Tail Rotor Problems

[jellycopter]

Stuck pedal - low power scenario

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]

Thanks JJ , i will def come and do that soon .
With ref to examining etc i dont think you have a choice .....you cannot let the lunatics run the asylum 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 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]

Fiddling with the RRPM.....

........ 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.

[topendtorque]

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]]

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

[Geoffersincornwall]

Crab

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.

[Hughes500]

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]

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

Hughes500 what type you examining on?

[Hughes500]

Hm

500's, 300's and 341/2

[jellycopter]

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]

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]

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]

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]

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

[Thomas coupling]

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.

[Thomas coupling]

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

[Dennis Kenyon]

T/R problems

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]

DK: Thanks very much for your contribution. Worth its weight in gold
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]

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 hangar. 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]

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'