TR malfunctions for beginners - a refresher lesson!

[Thomas coupling]

Tail rotor malfunctions for beginners
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.

TRF:
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 tail 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 in normal flight, 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 / EC135 / 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: auto / 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 known, 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 or 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 auto rotation and consider chopping the throttle(s). Yaw further reduces slightly if the engine is shut down (but on some helos may cause a counter yaw).
Flare (yaw further reduces), level and cushion touchdown by cross controlling for yaw offset and side slip. Always shut the engine down immediately after touchdown as the reducing Nr will cancel out any tendency for the a/c to counter 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 verse. 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 aircraft 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]

From DK:

T/R control loss
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.

[apb]

Thanks alot for this post, It will help me to improve my knowledge about TR malfunctions and my english too ;)

[SASless]

TC,

Without sounding too critical....you do make some statements that may not be as certain as you present. As written some of your comments border on deadly unless someone with experience knows when to skip along.

If you experience a Tail Rotor failure that leaves you with too much Tail Rotor Thrust.....doing an EOL is the exact wrong thing to do.

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.....

Not so fast.....you might experience a complete loss of control along with the shift in CG....but the Exercise is just beginning.....not ENDEX as you suggest.

An IMMEDIATE full reduction of Collective, and if that doesn't work well enough, Engines(s) to Idle.....and attempt to regain control of Pitch/Roll/and see if Yaw control no matter how limited is regained. If you get the aircraft upright and responding to cyclic inputs.....reapply power and collective and see if you can maintain flight(if necessary) and if unable....or if there is a very good place to land.....do an EOL expecting yawing at the Flare/Decel. That might be checked with a bit of collective.

Remember the Bell 412 Crew in the GOM that had a TRGB and Blades depart the aircraft at night over the water. They said they went inverted and spun.....but were able to regain control after bottoming the Collective and rolling the engines to Idle....then were able to fly the aircraft until they decided to carry out an Emergency Landing.

It might be extreme....but don't ever quit flying the aircraft....fly it to impact but fly it.


TRF in fwd flight results in uncontrolled yaw in the opposite direction to main rotor rotation.

I prefer the use of the word "Un-commanded" rather than "Un-Controlled" in that the weather vane effect of the Tail Fin might limit the Yaw to a large side slip angle but not wind up in a spinning of the aircraft with a bit of airspeed.....thus is is "controlled" but remains "un-commanded".


Analysis of Tail Rotor failures other than Loss of Components can be a lot simpler than what you set forth.

Power Pedal forward.....Good Day. Power Pedal aft.....Bad Day.

The problems that leave you with too much Tail Rotor thrust can be addressed by reducing Tail Rotor RPM then making a shallow approach to a hover.....during the approach the nose of the aircraft will be off to one side and as the airspeed lessens will begin to come back towards the direction of travel. Hold adequate airspeed to control keep the Nose off to the side until you can get within 1-2 feet of the surface. If you plan the approach to arrive at a very low hover (1-2 feet )....as the Nose centers.....simply lower the Collective and land on.

If you miss judge and the Nose swings past the direction of travel....simply accelerate and try again.

If you have too little Tail Rotor Thrust....the choices are a running landing or an EOL. As groundspeed might be excessive to maintain yaw control and thus make for a very dangerous situation (you are hitting the ground at a very high rate of speed, with full power on the rotor system, and your yaw control is very limited), the EOL offers a lot of advantages. In the EOL you arrive a little or no ground speed, the engines can be in Idle or shutdown, your ROD will be slight after cushioning the landing, and if the aircraft yaws and even if it rolls over....the resulting forces are much less.

Even without a Tail Rotor....the Pilot remains capable of "Controlling" the Yaw of the Helicopter by varying Torque on the Rotor System and Airspeed.

The failure most Pilots make....is not fully understanding the effect of the various controls, levers, throttles, and cyclic have on "Yaw" in their helicopter. This is something that needs practicing, so that when an failure occurs, the initial response is automatic and the subsequent actions are based upon a thorough knowledge of the way the Helicopter reacts to the controls without benefit of the Tail Rotor.

You laid out a lot of information that will certainly get the discussion going....and is an interesting topic for you to bring up.

[Hughes500]

TC

Curious and I know it will depend upon type but when is the tail rotor actually doing nothing ?
Is it in the cruise when pedals are together ? If that is the case to simulate the tail rotor actually stopping is it as easy to put pedals together and say there you go ?

[SASless]

Pedals in Neutral are at the point where the Tail Fin is working at its best to streamline the aircraft due to weather vane effect....thus requiring the least thrust from the Tail Rotor. The Tail Rotor is also helping in that regard despite being in a neutral position.

For training purposes I use two positions for the pedals.

To make it Ambidextrous....Left and Right Turning Rotor Systems.....Power Pedal is the one you push forward to counter-act increased Torque.

Power On.....Perform a Max Performance Takeoff pulling more than Hover Power.....and lock the pedals at the max travel required to trim the aircraft as you begin the takeoff and have the maximum power applied. Continue around the pattern and land.

Power Off (Loss of Thrust-Loss of Components)....apply up to 80-90% Power Off Pedal. Depending upon amount of yaw.....either a running landing or EOL as required.


For Bell Helicopters....using manual control of the throttle and flying down the runway following a Tar Joint or painted line....and controlling the Yaw by throttle alone is a very good exercise. Some parts of the USA....you can follow a fence line for miles and miles.

[jellycopter]

TC

you stated......

"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..... "

I'm very much with SAS on this one.

The following NTSB report should be mandatory reading.

http://dms.ntsb.gov/aviation/AccidentReports/ikbw1u55sdzxacmnw1vfn0y21/L02262013120000.pdf

It clearly shows that loss of T/R and TRGB (and half the vertical fin in this case) is still controllable!

JJ

[jellycopter]

SAS

Whilst I'm with you 100% on the rest of your post; this bit is a bit of a struggle for me -

"Power Pedal forward.....Good Day. Power Pedal aft.....Bad Day."

Great with a proper throttle ( especially if its on the collective), but these all too common modern aircraft with fancy FADECs don't allow any control of the RRPM.

JJ

[SASless]

True.....but you can beep the engine down as much as you can. In a twin...shut one engine down and minimum beep the other. The goal is to slow the tail rotor as much as you can.

I also remind folks that the Green Arc is "Normal Operations"....and that a Tail Rotor Failure is not a "Normal Operation".

In an ideal world....you should be able to reduce the ground speed to a point that you do a landing with some forward speed...controlling the yaw with collective input as you slowly lower the collective to "brake".

The key is to touch down as the nose comes in line with the direction of travel.

[407 too]

Jelly,
with FADEC (on the 407 anyways), you still have full control of the RRPM while in Auto -mode.., just roll the throttle back.

How else are you be able to acheive ground idle, or 100% from ground idle??

[SASless]

I used to hear a similar argument about Power Levers....but reminded folks I had done these drills in BO/BK/76's single pilot....fraught with peril but do-able as if one is to instruct in the machine....you do have to be able to do it and demonstrate the maneuver to the Student.

[Hughes500]

SASless
thanks for that but in reality is there actually a position where the tail rotor is actually doing no work. Lots have said look where it is in zero pitch which is actually easier said than done as it will depend upon how the blade is constructed

[victor papa]

On the AS350 Arriel 2(either 1 or 2 EECU) despite the electronics you can go to ground idle using the twist grip on the collective. That includes the old B3 with the single channel EECU as the flt to gnd on the throttle(twist grip) is always available and in all B3's ground idle is ensured via the forced idle microswitch which gives the pilot authority despite the computers so engine goes to ground idle. Biggest problem in the old B3 and B3+ premod is getting back to flight if the out of ground switch fail during training and the engine remains at ground instead of going back to flight and you didnt select a landing site as the ground idle position is always assured and priority if the pilot selects it incase of a emergency.

With the 350's pedals centred on a B2 and B3 definately and some BA's and B's dependant whether the 355 TR blade is fitted, you already have 10degree pitch as the blade is offset 10 degrees positive to the spar.

[Thomas coupling]

:ouch:
My thanks to the likes of SAS and JJ, one never stops learning and to discover several things whilst compiling this thread was amazing. First was that there are singles out there that drive the TR from the engine (AS350) - why Aerospatiale built it this way astounds me as it inherits more problems than it resolves and secondly, that some helos can survive a total loss of tail rotor structurally and still survive - thanks to all.

However guys, this particular thread was meant to be FOR BEGINNERS :ugh: - to gently break them in and you lot have descended on it with technical descriptions of this that and the other which is enough to put any R22, Enstrom pilot off for life!
By all means correct or add or whatever but please please please KISS guys, so the sub 500hr makey learney doesn't get blown away with the complexities of beeping back on one engine to reduce TRE so as to follow fences for half a mile whilst yawed off differentially in auto reading the oxford english dictionary :)

KISS

[500guy]

Not to beat a dead horse or beat up on TC but...
Just this month a R44 lost t/r gearbox in flight and landed safely:
http://dms.ntsb.gov/aviation/AccidentReports/zr5agpfc5h1hi0afhedh4k451/S02262013120000.pdf


Good info all around!

[victor papa]

TC, I appreciate what the thread stands for. However, it is important that low time guys also knows the facts and theory behind the basic and it is not that complicated. For instance, you make a stand against the 350 design that might scare some when reality is it is a multi million fleet hour design with plenty hours flown where no other machine can so surely they have the right to the facts as well. We have now established the myth that FADEC prevents the pilot going to ground idle is a myth on both 407 and 350! The 350 gets lots of critisism without taking into account the percentage of failures vs hours flown nevermind the circumstances in which the average machine flies. Younwanna fly a helicopter you need to know and understand it!

[jellycopter]

VP

There's no myth that you can't take a FADEC engine to idle when in flight. The problem lies with the bits in between idle and flight rpm when the pilot wishes to reduce the T/R RPM partially to reduce its thrust.

TC

Another helicopter that drives the T/R from the engine is, wait for it.......... pehaps the most prolific light turbine engined helicoper ever produced, the Bell 206 Jet Ranger.

So it's not peculiar to the AS350, far from it.

JJ

[RVDT]

Who knew? :O

First was that there are singles out there that drive the TR from the engine (AS350) - why Aerospatiale built it this way astounds me as it inherits more problems than it resolves

http://www.tsb.gc.ca/eng/rapports-reports/aviation/2000/a00w0215/Figure_1.gif

B206, B206L are effectively the same.

http://www.tsb.gc.ca/ENG/rapports-reports/aviation/2011/a11c0152/images/A11C0152-figure-1.png

Don't get started on the SK76 and its issues as well. Remember the days of one engine driving the MR and one driving the TR?

[Thomas coupling]

Victor Papa: chill out for goodness sake. I'm not slagging the bl**dy squirrel off, far from it.
Secondly my comments re the 350 are and remain outside the main thread. IF newbies had read this first post only as it was meant to be read, there would have been no reference to the 350.

Why can't readers appreciate this post for what it is - a beginners introduction only. If people like you want to make a meal out of it, go to the other tail rotor thread.


I've flown the Gaz and the twinstar for thousands and thousands of hours. I have 11 types on my licence, twins and singles and all but 1 of them drive their TRGB from the MRGB!

My observation is: why does an a/c designer complicate matters by driving the TR from the engine?
If you drove it from the MGB, you wouldn't need to worry about hi speed shaft failures as the MGB would carry on regardless and drive the TRGB. No need to design in coll anticipators/fadec bypasses/overspeeding FPT's - call it what you want. Shaft fails, enter auto, the end.
Whats strange is Aerospatiale built both the Gaz piece and the As350??

You seem to know everything - answer this?

[jellycopter]

TC

"My observation is: why does an a/c designer complicate matters by driving the TR from the engine?"

My own observation is that far from complicating matters, the AS350 and B206 actually simplify things. For a single-engined heli with the engine aft of the M/R transmission, in order to drive the T/R as you consider best, it would mean doubling up on drive shafts and complicating the MRGB to generate a T/R drive coupling. In the case of the Gazelle, as well as the extra drive shaft and the more complex MRGB, it also has an additional intermediate inclined g'box to route the T/R drive under the engine. All this adds extra components, building in failure points, but most importantly, extra weight. Furthermore, an independant T/R driveshaft passing under the engine means the engine has to be mounted higher reducing clearance from the main rotor, making the C og G higher and increasing panel sizes to cover the engine.

In the AS350 and B206, the design is simple, light-weight, compact and reliable. All the talk of Short shaft / High speed shaft failures is mainly theory as on the whole, as VP asserts, these failures are extremely rare in any case.

JJ

[sycamore]

Several decades ago in Borneo,I had a tail-rotor blade depart,followed by the other one and the t/r gearbox.The day had started with a troop lift to various LZs,and whilst landing to refuel,someone pointed out `something` under the aircraft,Ilifted off and moved away,revealing a large pool of oil on the ground,so parked,shutdown and inspected it.It was a broken transfer oil connection between the engine and reduction g/box. Short story ,called `home` ,who sent a standby aircraft and groundcrew ,who whipped out the engine/gbox and replaced the bit,whilst the other crew finished my task.A/C ground -run,so we set off as a pair to RTB,,with a couple of the mechs downstairs with an access panel removed to check if it was all OK at the back of the g/box.As we climbed,I checked down in the cabin to get a `thumbs-up `(no intercom) from the lads,when there was an almighty `Bang`,the aircraft yawed hard right,and pitched nose down.Pedalling produced no response,and the stick was on the back stop,to prevent it pitching further.I thought I had collided with the other aircraft,as I couldn`t see it,and had gone into autorotation instinctively about 800ft, looked for a place to land,as I had no idea of further damage,thinking I had collided,and put out a couple of `Maydays`,just in hope..As we were over a mix of primary and secondary jungle,I saw what appeared to be a small clearing,and continued a right spiral towards it,closed the HP cock,and flared as much as I could,finding that the area was actually a `small` hillock,or pimple,and then EOL`d at zero speed,but finding we were on about 10-15 deg slope.The mechs were OK,strapped in,and the only other damage was a burst tyre from a tree stump ,that was stopping the a/c from rolling off...Shortly after,the other aircraft appeared,and winched us out.The Board of Inquiry offered the local Headman,and villagers money for any bits they could find,and the blades and g/box were recovered a week later.It was found to be a fatigue crack in the threads of the blade spindle that had caused it,but it had taken some time to propagate.
Throughout the incident,I had poor roll control,probably due to a lowish speed 50-60 kts,and limited cyclic f/aft,but that may have improved as the mechs `down below` went aft to strap in.I don`t recall any yaw on touchdown,probably because the undercarriage was caught in the secondary ground foliage.
The aircraft was lifted out,and repaired at base,and the following day one of the other pilots gave me a USAF Flight Safety magazine, with an article about `How to handle t/rotor failures in the H-34!
There`s a pic or three on P15 of `Rotorheads around..Cockpit views`,#294...

[Thomas coupling]

Jelly: I thin k it's horses for courses. The Gazelle TR take off is simply another cog running from the planetary gear in the MGB simples.

In this type of a/c, when the engine stops for any reason including hi speed shaft failure - you enter auto and the MGB drives the TR. Simples.

In the 206/350, during auto the MGB drives a reduction GB which in turn drives the TR.. which in itself is 'awkward' and unnecessary BUT THEN you have to have the engine running AS WELL to drive the FPT to drive the reduction GB to drive the TR....in case you experience a hi spd shaft failure?????

Where in the design team @ Aerospatiale did they have a shift of ideas about single engine drive train layout...just out of curiosity. For better or worse :)

[victor papa]

If the MGB input driveshaft/high speed shaft fails, it can not drive the tail as the only shaft between it and the engine has failed and thus no relation between MGB and tail speeds. MGB no longer driven and tail will go with the free turbine due its connection.

[Thomas coupling]

That's just what I said VP:ugh:

That's the problem I see with 350/206 drive train layout: you need to keep your engine running to drive the TR during a hi spd shaft failure. In all other singles you don't.

[SASless]

Now as I see it....in the situation you are describing.....if you shut the engine down....you don't really need the tail rotor as you are going to be in autorotation. Granted it would be nice to have the tail rotor working....but in an autorotative landing...it isn't absolutely necessary as in a power on landing.

[victor papa]

Exactly! If the MGB is powerd the airframe wants to go opposite direction to the MR direction. If MGB is not powered the frame wants to go with the MR. There lies the trick with the 350 as the TR is countering a non existingnforce if the MGB is not powered and pushing the tail where it wants to go with the MR if say the MGB input shaft failed. Only way to stop is to take eng to grnd or shutdown as no use to MR?

[Thomas coupling]

SAS and VP.
TRE makes sense if during auto/eol you want to turn, say 180 degrees into wind. Without TRE you have to rely on side slip to get it round the corner and we all know that takes time and height. With TRE at hand you simply apply pedal during the turn to hasten the change in direction. You may want to dodge obstacles during auto, how do you do that comfortably during auto with no TR? It could be achieved but at the expense of a marked increase in height loss.
At the bottom of an auto/eol when you flare, the Nr goes up...so what would happen to the nose of the a/c if you didnt have a TR? It would put you in a yaw offset situation which would probably make it harder to cross control to land.
All in all it still strikes me as odd that designs like this exist when the other far simpler designs don't rely on the engine having to be kept running if the hi spd shaft fails.

PS: The designers obviously beleive there is a need to drive the TR when a hi spd shaft fails because they advise you to keep the engine running during this malfunction. So presumably my argument that you always need a TR running to assist flight is true?
[Yes - I know a/c survive without TR's, we've seen examples recently. But from a design perspective, AS350's and some others have designed in the need to keep your FPT running during a hi spd shaft failure????

C'mon u 2 ...you can do better

[SASless]

TC,

You are mixing up the argument. The question was about what happens if the engine is not driving the Tail Rotor following the other failure.

My response was to say that if the engine was no longer running....then there would be no Torque to counter act as it would be with the engine running.

Yes, maneuvering without a tail rotor is awkward and yes....at the bottom when you decelerate the nose of the aircraft is going to want to move to one side but that can be minimized by applying a bit of Collective.

Yes....if the engine is running....leave it running on the 350 and 206....but that was not the question as I recall.

Tail Rotor failures are not the bugger bear some folks make them out to be.....but that is due to way too many folks having no real certain grasp of how their aircraft responds in unusual circumstances like tail rotor failures.

The tendency these days is to discuss over a cup of Tea and not get out and do the drills that used to be normal fare. Our growing tendency to be "Risk Adverse" is not necessarily a good thing in all regards.

I remember being told of a conversation by the owner of a company I worked for as a TRE/IRE.....which stemmed from his seeing one of his BO-105's spinning around a fair old rate....as it passed by the office window that overlooked our practice area. As he spilled his Tea thinking something was seriously amiss.....he make a query to the Chief Pilot along the lines of "What the Hell is going on!" or other mild words to that effect. The CP never looked up and said....."Sasless is discussing Torque Control with Hose Nose." (Hose Nose was the Pilot's Nickname.).

I did have the benefit of US Army training back during a period of time where we got lots of practice in Tail Rotor failures and EOL's....as upon graduation from Flight School we were almost assured to need those skills in our first operational assignment. I lucked out and wound up in Chinooks where we did not even discuss Tail Rotor failures for simple reasons.

On all of my check rides under the UK system.....not once did we do a Tail Rotor failure drill....and when asked if there was anything I would like to do at the end of a check.....not one TRE ever took me up on a request to do some Tail Rotor Drills.

That kind of situation does not lend itself to building any Monkey Skills that would be very useful in coping with a real Tail Rotor Failure.

[Thomas coupling]

Concur wholeheartedly SASless. Hence the reason for my opening post, to stimulate activity in the ab initio world.
But without training and practice, it will continue to haunt pilots as a sinister malfunction.

[SASless]

When teaching in the Huey....one exercise I liked to do was to put the Governor in Manual....and have the Pilot fly a full circuit using the Throttle to control the Nr....then on the second circuit apply a bit of Left Pedal and have the Pilot remove feet from pedals and use the throttle to control yaw as we flew the full length of the runway at a slow airspeed and about 2-3 feet off the runway. Usually, by the time we got to the end....the Monkey Skill was in place. On the next circuit....quite a bit of left pedal was applied and we did the same drill but slowed the aircraft to the slowest speed the nose would stay centered.....until we determined the slowest speed we could use even if it was a steady hover. The final trip round the pattern was to set up for the same scenario....but this time I asked the Pilot to let the nose go all the way across.....accelerate till the nose swung back over....and repeated that drill as many times as possible before reaching the end of the runway.

That demonstrated to the Pilot the kind of control you might have in a real failure.

Demonstrations are not the real thing.....just as Simulators are good but not the real thing. What you are teaching is the Concepts so the Pilot is better equipped to deal with a real situation.

[krypton_john]

RVDT, thanks for posting the engine schematic. I note that it has a "Muff Coupling" which sounds a lot better to me than a Thomas Coupling.

Sorry TC - it is what it is!

[Thomas coupling]

Have another go Krypton. See if you can contain yourself and say something constructive to help all the ab initio's out there who clicked on this thread to only to read your musings. Well done. :rolleyes:

[[email protected]]

Sasless - in your EOL with no TR, what happens as you decay the Nr to cushion the landing?

I think Mr Newton would argue that the action of slowing down the rotor will cause a reaction by rotating the fuselage in the opposite direction.

I don't recall many EOLS (and I have done a few) where I didn't need the TR to keep straight during the landing.

[SASless]

Crab,

As you rightly state....there will be a yawing movement but that can be minimized somewhat by application of Collective ( a quick pulse), then as the aircraft touches down any other yawing will have to be followed by cyclic as the aircraft comes to a stop.

The point is even with no Tail Rotor Control by use of the Tail Rotor Pedals.....one does retain some control by application of Collective (if no engine power) and use of Collective and/or Throttle if the engine is running.

If you do not have a usable Tail Rotor....you will have to live with the fact the nose may not remain pointed in the direction of travel. The key is to have as slow a ground speed as possible....and follow what yaw you have with cyclic to keep the aircraft upright.

[[email protected]]

And therein lies the problem of a 'one size fits all' piece of advice.

In a light helicopter, achieving a zero speed touchdown for an EOL, whilst not being simple, is a reasonable expectation, especially if there is a bit of wind to help you.

In a larger helo, this becomes progressively more difficult and a run on (at some speed) is inevitable - usually because the amount of nose up required to wash off all the speed will, in all probability, smack the tail into the ground and rather aggravate the situation, or leave you quite high with decaying Nr. Whilst this (putting the tail in) might be a desirable technique when conducting the EOL to water (or perhaps into the tops of trees), in normal circumstances it is more likely to prejudice the outcome of the EOL than assist it.

This is where lots of lateral cyclic and friction from the skids or differential braking on the wheels will probably help.

[SASless]

In a larger helo, this becomes progressively more difficult and a run on (at some speed) is inevitable - usually because the amount of nose up required to wash off all the speed will, in all probability, smack the tail into the ground and rather aggravate the situation, or leave you quite high with decaying Nr.


Do you consider a Chinook to be a "larger" Helo?

Why is a Run On landing inevitable?

By "inevitable"...you mean to say every single time no matter the conditions?

[[email protected]]

When did a Chinook grow a tail rotor??????? Keep with the program Sas - this is a TR thread.

By "inevitable"...you mean to say every single time no matter the conditions? no, you might be operating with a very strong wind, you might be the best pilot in the world or you might just be really lucky - but for the most part, an EOL in a bigger helo (not a chinook or anything else without a TR) with a TR failure will result in a landing with some forward speed.

There, is that specific enough?

[SASless]

What is the smallest "bigger" helicopter you have in mind? Where do you draw the limit between smaller and bigger? Do you account for the difference in rotor system inertia dynamics when you draw that line?

When it comes to the Chinook....it does very much have a Tail Rotor....one that is as big as the forward rotor....but EOL landings were part of the syllabus.

Since you added the "bigger" helicopter discussion.....and the Sea King is what you are considering a "bigger" helicopter as that is what you fly....I can only assume you are providing commentary about your personal experience in the Sea King.

[Thomas coupling]

SAS I regard the EC135 as a suitable a/c for a running landing. don't confuse readers with tandem or other non tail rotor a/c. it's bad enough holding it together so far!
crab: you'll have your hands full with some of the conversations here:cool::zzz:

[SASless]

Crab was talking about Zero Ground speed EOL's and opined that "Bigger" helicopters could not do them without ground run. I was seeking his definition of "Bigger" so as to see if I had flown one of his "Bigger" aircraft.

As I have said before.....Wokka's and Tail Rotor Failures don't take much discussion.

[MightyGem]

Just to show that it can be done, the(poor quality) video shows a AS355 landing after a tail rotor drive shaft breakage. If you look carefully, you can see a stationary tail rotor. To prevent any doubters asking, we have the tail rotor on our crewroom wall.

Tail Rotor Failure - YouTube

[jellycopter]

MG

Do you have any background on this incident? I use this video in my T/R Malfunctions briefing at work and some more info would be helpful.

JJ

[[email protected]]

As I have said before.....Wokka's and Tail Rotor Failures don't take much discussion. so why introduce the subject here?

For clarification - small= Gazelle, Squirrel, R22 (at a push because of the low inertia rotor), Jetranger - with good conditions and elements of skill and luck zero speed EOL with no TR feasible and survivable.

Larger - UH1 - yes (big, high inertia rotor allowing a big flare, a high level and enough Nr for a vertical cushion); Wessex - marginal but the tail wheel allows for a lot of nose up whilst protecting the TR; Lynx - no, small disc with great response but the TR would strike first and the Nr decays very quickly. Sea King - no, long tail and no protection for the TR.

[Thomas coupling]

jelly:the AAIB report is online. If you really need info from the horses mouth, I can put u in touch with the pilot. send me your e-mail addy in a PM and I'll pass it onto him.

[[email protected]]

And no attempt at a zero speed EOL, surprise surprise.

[SASless]

So he chose option B....and carried it out quite well.

You reckon he did a handling check and decided Plan B would work?

As to "Zero Speed" which you are quite hung up on.....if you will read the prior posts you will see it was "Zero speed to Minimum/Minimal/slow forward speed".

[cyclic flare]

TC

Your best post to date well done

Cyclic

[Thomas coupling]

Thank you cyclic:suspect:;)

[JohnDixson]

Crab, in the autos done on the S-92 as an example, after leveling, and during the collective application, there was a distinctive right yaw tendency, thus a necessity to add a bunch of left pedal to keep the nose aligned. As I recall, we used almost all of the available range. Of course, the TR Nr is going down and the lift/V squared relationship is in play.

[Thomas coupling]

John,
The speed of the TR doesn't change during an auto?
[Hence why you get yaw in the opposite direction to MR rotation].

In an EOL however...........................

What was your message anyway?


SAS: Please try and remember your heritage;) Not everyone was born in a helo!
The purpose of this thread was to prepare ab initio's and those 'out of touch' that TR failures / control restrictions are VERY serious in helicopters UNLESS one is VERY familiar with the mechanics.

It is probably the LEAST discussed/rehearsed/trained evolution in the rotary world. AND the most damaging if handled badly:=

[jellycopter]

TC

I'm responsible for briefing T/R malfunctions on one of the ab-initio squadrons at DHFS. This brief is given prior to going solo and at the first circuits phase.

I split the subject into 2x 2-hour briefs as there's so much to cover!

One brief deals with Yaw Control malfunctions (T/R control failure), the other covers uncommanded yaw incl. T/R Drive Failure.

I have found that the best way to foster a depth of understanding wrt to 'uncommanded yaw' is to ellicit the possible causes. These include, T/R Drive failure, LTE, Hydraulic Malfunction (we teach on AS350 with Hyd assisted T/R), wrong pedal input and weathercocking. By breaking down each scenario into it's components and likely symptoms and recovery actions the students seem to lap it up. A generous smattering of helicopter porn (relevant accident videos) and accident reports spice things up a bit. It's one of the few briefs that I give where theres a 'buzz' in the canteen during the coffee break.

There have been numerous training accidents worldwide (IIRC incl 2 at DHFS) as a consequence of poorly handled yaw. Rarely was there anything wrong with the helicopter. Understanding, recognition and correct diagnosis are the key to a successful outcome.

JJ

[SASless]

This part of your post I fully agree with.


TR failures / control restrictions are VERY serious in helicopters UNLESS one is VERY familiar with the mechanics.

It is probably the LEAST discussed/rehearsed/trained evolution in the rotary world. AND the most damaging if handled badly

The other thing I see wrong is too many folks fail to understand just how much "Yaw Control" they do have even when the "normal" control system is not working as it should for any number of reasons.

[Thomas coupling]

JJ, I've heard you do a very comprehensive TR brief, both from MG of the long moustache clan and from AH ;)

[[email protected]]

John Dixson - I am curious, the S92 autos should have required 'non-power pedal' as the Nr reduced on an EOL which would have been right pedal for a counter clockwise rotor.

With a power assisted 'flare recovery' I would expect power (left) pedal to be required but if the engines are shut down or at ground idle, the opposite should be true. I base this on many years of Gazelle (clockwise rotor) EOLs where lots of left(non-power) pedal was required during the Nr decay phase of an EOL.

[MightyGem]

there was a distinctive right yaw tendency, thus a necessity to add a bunch of left pedal to keep the nose aligned.
Transmission friction? (If I've got the blade rotation direction correct).

[RVDT]

For those with failing memory and in answer to the question lets roll back 5 years or so.

ATPL Theory Question (http://www.pprune.org/rotorheads/268244-atpl-theory-question.html)

On helicopters with a large fin (fenestron) be a bit careful about getting too slow. Life gets a little interesting if you get in a position when the thing stalls suddenly.

Those of you that are fortunate enough to have access to a sim should try an OGE hover and get the instructor to drop the TR off line. Prior to this it is your call as to what height you think you can recover to straight and level from.

Answers on the back of a postcard please.:eek:

[Thomas coupling]

Whatever happened to KISS:=

[[email protected]]

RVDT - at least 1000' - and that is to get enough control to make an EOL, not to recover to straight and level (because I don't think you can unless you have a big tail fin).

[JohnDixson]

MG, that was our understanding as well.

Re the large vertical fin/fenestron stall observation: the solution there is akin to the solution for keeping LTE off the table for a particular machine: design for a higher standard of available tail rotor thrust than the minimum FAA crosswind landing req'ts would lead to. Much higher. Hence as an example, the RAH-66 was designed to ( at mission weight ) fly level at 80-100 KIAS, do a snap 90 degree yaw ( either direction ) and hold that heading whilst flying sideways at the entry speed. Mr. Lappos demonstrated that capability to the Eurocopter Chief Pilot in the S-76 fan-in-fin demo machine at the Paris Airshow. In fact he went a bit further and went around to flying backward, all the while staying in formation with me ( flying a UH-60 ) doing the heliroute from Le Bourget back to Issy. We took video from the Hawk. I do not recall the last name of the Eurocopter Ch. Pilot, but he had just put on a spectacular demonstration in their Panther. Nick said that his guest looked at him, smiled, and said one word: " formidable ".

[Spunk]

TC`s way of explaining T/R failure and/or T/R malfunction reminds me of my old flight instructor who taught me the very same way. This plus the ongoing discussion about whether it makes more sense to do it one way or the other reminds me of the most important lesson some of us tend to forget:


maintain aircraft control
analyse situation
take proper action


If you can't slow it down to a speed suitable to make a running landing you better start looking for an alternative solution.

[Thomas coupling]

You can come again Spunk:E

[Spunk]

TC, I didn't expect you to be in your mid 70's by now:}

[[email protected]]

I think jokes like that show that TC is firmly stuck in his mid-teens, not mid-seventies - although his mid-teens probably were in the mid-70s:E

[212man]

I do not recall the last name of the Eurocopter Ch. Pilot

Probably Guy Dabadie

[JohnDixson]

No, it wasn't Guy. We had traded rides between Blackhawk and the prototype Super Puma at the 1991 Paris Airshow, and knew Guy from having done that. I believe the Ch. Pilot's first name was Etienne.

[Hot and Hi]

TC

you stated......

"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..... "

I'm very much with SAS on this one.

The following NTSB report should be mandatory reading.

http://dms.ntsb.gov/aviation/AccidentReports/ikbw1u55sdzxacmnw1vfn0y21/L02262013120000.pdf

It clearly shows that loss of T/R and TRGB (and half the vertical fin in this case) is still controllable!

JJ
This report can now be found here: https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20130211X13953&AKey=1&RType=Final&IType=LA

[Thomas coupling]

Hot and Hi.
Not sure what is what here. Did I state the part in quotes, or all of it?
The accident report shows a guy landing a helo with a damaged TR, what's the problem??

[Hot and Hi]

Hot and Hi.
Not sure what is what here. Did I state the part in quotes, or all of it?
The accident report shows a guy landing a helo with a damaged TR, what's the problem??
In his post from 2013 Jellycopter linked a NTSB accident report. The URL has since changed.