Agree - it would be interesting to see if that was even worse than having zero rotor thrust -Just wondering whether you can get negative pitch on the tail rotor at extreme pedal inputs?

Yes, of course you can get negative pitch! Any helicopter has to be able to turn in both directions, both at high power and in autorotation. As I stated a long way back in this thread, the tail rotor of a Puma helicopter, as one example, can go between +35 degrees to -17 degrees of pitch.

That is why a tail rotor control failure can be far worse than a tail rotor drive failure.

Perhaps there should be an explosive bolt or a quick release dog clutch on the tail drive shaft then...

None of you guys have ever tried this??? With training aircraft like the B47 we'd do lots of simulated tail-rotor failures from 500' by jamming in full right pedal, pointing out the pedal position was not going to move again until on the ground, and then slowing down until keel effect was lost and it snapped around. Let it go around a time or two (any more and the student would lose horizontal plane reference and bad things would happen) then roll throttle off to stop rotation, bottom collective to hang onto rpm, and pitch for some airspeed. From 500' you'd get about 10 mph per 100, so you'd have enough to flare and skid it on full-auto. It would veer a little to the right but not bad, at which point you'd point out to the student that the pedals hadn't moved. Most pedal goes to counter right yaw, in an auto it's almost full right. 212's weren't much different, but all this stuff used collective throttles.

Who knows whether a sim is going to replicate reality or not. You have pilots chatting with programmers on what they think will happen, nobody has hard data. And after all, the only failure we're concerned about now is an engine failure (but only if you have two), anything else is as JimL describes beyond the probability where it is worthy of much attention. On the S76 we tried jamming right pedal in a hover and having the copilot chopping throttles but it didn't work all that well with experienced trainers and there was low probability of success with a plebeian crew, hence the new world order of just lowering collective. We never did zero airspeed simulated by right pedal tail rotor failures at altitude either, too hard. So we always trained with enough speed for keel effect where the lowering of collective had some merit an gave time to bring the throttles back.

Discussions on takeoff profiles again, comparing the statistical exposure time. Jungle ops we started out verticalling some distance above the canopy before transitioning to forward flight, but gave up on that due to what felt like a lifetime of exposure for that extra hundred feet and the reality that you weren't going to find your way back down that keyhole anyway. Judgement call based on wide consideration of environmental factors, something we can't do anymore - each specific case has to be in the RFM.

In this one, I saw nothing wrong with Eric's departure profile. In the absence of any RFM or regulatory Class 1 guidance I would have flown the same. Raising the gear, I don't know if it had any significance or not. The S76 has a priority valve to deal with extra demand on hydraulics during gear retraction. The 139 deals with it by bolting on an extra hydraulic pump.

Yes, if full un commanded pedal is worse than loss of drive, perhaps a "drive disconnect" switch could be an option on the way down.

Malabo
Just to eliminate your query on landing gear vs hydraulics. The landing gear on an AW169 is moved by electric motored actuators. No hydraulics involved at all.

Another EASA AD issued with expansion -

so you want to add to the complexity of the TR system, adding weight and more potential points of failure, into something that already has a low probability of failure in any form, just to guard against an even more remote type of failure.............c'mon chopjock- get real!

If you have an un commanded stuck full pedal and you don't have easy throttle control or perhaps a bird strike on a tail blade causing severe vibration with tail rotor imminently about to fall off etc and high up in the cruise, how do you maximise your chances to land safely? Presumably this is when you do not want a turning tail rotor? Just for discussion ...

I love you choppy - you should have your own show. The last dozen posts have all been about real professionals tying themselves up in knots trying to win you over. :=
For someone who actually poles one of these contraptions, your technical and practical insight and knowledge of rotary wing operations is excrutiatingly lacking.
You've been like this for years on Pprune and I for one love watching proper pilots froth at the mouth trying to convince you .
Great show choppy - love it.

Just an ordinary SLF here and I wonder if anyone can explain why the grease containers are also required?

If things are wearing unexpectedly then it may leave traces in the lubricant that can be found by analysis.

unlike oil, where you can run a chip plug and filters to collect debris, grease holds onto those particles.
if a bearing has started to fail, those chips/flakes will continue to beat around the bearing. collecting the grease for inspection will help determine whats breaking down. whether its the bearing cage, the rollers themselves or the races. It may even determine that it's a foreign material that shouldnt be in there to begin with.

Or even if there was no grease or the wrong grease....or other kinds of contamination.

The scientific name is 'tribology'. It studies lubricants, and effectiveness. If there is a failure of a lubricant, the examination of the fluid(grease) and contaminants may point to a specific failure mode.

Thanks to all that answered my query.

Further info on AAIB website released today at 1400

AAIB Special Bulletin here ; https://assets.publishing.service.go...018_G-VSKP.pdf .

Already reported on the BBC .

If I read this right, the yaw went full travel to the right. Since the limit of right yaw is usually set for the ability to yaw right in autorotation, it would probably mean it was still yawing right even in auto with engines shutdown (normal recovery actions for a tail rotor drive fail in the high hover). Thoughts?

My reading of this is that the duplex bearing was overheating, binding and then seizing, causing rotation of the control shaft which undid the castellated nut and removed the pitch control of the TR allowing the servo to go to the 'full right pedal' position.

I don't care how many times you practice in the sim, that one is pretty unrecoverable from that height and speed.

The question is - is this an assembly issue (bearing end nut over torqued) or a maintenance issue? The aircraft was quite young.......

Looks like it should have been a left hand thread, although that might just have postponed disaster.

​​​​​​From the Special Update "The observed condition of the duplex bearing and the increased torque load on the castellated nut that remained on the spider end of the shaft is consistent with rotation of the tail rotor actuator control shaft."

I read that to mean that they think the increased torque on that nut was caused by the rotation of the control shaft - for the reasons you gave. Maintenance issue or defective bearing ??

What sober and horrible reading the S2 bulletin is - if only they had a few more minutes to transition into forward flight.... RIP!! Horrible sad loss for all involved

The report seems to be a little confusingly worded. I agree, the duplex bearing at the spider end seems to have failed and then spun the shaft. Result, the castleated nut at the input end of the shaft has spun, with the shaft, against the pin-carrier, until the nut friction-welded to the pin-carrier and then the split pin was sheared and the nut unscrewed and disconnected the input. That is the way I read it. There does not seem to be any prejudice against the assembly in that. The duplex bearing failure looks to be the culprit from my interpretation. :sad:

OAP

So, if my understanding is correct, it appears that the duplex bearing seized solid, causing what should have been a non-rotating component to become physically welded and locked to the rotating part of the shaft, so that it also rotated, unscrewed then threw off the locking nut, overwhelming its thread locking methods. Effectively, the pilots yaw pedals were no longer connected to the tail rotor pitch change mechanism and the servo went to full travel, pushing the tail rotor to a position where it would give maximum right yaw.

Unfortunately, JetScream32, transitioning to forward flight would not have made any difference. The aircraft would still have yawed hard right because the tail rotor made it do so as hard as it physically could. Shutting down both engines and entering autorotation would not have stopped it from yawing right. As I wrote earlier in this thread, the closest equivalent failure on a twin engined aeroplane would have been a full right rudder deflection with asymmetric thrust pushing it in the same direction, making it worse. If the "outer" engine of said aeroplane were shut down to remove the asymmetric thrust, the rudder was still at full deflection.

Irrecoverable and horrible to contemplate.

A balance-spring bias on the servo unit towards left pedal( power input) would have been useful...

Low Blade Angle
 

And that is why blade designers strive to get the overall c.g. of the blade forward of quarter chord or the center of lift, such that if one loses pitch link input on one or all of the blades they will drive themselves to zero (or neutral) pitch. If the c.g. is aft of quarter chord the blade drives itself to maximum blade angle which is a divergent condition and does not help a disastrous situation. If other control failures are present such that the blade is driven to some adverse pitch angle, the c.g. effect may be over-ridden. Otter

Otter - in this case the TR servo was allowed to drive the pitch change mechanism all the way to full travel which would easily oppose any aerodynamic backloads..

Sycamore - the Lynx had such a spring bias unit but it wouldn't have helped as it would, if designed in the same way, have gone past top dead centre and pushed to the right instead.