitch input
TRF's will result in the a/c yawing in the opposite direction to the rotation of the blades.
(Whereas an engine failure causes a yaw in the same direction of rotation).
More often than not, a TRF in the cruise where the tail rotor is still attached to the airframe, will result in some form of controlled landing (as opposed to a crash). Invariably a running landing or an EOL. The controlling factors for a successful landing (for the pax not necessarily the a/c) are:
Height at which failure manifested itself.
PRT (pilot reaction time).
Level of experience of the flying pilot.
Training.
Speed of a/c.
Stiffness of airframe (weathercocking ability).
(To name but a few.)
Height: The higher you are, the longer you have of regaining control.
PRT: The average PRT is estimated at 2 seconds for most instances. Any longer and the a/c could exceed airframe stress limits and in flight break up could occur.
Experienced pilot: Speaks for itself – they have trained and practiced for these occasions so some of it shouldn't come as a surprise. They are therefore less likely to 'overload'.
Speed of a/c: The faster you are travelling the more likely the a/c will remain cocked in the direction of travel. Atleast the yaw disturbance would be minimised.
Stiffness: Some a/c are more benign to TRF transgressions [Dauphin / Fenestron a/c / Squirrel etc]. Others may not be so forgiving [R22, Alouette etc]. The stiffer a/c will reduce the amount of yaw making it more likely to prolong fwd controllable flight in an attempt to reach a LZ.

These factors combined will decide the eventual outcome: EOL / running landing. The general 'expert' advice gleaned from actual and simulated experiences, suggests that a TRF in fwd flight and at height should be concluded with an EOL with the minimum of delay. This - because you are purposely putting the a/c into a know, tried and tested flight regime (auto) which also reduces the offset of yaw and pitch and culminates in a minimal fwd speed landing reducing the chances of high impact trauma. Prolonging flight leading to a resultant running landing could exacerbate the fault, cause unrecoverable departures from flight at height and below a minimum speed or increase the chances of a traumatic ending if speed and/or heading are uncontrolled at touch down.
In summary then it is suggested that a TRF in fwd flight and at sufficient height should be handled thus:

TRF in fwd flight results in uncontrolled yaw in the opposite direction to main rotor rotation.
Playing with the collective results in more yaw (applying collective), or less yaw (lowering collective). Regain some form of fwd controlled flight accepting yaw and or height loss. Continue fwd flight only if absolutely necessary. (IE: flying over hostile terrain).
Turn into wind if possible remembering to turn away from direction of yaw.
Enter auto (yaw reduces), establish autorotation and then chop the throttle(s). Yaw further reduces slightly.
Flare (yaw further reduces), level and cushion touchdown by cross controlling for yaw offset and sideslip. The reducing Nr will cancel out any tendency for the a/c to yaw substantially thereafter. Attempt as close as possible a zero fwd speed landing.

-------

TRCF: This is where it gets tricky for the incumbent, because it allows the pilot time to ‘experiment’ and we all know what happens then :{
The plan therefore is to apply KISS once more. Here goes:

The pitch on the TR blades is controlled (normally) by cables or rods. The pitch changes the amount of anti torque available to offset yaw due to the rotation of the main rotor.
If you RAISE the collective, the helicopter yaws in the OPPOSITE direction of main rotor rotation. And visa versa. When you RAISE the collective, I call the pedal that is pressed to keep the a/c pointing straight: POWER PEDAL. When you lower the collective, you ease off the power pedal to keep the a/c straight..or apply pressure to the other pedal, dependent on helo type.

When the ability to change the TR pitch stops, the a/c will yaw in direct proportion to the amount of collective lever movement and the pilot will not be able to prevent this happening.
Raise the collective without applying power pedal and the a/c yaws in the opposite direction to MR rotation. Lower the collective and the a/c yaws in the same direction as MR rotation.

For me, as I was learning about permutations for power/speed/heading offset/collective position/fiddling with throttle settings (and then changing to a new helicopter where the MR rotation was in the opposite direction! I felt I needed a ‘crutch’ to rely on, that would remain a constant. Here it is:

The moment I discover I have a possible TRCF, I remember the torque setting it happened at. I remind myself that a millisecond before the a/c became ‘unserviceable’ it was under control and flying in the direction I wanted it to fly in. Therefore if I can get back to that Torque setting prior to touchdown, the a/c will be pointing in the direction I require.
Example 1:
Flying straight and level – everything appears normal. I decide to descend (by lowering the collective) and in so doing, the a/c immediately yaws in the same direction of MR rotation without permission. I raise the collective back to the original torque setting and the a/c resumes ‘normal’ again.
I then commence my procedure for dealing with a TRCF i.a.w. my aircrew manual and my experience (or lack of it). However, when I finally wish to land safely at the end of this procedure, I know that by attaining that original torque setting, the a/c will be under some form of ‘recognisable’ flight regime (ie: straight and level).
In the above instance, the malfunction manifests itself at cruise power (say 50% Tq). I must therefore find a way to reproduce a relatively sustained profile at 50% Tq immediately prior to touchdown to achieve a survivable outcome.
Example 2:
I am in the climb where everything appears normal (70% Tq). As I lower the collective to level off, there is an undemanded yaw in the direction of MR rotation. I have a high power TRCF @ 70%. .
Example 3:
I am in the descent to land (30% Tq). I level off and there is an undemanded yaw in the opposite direction of the MR rotation. I have a low power TRCF @ 30%.

In all of the above examples, provided I can reproduce those ‘safe’ Tq settings (30, 50, 70), I will be in a relatively safe flight configuration. Now all I have to do is reproduce those steady states long enough to land:

Low power TRCF:
Where do I witness low power settings in my helo?
Bottom of a normal approach prior to arresting the descent.
Bottom of an auto prior to levelling.
Or if you look at the “KISS” power required curve below – Vy .
Vy varies for different helos but is around 60-70kts.

https://www.dropbox.com/s/hzt232ckxgb7ieq/Prfig2.gif

So, with a low power TRCF, I would initially endeavour to level off by raising the collective and accept the offset yaw in the opposite direction of MR rotation. I would then fly at Vy (minimising power, thus minimising this offset yaw) to a suitable landing strip. On arrival at the landing strip I would then initiate a SHALLOW descent at 60-70 kts (offset yaw reduces even more because you lowered the lever to descend). Prior to and just above the touchdown point, whilst still at 60-70kts, initiate a very gentle and prolonged nose up attitude (I specifically did not mention the word ‘flare’!). Allow the speed to decrease whilst still maintaining a gentle descent to land.
[*WARNING: If you reduce speed too much, you could end up dropping through your descent angle and have to apply collective to restrain the increasing R.O.D which could lead to uncontrollable yaw in the opposite direction to MR rotation and subsequent loss of control of the helo. In low power TRCF therefore be prepared for a relatively fast (50+kt) running landing].
Speed should now be inside the safety parameters for a running landing and either the skids / wheels can be used to maintain straight as you raise the collective to cushion the remaining touchdown, or if you are able (in a single pilot helo) bring the throttle back commensurate with the rate of change of yaw to remain relatively straight. In a twin pilot a/c the other pilot can play with the throttles at this stage.
[The bottom of this approach requires practice if it is to produce a polished outcome, but in an emergency, an unprepared landing without previous practice should still save your life].

Cruise power TRCF:
In the above low power TRCF scenario, I was trying to mimic the Tq I had when the TRCF manifested itself (30%). On the graph, 30% Tq or SHP relates to a minimum speed of Vy (ish).
This time though, with a cruise power TRCF I am looking to achieve 50%. From the graph it appears that the respective speed for this power is slower than Vy (as I move up the y axis from 30 – 50 Tq). So if I employ the same tactics as in the above scenario, at the bottom of the SHALLOW approach I can afford to slow down even more, prior to touchdown by raising the collective further to try to achieve 50% Tq/SHP. The result being that my touchdown speed is even lower than the previous scenario’s final running landing speed.

High Power TRCF:
The “easiest” outcome of them all. Using the same scenario as both of the above and in the best interests of KISS – I now have to go even further up the y axis to find 70% Tq/SHP. This means that at the bottom of my SHALLOW approach, as I begin my progressive nose up attitude immediately prior to touching down, I can bask in the satisfaction of knowing that the collective will need to be raised significantly as I slow down more, and more and more en route to achieving 70 Tq/SHP. In fact I might even (wind permitting) be able to come to a hover, or a (trickle) fwd drift.

In summary:

TRF: Provided the gearbox assembly remains onboard the probable outcome is a land as soon as possible (LASAP) auto and/or an EOL.

TRCF: Memorise the shape of your helicopters power/drag curve and picture where on the Tq axis your malfunction manifested itself and how you are going to return to that figure immediately prior to touch down. LASAP.

Remember: the more you practice this manoeuvre the more likely you will be able to survive it on the day of the race.:ok:

Thomas coupling

20th Feb 2013, 23:33

Nigel: When on deck stationary. Play with the tail rotor blades until they are in the zero pitch position, then memorise where the pedals are:ok:

Dennis Kenyon

21st Feb 2013, 18:31

Mainly for TC please ... congrats on a neat, informative and not too technical resume of the dreaded T/R problem. Can I suggest that all new and less experienced ppruners "mark, read, learn and inwardly digest TC's words" ... as my old RAF aerodynamics tutor would say.

But TC, having suffered two total failures, (one T/R drive-shaft shear and one T/R gearbox seizure) can I ask you to note, and as I wrote in my earlier contribution, that simply entering autorotation and closing the throttle (in the Enstrom 28/280 series) does not just REDUCE yaw opposite to the direction of M/R blade rotation ... having accomplished that ... a continued throttle closure produces an INDUCED yaw in the direction of M/R blade rotation as main rotor gearbox frictions take effect and at low speed would result in loss of control to left yaw/spin.

Once in steady state autorotation, my fix has been to use engine power (collective) combined with air speed to allow the nose to hold at the '10 o clock' position in the descent until just before the flare when the procedure you outline should get the machine safely on the ground with little residual run-on speed.

I agree with the axiom ... plenty of study of the principles and plenty of practice offers a significant increase in the chances of coping should the lights go out!

'Twould be good to hear from others who have experienced total T/R failure especially for we guys mostly flying on the bottom rung of light helis.

Dennis K.

Thomas coupling

21st Feb 2013, 23:01

DK: Thanks very much for your contribution. Worth its weight in gold:ok:
Can I confirm that for the Enstrom (where a/c 'stiffness' is lacking) , you established auto shortly thereafter the TRF and kept the engine running to retain a modicum of left yaw (in direction of MR rotation all the way to the deck, or did you convert to an EOL just prior to landing on as fwd speed was virtually eliminated?

Having said all that I think we are striving to achieve the same aim - go for an auto/eol rather than a running landing - yes?

Thanks again for your time.

cattletruck

21st Feb 2013, 23:02

Had a minor TRCF as a stude. After pulling to a hovering stop on the helipad I turned the H300 around and began a hovering taxy towards the hanger. As I picked up speed and started relaxing the left pedal I got no response, a slight push on the right pedal did nothing, so I tapped the left pedal a few times and normal control resumed for an uneventful landing. Although the problem was a binding cable it didn't seem like a big deal at the time. Reported it to maintenance nonetheless.

TC in your notes would you include something in there about short shaft failure as on some models full T/R performance is still available when it happens. Does anyone know if the engine surges and overspins the T/R before the fuel control eventually compensates for the sudden loss of engine load?

Thomas coupling

21st Feb 2013, 23:07

Cattle can u explain 'short shaft failure' please? Also what are you trying to say w.r.t. TR overspeed????
I'm doing TRCF later - much more 'complicated':eek:

cattletruck

21st Feb 2013, 23:21

TC, as you already know, all helicopters have a short shaft that connects the engine to the MGB. Most helicopters have a TR shaft that connects the MGB to the TRGB. Some helicopters have a different TR shaft that connects the engine to the TRGB instead, it's usually a continuation of the short shaft but goes to the back of the helicopter rather than to the front.

1. TR shaft from MGB to TRGB
When short shaft failure occurs, TR RPM and MR RPM remain coupled with 0% engine power available to both TR and MR.

2. TR shaft from engine to TRGB
When short shaft failure occurs, TR RPM and MR RPM decouple with 100% engine power available to the TR, and 0% available to the MR.

Thomas coupling

22nd Feb 2013, 07:21

Maybe it's too early in the morning but, I am not sure what you are describing.
The shaft from ECU to MRGB is called the high speed shaft, over here. On some a/c failure of this causes the engine to either trip offline or sit at max cont. If you are a twin engine cab you can still fly.

Can you tell me which helo(s) have a shaft from the ECU t the TRGB, out of curiosity? I am curious as to where the speed gearing takes place. Surely the small TRGB doesn't do all the speed reducing?
Don't understand your point number 2? Sorry.

cattletruck

22nd Feb 2013, 10:26

Ok, best to use pictures e.g. the Ariel 1B as installed on the AS350.

As you can see from this schematic, the engine reduction drive turns one integrated drive shaft that has a connection spline fore and aft. The foreward spline attaches to a shaft that connects to the MGB which is called the short shaft. The rear spline connects to a shaft that connects to the TRGB.

http://blog.aopa.org/helicopter/wp-content/uploads/2013/02/Picture123.jpg
(Credit: Tim McAdams, AOPA Hover Power)

Here is a nice pic showing the TRGB shaft connected directly to the engine drive shaft.

http://images2.jetphotos.net/img/2/4/9/5/69699_1233818594.jpg
(Credit: Hendrik Kruger, Jetphotos.net)

Hope this helps, even if short shaft failures are quite rare.

Matari

23rd Feb 2013, 22:28

cattletruck,

I've never worked on the Ariel, so forgive my lack of understanding. You wrote that...."The foreward spline attaches to a shaft that connects to the MGB which is called the short shaft. The rear spline connects to a shaft that connects to the TRGB." (bold italics mine for emphasis)

When looking at the cutaway that you posted, it seems that the transmission shaft is driven through a reduction gearbox by a free turbine, and the same transmission shaft drives both the MGB (forward) and the TRGB (aft).

I do not see a "short shaft" except maybe the small shaft driven by the compressor shaft, which drives the accessory gearbox (AGB). But that shaft does not drive the MGB, just the AGB.

What am I missing?

315B

23rd Feb 2013, 22:57

It is my understanding that the 'short shaft' is not depicted in the cutaway that cattletruck has provided. It must be located where the power drive is pointing towards.
Great food for thought TC.:ok:

cattletruck

24th Feb 2013, 00:26

Hard to find a good picture of what I was trying to illustrate, their is a freewheeling unit connected ahead of the forward spline that then goes into the MRGB, so in the case of the Arriel engined AS350 I guess you could argue the transmission shaft is the short shaft (I'm not an engineer).

The point I was trying to make was about the potential for decoupling of MR and TR RPMs with these types of configurations (engine drives MRGB and TRGB directly).

I think I have confused myself now, with the engine rather than the MRGB driving the TRGB, what happens when the engine stops and the freewheeling unit separates the engine from the MRGB? There must be some other magic to this.

Thomas coupling

24th Feb 2013, 07:32

Cattletruck: Some helos have clutches others have freeewheels. All helos allow the MRGB and or the TRGB to disconnect from the engine in the event the engine stops. Simples.
Engine stops: both MR and TR continue to spin (provided you are in auto!
TRGB stops, engine and MGB continue to work.
MRGB stops - you die!
"short shaft" snaps/shears, engine runs up out of control and either switches itself off or hangs at max cont. MGB/TRGB continue to work provided you go into auto.

Beautiful photos though....

jellycopter

24th Feb 2013, 07:49

TC

Wrt the Ariel engine.

"short shaft" snaps/shears, engine runs up out of control and either switches itself off or hangs at max cont. MGB/TRGB continue to work provided you go into auto.

Not quite......

The governor senses Nf at the accessory gearbox on the engine, not the MGB. Therefore, there would be a momentary rapid increase in Nf at the moment of short shaft breakage but this would be quickly countered by the governor bringing the engine down to idle. Now the MRGB needs autorotative flow to keep turning but the TRGB is still driven by the idling engine so yaw control is still possible. When conducting the cushion on at the end of the EOL, the collective anticipator will throw a bucket load of fuel into the engine and the T/R will increase RPM rapidly and possibly uncontrolably with ensuing 'unexpected' yaw.

Best advice, I believe, is to keep the engine running on the way down but shut it off prior to commencing the landing phase.

JJ

AnFI

24th Feb 2013, 14:32

is it true to say that the Ariel set to idle under those circumstances would still turn the tail rotor (at less effective TRPM) and negate the anticipator problem on collective raise?

Thomas coupling

24th Feb 2013, 16:00

JJ: Thanks for this. Are you saying that the TR runs down to the equivalent of ECU flight idle speed during a high speed shaft failure?
How much a reduction does the TR experience as a percentage of its normal operating speed?
TRE diminishes to below acceptable on 'normal' helos as the TR speed slows even by as little as 5%, never mind even slower:eek::eek:

On face value this sounds a very archaic way of driving a gearbox (directly from engine speed.

Would it not be wiser for the TRGB drive shaft to be driven by the accessories gearbox (as it is now) BUT in the event of a high speed shaft failure, the engine could run to idle or even shut down and the MGB could drive the accessories in auto thus allowing TRGB speed to be maintained inside normal operating limits.

I can't believe the TRGB slows to ECU Flight idle speed and still allows any elemnet of authority:eek::eek::eek:>

AnFI

24th Feb 2013, 17:08

TC - I don't think he meant idling in the sense of Nf rather in the sense of No Significant Load - I was suggesting going to 'Idle' ( instead of shut down ) and I should think that 40% TR authority would be a luxury (lad) and easily enough to overcome MRGBx drag and keep ... etc

and asking (confirming?) that the 'anticipation' is not effective at all in Idle

jellycopter

24th Feb 2013, 18:17

AnFI

Very good point re anticipator. And without digging the manuals out I can't confirm one way or the other; I'll check at work tomorrow.

As regards the drive to the main rotor and tail rotor, they share the same shaft that runs under the engine (see Cattletruck's link). Forwards to the MGB, rearwards to the TGB. However, when the short shaft (more correctly called Main Gearbox Input Drive Shaft) shears, the engine instantly off-loads to the MGB, however, due to the configuaration, the shaft is still driving the T/R. In the cruise, since the T/R is doing very little due to the fin producing the anti-torque force, the Nf and therefore T/R RPM increase rapidly but are quickly reduced again because the governor reduces Ng down to it's idling minimum fuel flow. I'm not sure what the Nf or T/R RPM are in this condition as I've (thankfully) never seen one. My guess, is that even with minimum fuel flow, the Nf and T/R may well be overspeeding in the cruise as I would imagine that the residual gas generator thrust, even at idle, would be more than sufficient to turn an off-loaded T/R.

I do recall an incident in the States, sadly fatal, where the short shaft failed and the pilot lost control during the landing due to the anticipator rapidly spooling the engine and therefore T/R up.

JJ

victor papa

24th Feb 2013, 18:24

The Arriel have a reduction gearbox(MO5) which reduces the free turbine rpm to app 6000 rpm varying between the Arriel 1 and 2 and variants. The MO 5 is at the back of the engine. On the 350 the MGB is driven by the torque shaft in the eng MO1(the silver circular bit below the gasturbine and free turbine) which drives the MGB driveshaft between the eng and MGB. The tail rotor is driven directly off the back off the MO5 via the short shaft then long 5 bearing tail driveshaft to the TGB which reduces from 6000 something to 4000 something. The purpose of the short shaft between the MO5 and tail driveshaft is to absorb misalignment and movement as on the 350 the eng is not fixed mounted but on elastomers so it absorbs vibrations. On a single there is no overspeed protection and the Arriel 2 with its FADEC by default will freeze where it thinks a failure occurs and not go to grn idle or shutdown by itself. The EBCAU another airframe comp will take over now but lowestN1 will be N1 at failure so you must continue to load the disc if lowering collective to prevent overspeed. So, if the MGB driveshaft fail between eng MO1 and MGB the engine will be offloaded due MGB not driven, yet the TR will continue at free turbine speed due direct drive. This could cause a TR and eng overspeed whilst MR in not driven mode-disaster! Dont know of a case of MGB input shaft failure yet but ASB's on checking couplings with no noise during rotation allowed. Too much typing on a Sunday night.

The freewheel is in the front of the MO1 between the torque shaft and the MGB driveshaft so no connection to the tail driveshaft. Also the Arriel 1 free turbine flyweight in the FCU is driven by the free turbine and Arriel 2 is 3 sensors-2 to eng comp lane 1 and 2 and 3rd to VEMD. Anticapator on Arriel 1 is teleflex rod from collective slowing down N2 before it actually does and on Arriel 2 digital signal from mixing unit collective input resetting N1 instantaneously.

AnFI

25th Feb 2013, 06:55

Thanks Jelly

and good info VP ( you are clearly a knowing man )- SO VP you say "free turbine flyweight in the FCU is driven by the free turbine" - So it will attempt to govern still even though the load is only the tail rotor?
This leaves us with an interesting question which Jelly raises: is it possible for the FADEC to command a sufficiently low fuel flow to prevent overspeed of Nf (ie T/R)?

[and since you are a knowing person - can I check with you what EBCAU does - surely it can't just freeze N1 (Ng) - it would mean major overspeeds on lever reduction - I think I tried to use the EBCAU and think I remember it doing some Nr (Nf) governing ? Do I?]

victor papa

25th Feb 2013, 07:36

EBCAU is a independant Eurocopter supplied computer that governs the free turbine between 98 and 102 perc or somethingnlike that utilising a seperate independant valve from the stepper motor metering valve and electronic logic in the ASU 3 situated to the right of the coplts feet. The EBCAU is automatically activated by the FADEC incase of total stepper motor or metering valve failure which will be sensed bynthe resolver or both comp fails. No action req by the pilot. This is for the 2B1 and 2 D, the 2B will unlock the sliding lock on the throttle to allow manual control. In both cases the engine comp stops the metering valve where the failure occured thus the N1 and the EBCAU or throttle can control free turbine speed around that position.

Overspeed control is called the pilot on the singles. Not sure how the DECU's will deal with it and not going to try and see what happens if the MGB input shaft fails.

AnFI

25th Feb 2013, 11:31

VP - that's great information - so EBCAU will govern Nf then so even if DECU's 'flip out' the basic EBCAU should stop overspeed ... ? and so T/R might not overspeed if that all works ...

as an aside (whilst we have an expert on the line) people run helis up without blades sometimes - does DECU have a problem with this ? Is there an issue with feedback/resonance/oscillation ? Is it an 'ok' procedure to run with out blades?

Thomas coupling

25th Feb 2013, 13:05

ANFI / VP: Let's keep it simple for simple folk like me eh?

Taking only the AS350 as an example (because other SE helos don't follow this concept):

In the event an AS350 eperiences an engine failure, the a/c has to enter auto immediately for obvious reasons! The MRGB then 'reverse' drives the reduction gearbox off the engine, which in turn drives the TRGB at normal speed - correct?

Now:

In the event the short shaft between the MRGB and engine - fails (high speed shaft failure), the engine reduces to Flight Idle (or similar). The a/c has to enter auto/eol and the MRGB maintains its speed. The TRGB is dramarically reduced in speed - correct?

At the bottom of the auto, prior to touchdown the MRGB increases in speed initially due to flare effect but then reduces as the collective is raised - but the tail rotor speed remains at tick over throughout.
How, then do you stop the undemanded yaw of the a/c due to the TR not being able to offer any anti torque - during the flare effect (provided the engine is at tickover speed)???

Thanks

jellycopter

25th Feb 2013, 13:06

AnFI

Why I needed to go to work to check the books, i don't know. Of course throttling back to Idle will inhibit the anticipator. Every time we practice EOLs, the throttle is at idle and therefore there's no problem with the anticipator spooling the engine up.

JJ

jellycopter

25th Feb 2013, 13:17

TC

The Ariel has a Free Power Turbine.

In the case of an MGB Input Drive Failure, the governor will sense Nf (N2 on Allisons) being High so will command the Gas Generator (Ng / N1 / CRPM) to minimum RPM. However, as the T/R is driven from the aft portion of the shaft, and the M/R is driven from the forward portion of the shaft, there is a potential disconnect between T/R and M/R rpms. The M/R will require you to enter auto, as per any helicopter that loses drive to the MGB.

The difference, is that the Ariel will still power the T/R in this configuration. The big questions are, since the Gas Generator is at minimum fuel flow, and therefore minimum Ng (CRPM / N1) what is its residual thrust? Will it accelerate the Nf ( and therefore T/R RPM) beyond normal limits as it won't be loaded by the tail rotor at close to min pitch? Will it start to govern effectively again if the tail rotor is loaded for any reason (I suspect it will)?

Any clearer?

JJ

victor papa

25th Feb 2013, 13:20

As said, I am no expert and am definately not going to try a MGB input shaft failure! Dont wanna be close just for incase. Before the 350 doom prophets climb onboard another possible flaw, I think the chances are very slim seeing the fleet has got millions of hrs and we are speculating only.

The Arriel is extremely fast in excelerating especially the Arriel 2 so I do not know whether I would run without blades.

jellycopter

25th Feb 2013, 13:24

TC

I've just read your earlier post where you state "TRE diminishes to below acceptable on 'normal' helos as the TR speed slows even by as little as 5%, never mind even slower"

I don't think that's helpful in understanding the dynamics and capability of a tail rotor. If you're demanding max anti-torque thrust from your T/R, say high AUM, OGE Hover and spot turn, then yes, the TRE may be insufficient.

However, the T/R is perfectly capable of maintaining effectiveness at reduced RPMs if the pilot reduces the thrust demand of the T/R. In the case of an autorotative descent and landing, the T/R will be effective at massively reduced RPMs because there's not much work for it to do.

JJ

Thomas coupling

25th Feb 2013, 14:00

Hi Anfi: Its a scary thought that operators of said aircraftdonj't really know what's going on after a hi spd shaft failure, though isn't it. I have my "research team" onto it and if anyone knows whats going on - they will. I will revert.

Meanwhile:

Your last sentence concerns me a little. It may be the printed word that is relaying the wrong message but when the 'propeller' called a tail rotor, slows down by a relatively small amount [In the Sea King, when Nr decays from 102 to 82% - below that TRE is almost zero:eek:], the TR becomes a useless piece of rotating junk (to exagerate)...

Now my previous question was/is:

Yes, I agree that during auto, a TR needs to do not a lot, as a matter of fact it may just as well be stationary. BUT:

During the descent, if the pilot wishes to manouevre in the descent either to avoid something or to turn, say 180 degrees round the corner to get into wind, he/she will need directional assistance.
also
at the bottom, when they flare like a ding bat to wash off descent, the Nr goes up and (normally) the pilot maintains directional control with pedal BEFORE this directional control rapidly dies to zero as the Nr decays (engine off or at idle).

How then - in an AS350 helicopter does the pilot achieve any of these objectives above, if the TR is outside its TRE????????:eek:

AnFI

25th Feb 2013, 18:41

Hi TC

No problem with T/R thrust at reduced T/R RPM - there's no major torque to counter - and the flare/level/lever part won't vary the gearbox drag all that much (not significant) - the reduced effectiveness of the T/R will easily be enough... (especially not a problem in manoeuvering turns on the way down - any kind of airspeed (30kts +) will keep that tail near the back - only tiny T/R thrust to stay in balance if you really want to...)

T/R thrust is just about proportional to T/R RPM squared - so 50% RRPM implies 25% of max thrust available - easily enough.

If you think about a running engine off landing you do need much larger T/R pedal inputs to make T/R thrust changes...

The interesting points in this thread is how many people think cutting the throttle is the answer to achieve reduced torque on loss of tail rotor. (as opposed to putting the lever down - Brazilian Squirrel)

AND

The greatest danger when teaching T/R loss is that the student has afterwards an increased chance of mis-diagnosing a tail rotor failure when really they were just caught out by the wind etc

Dennis Kenyon

25th Feb 2013, 19:56

Mainly for TC ... thanks for the note and wish I could exchange my words for a couple of gold bricks!

To answer your note.

Following recognition of the TRF situation, I teach an immediate reduction in power ... (lever fully lowered) My use of the word 'immediate' depends on the power in use and airframe speed at the moment of failure. High power, low speed ... no upper limit. Low power ... highish speed, the lever can be lowered more leisurely. To enhance the simulation, I also 'induce' some airframe fibration with collective lever 'fluttering.'

Either way, the aim is to regain control and prevent further RIGHT yaw (USA machines) by allowing the airfame to return from 'right to left' past the 'dead ahead' position to establish a steady state descent in the '10 o Clock' position which requires right cyclic. With control regained, a combination of speed and power allows stable flight in the '10 o Clock' position to be established AND/OR a variation in distance flown and rate of descent as necessary.

With a suitable landing site available, a descent is initiated to around 20 - 30 feet AGL at the lowest achievable speed. As the airframe sinks to the surface, collective lever is raised to arrest ROD with a cyclic flare used to reduce forward speed. With the increase in power, airframe commences the original yaw to the RIGHT which is allowed as far as the 'One o Clock' position when throttle is closed for a low speed EOL. Cyclic is pushed forward to co-incide with the airframe approaching the dead ahead 'twelve o Clock' position while allowing the aircraft to partially touch down on the rear skids to assist a dead ahead 'run-on' landing. Even so the airframe is still likely to continue its left yaw, which as noted in a post above, can be minimised using a right cross wind giving greater drag on the right skid.

I hasten to add that my notes must neccessarily apply to the type on which I've experienced my two total T/R failures, being the cable operated Enstrom T/R control system where the failed cable wrapped itself around the T/R to seize the transmission. The other total failure being a straightforward break in the T/R drive shaft.

My 1999 Biggin Hill Air Fare failure being the less difficult left hand CABLE breakage which allowed use of left anti-torque pedal, but leaving no right pedal control for left yaw. I hope these somewhat neccessarily convoluted notes help an understanding of these failures. and I'd happily chat further with interested parties by PMs. Safe flying to all. Dennis K.