Go Back  PPRuNe Forums > Aircrew Forums > Rotorheads
Reload this Page >

Tailrotor failure - is it always unrecoverable ?

Rotorheads A haven for helicopter professionals to discuss the things that affect them

Tailrotor failure - is it always unrecoverable ?

Old 20th Sep 2017, 04:33
  #1 (permalink)  
Thread Starter
 
Join Date: Jan 2011
Location: Arizona
Posts: 542
Tailrotor failure - is it always unrecoverable ?

Newbie question here, so please forgive my ignorance.

All the tailrotor failures I have seen training videos of, invariably seem to show a high sink rate often accompanied by a spiralling nose down attitude until impact.

Is there a best practice recommendation for cyclic and collective use in such a situation, or are you really just along for the ride at that point?
Fonsini is offline  
Old 20th Sep 2017, 05:14
  #2 (permalink)  
 
Join Date: Jun 2003
Location: PUDBY
Posts: 690
I'm sure you'll get plenty of answers from the professionals here, but even as low-time heli pilot I know it's not unrecoverable - though when it happens IRL it may be a bit of a shock, like any emergency.

You can enter autorotation, supposing of course you have somewhere to go. Or in the R44 (yes, I know), and I guess most other types, you can fly in a peculiar feeling slip at about 70 knots until you can find somewhere safe to autorotate to.

This is when your instructor has spent half an hour prepping you for the exercise, then counts down before putting the pedals in a more-or-less neutral position. How it works when the TR suddenly parts company with the airframe, or the drive snaps, is another matter. By the time you've realised what's happening, instinctively tried to counter it with pedal, realised it's getting worse, had the "aha! TR failure" light bulb go on, and dropped the collective... you'll be going round quite fast, so your reactions may not be quite that good.
n5296s is offline  
Old 20th Sep 2017, 05:49
  #3 (permalink)  
 
Join Date: Apr 2010
Location: Kansas
Age: 33
Posts: 89
A loss of tail rotor thrust is all about the balance of torque and anti torque. In normal flight we have some amount of torque applied to the main rotor, and we supply the anti torque from a combo of tail rotor thrust, and the stability that comes from the vertical stabilizer. When the tail rotor fails, we must either have enough speed to allow that vertical fin to offset the torque, or we can reduce the torque to the main rotor so that the airspeed can help offset. The absolute fastest way to reduce torque is to lower the collective. By reducing torque this way, we leave the engine fully functional and available, we just remove the torque by reducing the rotor drag. We are then free to increase speed to a point at which that vertical fin can offset torque. then the pilot increases main rotor torque and flies away to a safe run on/ forced landing. If this balance is lost, the aircraft will spin... at that point, it can quickly become a ride along.
army_av8r is offline  
Old 20th Sep 2017, 06:35
  #4 (permalink)  
 
Join Date: Jun 2016
Location: Brantisvogan
Posts: 513
It largely depends on what fails and how and where.
If a driveshaft fails, or there's a gearbox failure, you could potentially also lose part of the tail. In this instance the change of CoG could make the aircraft unrecoverable.
There's theory and there's reality.
It's always better to not stick the tail somewhere it doesn't belong, since that seems to be a leading cause of "loss of thrust".
Bell_ringer is online now  
Old 20th Sep 2017, 07:34
  #5 (permalink)  
 
Join Date: Dec 2006
Location: UK and MALTA
Age: 56
Posts: 1,202
Army AV8R I think your explanation is spot on and very clear. Might I add some value to your post by saying, the amount of "Thrust" (antitorque) available at a specified IAS by the Fin/Tailboom varies considerably as a result of the design. I.e. fenestron fins tend to provide a lot of thrust.

And of course, the lower the IAS when the event happens the more height needed to achieve an IAS whereupon the Fin thrust can be exploited.

Sadly we tend to be at Low IAS during approaches and departures where height is often insufficient to effect recovery.

This post and AV8Rs relate only to a loss of tail rotor thrust. (All the physical components remain on the Helicopter). Flight Manual procedures generally only provide guidance for this type of failure.

As Bell Ringer points out, if a lump of mass departs the airframe at the end of the tailboom the effect on the CG could cause departure from controlled flight. The TR and its GBX are a long way from the rotor centroid so not a lot of mass needs to depart to create this loss of control. For this reason I am not aware of any flight manual that provides a solution for such a condition.

Hope this helps the original poster.

Last edited by DOUBLE BOGEY; 20th Sep 2017 at 07:44.
DOUBLE BOGEY is online now  
Old 20th Sep 2017, 07:49
  #6 (permalink)  
 
Join Date: Feb 2015
Location: UK
Posts: 197
http://www.pprune.org/military-aviat...ssue-19-a.html

FYI
Rotate too late is offline  
Old 20th Sep 2017, 08:27
  #7 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 7,485
Fonsini - the immediate survivability depends a lot on your height and speed at the time of failure - a low hover or highspeed, high altitude flight should, in the initial stages, be very survivable. Having said that, it depends on whether you identify the nature of the failure quickly and correctly, especially in the low hover.

Altitude and airspeed are your friends when dealing with a TR failure and you may be able to establish a power/airspeed/sideslip condition to allow you to transit to a safe area for EOL or, on some types, a fast running landing.

The EOL may in itself be an interesting one since you have no yaw control and, as you decay the Nr by pulling pitch at the bottom, the aircraft will want to yaw. The friction of the skids on the ground will help keep you pointing in the right direction and a crosswind can make things easier still.

Low speed, lowish altitude failures are usually the worst - if you can gain speed without spearing yourself into the ground then it may help you regain directional control. The worst condition would be a still air hover at 100 -500'.
crab@SAAvn.co.uk is offline  
Old 20th Sep 2017, 08:39
  #8 (permalink)  
 
Join Date: Jun 2010
Location: Germany
Posts: 87
And besides "loss of tail" (with loss of CoG) and "loss of tail rotor thrust", you can also experience "loss of tail rotor control". In this third case, you usually will be able to fly along with the help of the vertical stabilizer and then make a running landing on any surface. This can easily be trained. Just get off the pedals and enjoy.


It gets tricky when it comes to "stuck pedal" (either low power stuck or high power stuck).


Thracian
Thracian is offline  
Old 20th Sep 2017, 11:54
  #9 (permalink)  
 
Join Date: Oct 2006
Location: UK
Posts: 4,649
Another problem with a drive failure in forward flight is that the tail rotor will go into autorotation. This could lead to flapback and large pro torque moment. The only way you can overcome this is to enter autorotation.

With old helicopters like the S55 and S58 you could get away with finding a power/speed combination to either a fast run on or an autorotation to the ground. Modern helicopters, especially those with a stabilisation system, don't have the aerodynamic qualities to continue flight so you are committed to landing fairly rapidly.
Fareastdriver is offline  
Old 20th Sep 2017, 12:59
  #10 (permalink)  
 
Join Date: Aug 2000
Location: UK
Posts: 3,666
Probably a pilots worse nightmare.
Probably the least practiced malfunction (for obvious reasons).
Probably the least understood malfunction.
Probably a helicopter's achilles heel (unless you're NOTAR).
Probably not a lager, then................

http://www.pprune.org/rotorheads/422...+rotor+failure

Enjoy.
Thomas coupling is offline  
Old 20th Sep 2017, 13:14
  #11 (permalink)  
 
Join Date: Aug 2000
Posts: 1,630
TC

Notar has a TR but it is hidden in the tailcone ! The shaft and straps can fail !
Hughes500 is offline  
Old 20th Sep 2017, 13:37
  #12 (permalink)  
 
Join Date: Sep 2016
Location: UK
Posts: 88
Originally Posted by Thomas coupling View Post
Probably the least practiced malfunction (for obvious reasons).
I didn't realise this while practising it during training, until considering that it's always practised at height, with sufficient forward airspeed.

The discussion the other week about the "pumping" of the collective if experiencing TR failure in the hover or taxiing was a good clue as to how little the typical PPL training covers this problem.
WillyPete is offline  
Old 20th Sep 2017, 13:41
  #13 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 7,485
In the hover or hover taxy, if you can recognise it, reduce the yaw (lower the lever and or close the throttle - twist grips have a distinct advantage here) and cushion the touchdown then you should survive it.

I didn't realise this while practising it during training, until considering that it's always practised at height, with sufficient forward airspeed.
that's because it can be very dangerous so best done in a simulator if you can get in one.
crab@SAAvn.co.uk is offline  
Old 20th Sep 2017, 14:08
  #14 (permalink)  
 
Join Date: Jun 2013
Location: Thaïland
Age: 62
Posts: 183
First, you have two know about the main failure about Tail rotor :
1/ lost of control = pedal, cable, anything change the pitch of the blade but blade runing always
2/lost of part or totally Tail rotor : driving shaft, blade, tail gear box =blade no more runing properly.
Two cases totally different to manage, but you have to identify what happen first..
in case 1, you only lose the "pedal" control of your tail rotor, that'is the simpliest case. Take a look on your T/r if you can to understand whathappen, then, take your time, choice a big and flat safe place to land. On most of the helicopter manual, they say "Perform autorotation". It's trus, but you have another way : make a very flat approach, accept the yaw and keep the nose be to the right or left (depend the rotor clockwise or not). when you are nearly to the touch down low rate of descent low speed, apply slowly the pitch to put the nose "on line" with the frame and slide slowly until stop. Then put the pitch down, shut down and go to the Bar drink a Beer....
In the case No. 2, it's a little more creepy. i got a training by a pilot who's had experiment IRL .. on after the training, it and i'm not sure to do the same thing if happen really to me...
So, when you lose part or totally of the blade the CG change and you lean and you have the nose to go down . All of that happen fast...very fast.
At this time, reduce the speed if needed. Remind that to go straight and maintain altitude, you have to admit the bank and the nose down Take your time ( not too long, you don't know if the T/R maybe destroy the rest of your tail...) to choice a really good place to perform an autorotation...And do it......BUT, you have to know something : All your instrument byair managed are totally false ! Speed/rate of descent/ altitude..The bank and the nose down change the wind in the static plug and dynamic.. So if you have a GPS or another electronic system, trust him for the speed...If not : trust yourself for manage the speed, your rate of descent and perform your autorotation.....Good luck....
I never see this procedure on any manual and nobody train me on , except a old Jungle pilot who's experiment that the sling rope and the hook bump and destroy his Tail rotor. Thanks to him
Sure, to explain the both procedure i take the best case : some speed, and not to close to the ground...
If no speed, try to gain some speed to put some air on your fin and be little be "online" to perform case 1 or 2.
If close to the ground, look like final approach, ( i had experiment it IRL ) PITCH DOWN ! if not you make the weathercock very fast.. It's better to crash on your skids than turning until not controlled crash...
BOBAKAT is offline  
Old 20th Sep 2017, 14:26
  #15 (permalink)  
 
Join Date: May 2002
Location: "Deplorable but happy as a drunken Monkey!
Age: 71
Posts: 16,340
Making "absolute" statements. Where generalized comments are more appropriate would be a better approach to this.

Example...a Gulf of Mexico event where a Bell 412 had a total upset at night due to a tail rotor failure in cruise flight.....the Crew regained control and successfully landed the aircraft.

It is not the presence of SAS that determines the outcome but rather the ability to match weather vaning effect to main rotor torque by reducing Collective and reaching an airspeed/Pwer setting that allows that.

The biggest hindrance probably is far too many helicopter pilots just do not understand the interaction of the flight controls and rpm control and the affect they can have "Torque" applied to the airframe.

As there are many different kinds of tail rotor failure....in all kinds of situations....in all kinds helicopters....there can be no single response.

The key training issue is to explore control reactions in controlling "Torque" without use of the Tail Rotor.

Also....one has to grasp the fact "Green Arc's" are for NORMAL OPERATIONSand there is nothing Normal about dealing with a tail rotor failure.

One has to be able to instinctively react in some situations and then be able to follow up with learned concepts.....and doing whatever it takes to control the aircraft.

I have had one tail rotor failure at a hover....we landed with no damage.

I have had one Stuck Pedal situation in flight....and landed safely.

I attribute both to the quality of training I received in the US Army.

We did lots of practice in Huey's....and got to see the effect of the controls including throttle. I used those same techniques in other aircraft I flew later.






Originally Posted by Fareastdriver View Post
Another problem with a drive failure in forward flight is that the tail rotor will go into autorotation. This could lead to flapback and large pro torque moment. The only way you can overcome this is to enter autorotation.

With old helicopters like the S55 and S58 you could get away with finding a power/speed combination to either a fast run on or an autorotation to the ground. Modern helicopters, especially those with a stabilisation system, don't have the aerodynamic qualities to continue flight so you are committed to landing fairly rapidly.
SASless is offline  
Old 20th Sep 2017, 15:07
  #16 (permalink)  
Thread Starter
 
Join Date: Jan 2011
Location: Arizona
Posts: 542
I have some key takeaways from your responses that I had not considered. I suspect that many pilots instinctively pull on more collective during a TR failure, exacerbating the spin and loss of control. I fully understand why perhaps only the military routinely accept the risk of training their pilots to deal with such an emergency, as it is far more likely to happen when people are shooting at your aircraft.

Gold standard information as ever - thanks guys.
Fonsini is offline  
Old 20th Sep 2017, 16:32
  #17 (permalink)  
 
Join Date: Jun 2013
Location: Thaïland
Age: 62
Posts: 183
I agree with SASless, I make my training in the French Army and we perform all kinds of T/R failure....Training is the key
BOBAKAT is offline  
Old 20th Sep 2017, 16:33
  #18 (permalink)  
 
Join Date: May 2002
Location: Wanaka, NZ
Posts: 2,059
When teaching TR malfunctions just keep it simple. There are 2 types. Loss of drive. Loss of control (stuck pedal, broken cable, jam before or after mixing unit). Easy to diagnose which is which. Loss of drive gets your attention immediately. Loss of control you might not notice at all initially. Loss of drive = lower collective, autorotation, shut down engine/s before the landing flare. Loss of TR control, the technique is the same whatever is causing the problem. Fly at 60-70 kts adjust collective until the ball is balanced and note the power (this will be very close to the power at touchdown). If ball centered with high power, lucky, expect slow speed run-on or hover landing. If low power, bad luck, expect a faster run-on landing. With nose left of centre and aircraft aligned with runway reduce speed to 45 kts just before touchdown but don't land. Fly level 5-10', slight flare to slow down, as the aircraft slows down it will descend, but don't let it land. Slowly increase power whilst flying level and continuing to slow down (the nose will come right as you increase power). When straight, keep that attitude and power setting and the helicopter will continue to slow down and land itself. Do not let the nose go right of centre (for anti-clockwise main rotor, eg Bell). Keep it straight during any ground-run with throttle and some opposite cyclic. That's it, easy. This technique will work for most helicopters.

As long as the TRGB or components haven't departed the scene and put you out of CofG limits, and you have a suitable landing site, you should be able to land without a scratch. I've practiced this many hundreds of times. The key to stuffing it up is letting the nose go right of centre when you're close to the ground. When this happens students tend to react by increasing power, and when you do that it very quickly becomes unrecoverable and you'll turn it into a mangled mess.

Last edited by gulliBell; 20th Sep 2017 at 16:55.
gulliBell is online now  
Old 20th Sep 2017, 17:24
  #19 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 7,485
Gullibell - you should also stress how helpful a crosswind from the (left in your case) is as it helps keep the nose from swinging right for longer and gives a slower touchdown speed. The bigger the fin/vertical stabilisers, the more effective this becomes.
crab@SAAvn.co.uk is offline  
Old 20th Sep 2017, 17:32
  #20 (permalink)  
 
Join Date: Mar 2007
Location: Hobe Sound, Florida
Posts: 699
F.E.D. re your post #9. I can add a historical data point.

During initial commercial S-61 FAA testing at the forward CG limit, the test crews had trouble making the required control delay time after a forward AFCS pitch channel hardover at max power, min speeed ( 40KIAS as I recall ).

The Ch Exp. Pilot got involved and went out to get the data point. Set up the condition right after takeoff, hardover was introduced and nose went down.....and down. Well, the aft stick input was finally made ( with authority ), easily meeting the required delay time, but the tail drive shaft suffered an intervention by a main blade, and an autorotation into an open field with 2 inches of snow resulted. A picture exists showing a perfectly straight line thru the snow made by the tail wheel. Instrumentation indicated a tail rotor Nr that could only be explained by autorotation.

So, the tail rotor autorotative state may not be all bad. The copilot for the flight was Dmitri ( Jimmy ) Viner for the historians out there.
JohnDixson is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.