Tail rotor drive failure video
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Crab,
I am speculating (and that is all) that the failure is an instability in the tail control system, not a stuck control, that is why the TR drive was possibly damaged enough to separate. The failure mode is a divergent oscillation of the yaw controls that leads to extremely high left pedal, then right then left at perhaps 2 Hz. Note the wiggles and left yaw just before the fun starts. Do not think of this mode as a stuck anything, it is a divergence that ends up failing the TR drive.
Regarding engine controls, all S-58's, recip and turbine, have throttles on the collective.
Regarding Blender pilot's impossibly large type comments, I find that the larger the type, the smaller the thoughts. It is wonderful to see how pilots might redesign helos to solve all the rare and almost impossible problems and not even think about the ones that get us. Should blender pilot ever actually read any accident statistics, I would hope he would use even larger type to ask for CFIT protection, or IFR capability and instruments for all helos, or fixes for all the things that actually get helos. But that would be a stretch, wouldn't it?
Asking for twist grip throttles to solve a TR problem is like donating $1,000,000 to the Cancer society not for a cure, but for fancy caskets. Sort of forgets that we might actually just fix the baseline problem, huh?
I am speculating (and that is all) that the failure is an instability in the tail control system, not a stuck control, that is why the TR drive was possibly damaged enough to separate. The failure mode is a divergent oscillation of the yaw controls that leads to extremely high left pedal, then right then left at perhaps 2 Hz. Note the wiggles and left yaw just before the fun starts. Do not think of this mode as a stuck anything, it is a divergence that ends up failing the TR drive.
Regarding engine controls, all S-58's, recip and turbine, have throttles on the collective.
Regarding Blender pilot's impossibly large type comments, I find that the larger the type, the smaller the thoughts. It is wonderful to see how pilots might redesign helos to solve all the rare and almost impossible problems and not even think about the ones that get us. Should blender pilot ever actually read any accident statistics, I would hope he would use even larger type to ask for CFIT protection, or IFR capability and instruments for all helos, or fixes for all the things that actually get helos. But that would be a stretch, wouldn't it?
Asking for twist grip throttles to solve a TR problem is like donating $1,000,000 to the Cancer society not for a cure, but for fancy caskets. Sort of forgets that we might actually just fix the baseline problem, huh?
Nick I have difficulty understanding how a basic cable operated TR control system can suffer a fault such as you suggest. The system only comprises a quadrant to change the yaw pedal input from rod to cable at the front end and a similar arrangement at the end of the tail to convert it back to rod and route it up the pylon to the pitch change lever on the TR gearbox. From there it relies on a magill bearing to transmit the pitch change to the TR spider. The only divergent mode I can imagine is a sort of 'bounce' in the TR cables which would mean the tensioner has failed and in many years of the brit mil operating the Wessex, I have never heard of such a failure mode. It would take some fairly major and very rapid pitch change reversals ( more than an autopilot with limited authority could manage) to trash a fairly beefy TR driveshaft like that on the S58.
I don't know what caused the initial left yaw before the failure (turbulence, control input, power change, autopilot malfunction) but a divergent 2Hz oscillation would surely have produced more than one wiggle before drive train failure occurred.
It matters not anyway as the pilot did a very good job, my friend who had the TR failure over a lake in Wales had the same difficult job assessing a suitable height to chop the throttles and ended up going in backwards with low Nr.
I don't know what caused the initial left yaw before the failure (turbulence, control input, power change, autopilot malfunction) but a divergent 2Hz oscillation would surely have produced more than one wiggle before drive train failure occurred.
It matters not anyway as the pilot did a very good job, my friend who had the TR failure over a lake in Wales had the same difficult job assessing a suitable height to chop the throttles and ended up going in backwards with low Nr.
Thread Starter
Originally Posted by [email protected]
It matters not anyway as the pilot did a very good job, my friend who had the TR failure over a lake in Wales had the same difficult job assessing a suitable height to chop the throttles and ended up going in backwards with low Nr.
What amazes me is the ability some have to view a video and not only tell me what the pilot in the cockpit did....but also tell me what the guy was thinking and planning to do.
That is pretty darn amazing!
That is pretty darn amazing!
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SASless, I accept the jibe, but he still did a great job, even if he was thinking of you when he did it!
Crab,
You must know that the TR servo is not at the TR but on the upper deck, and the force from the TR propellor moment is conveyed along the TR cables to the upper deck. The frequency of the cable tension response is set by cable tension, if the tension is very low, the natural frequency of the cable can tune down until it matches the natural frequency of the TR collective pitch and excite the "umbrella" mode of the rotor, sort of like collective bounce in a H-1. Since the TR stops are on the upper deck, the TR pitch can go to very big numbers, the ride is short, exciting and often leads to a TR failure. Cable tension is a nice thing. All this is speculation, of course, but I cannot explain the yaw twitches and obvious left yaw just prior to the TR failure any other way. I could be very wrong, it has happened before!!
Crab,
You must know that the TR servo is not at the TR but on the upper deck, and the force from the TR propellor moment is conveyed along the TR cables to the upper deck. The frequency of the cable tension response is set by cable tension, if the tension is very low, the natural frequency of the cable can tune down until it matches the natural frequency of the TR collective pitch and excite the "umbrella" mode of the rotor, sort of like collective bounce in a H-1. Since the TR stops are on the upper deck, the TR pitch can go to very big numbers, the ride is short, exciting and often leads to a TR failure. Cable tension is a nice thing. All this is speculation, of course, but I cannot explain the yaw twitches and obvious left yaw just prior to the TR failure any other way. I could be very wrong, it has happened before!!
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Why didn't he enter an auto once he was above his point of landing instead of descending then trying to hover auto? Wouldn't he of had enough rotor RPM to use? 
Nick,Perhaps you can discuss "Tail Rotor Buzz" as used to apply to the 58T. Could that have played a role in this....I would think it is different to the cable tension issue you note already.
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Perhaps the drive system disengaged and then re-engaged a few times before it ultimately failed. That would explain yaw both left and right, and the endstate of a stopped tail rotor.
Any type experts that can add comment?
Any type experts that can add comment?
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S58T Auto
The aircraft suffered a loss of tail rotor drive due to a failed tail driveshaft coupling. The S58 uses two opposing jaws with bonded rubber separating then the teeth letting go, this is way before the Thomas Coupling, simple and effective, but you had to kep an eye on the rubber segments. Pilot did a heck of a job, but barely missed being impaled on a pole from the chain link fence. 
Aser - yes that's him - he and another good friend survived, sadly some cadets who were on air experience did not.
Toolguy, thanks for that - it is as I suspected, the disconnect coupling needs to be well maintained - even using the wrong kind of grease can cause the spring loading to stick and prevent proper location of the drive teeth. The Wessex drive disconnected and the TR started to slow down, when the drive bit again the shock loading twisted the TR driveshaft like a coke-can and the video posted by aser was the end result.
Toolguy, thanks for that - it is as I suspected, the disconnect coupling needs to be well maintained - even using the wrong kind of grease can cause the spring loading to stick and prevent proper location of the drive teeth. The Wessex drive disconnected and the TR started to slow down, when the drive bit again the shock loading twisted the TR driveshaft like a coke-can and the video posted by aser was the end result.
Mr. Lappos,
I too am wondering about the option of flying away from the TR failure.
Earlier you stated "...he saw that the rotation rate was not unmanageable."
What made it so? Is the lowering of collective, and therefore torque, the key to keeping the yaw rate manageable? I would have expected a helicopter in a high power required HOGE to start yawing very rapidly and almost immediatley become unmanageable.
In cruise flight I understand that the fuselage will streamline with a degree of sidelsip and roll. A run-on landing and/or autorotation (depending on type) are then your two available options. Somewhere between cruise and hover my little mind is telling me that I might be able to reduce collective and spiral to attain the streamlining effect, altitude permitting.
Would the rule-of-thumb be that one would initiate a descent (with rotation)and a hovering auto once the helicopter has begun swapping ends? Or is there any merit in attempting to sweep the cyclic while trying to attain enough forward airspeed for it to streamline?
My RFM only states to execute a hovering auto if TR thrust is lost while hovering.
Thank you for your time. Your willingness to share your knowledge is much appreciated.
Todd
I too am wondering about the option of flying away from the TR failure.
Earlier you stated "...he saw that the rotation rate was not unmanageable."
What made it so? Is the lowering of collective, and therefore torque, the key to keeping the yaw rate manageable? I would have expected a helicopter in a high power required HOGE to start yawing very rapidly and almost immediatley become unmanageable.
In cruise flight I understand that the fuselage will streamline with a degree of sidelsip and roll. A run-on landing and/or autorotation (depending on type) are then your two available options. Somewhere between cruise and hover my little mind is telling me that I might be able to reduce collective and spiral to attain the streamlining effect, altitude permitting.
Would the rule-of-thumb be that one would initiate a descent (with rotation)and a hovering auto once the helicopter has begun swapping ends? Or is there any merit in attempting to sweep the cyclic while trying to attain enough forward airspeed for it to streamline?
My RFM only states to execute a hovering auto if TR thrust is lost while hovering.
Thank you for your time. Your willingness to share your knowledge is much appreciated.
Todd
Tweedles you have a lot of height, around 1000' feet or so you might stand a chance of getting into forward flight but if you watch the video of the yellow wessex, he tries this but cannot gain sufficient airspeed and he was doing around 60 kts at the time of the failure.
I do not believe there is a rule of thumb for this manoeuvre but hovering at heights up to about 500' it may be worth lowering the lever sufficiently to get a manageable rate of descent and a manageable rate of rotation but there are no guarantees and it will vary from type to type. Having a slab sided tail boom like the S58 will help as it creates a lot of drag and reduces the rate of rotation a bit. You have so little control in this situation you can only hope you get to a position where you can chop the throttles and carry out a hover EOL.
I do not believe there is a rule of thumb for this manoeuvre but hovering at heights up to about 500' it may be worth lowering the lever sufficiently to get a manageable rate of descent and a manageable rate of rotation but there are no guarantees and it will vary from type to type. Having a slab sided tail boom like the S58 will help as it creates a lot of drag and reduces the rate of rotation a bit. You have so little control in this situation you can only hope you get to a position where you can chop the throttles and carry out a hover EOL.
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I am reminded of this thread.
Perhaps a different failure (pitch rather than power), but still not the reaction outlined in the manual... And he avoided swimming!
Perhaps a different failure (pitch rather than power), but still not the reaction outlined in the manual... And he avoided swimming!
Thread Starter
I can't remember exactly but I was having fun in a 412 sim in hover maybe 2000' agl when the tail rotor departed (funny instructor)... everything went crazy (helo spining like mad) I put 30º nose down, lowered collective and I got airspeed to streamline the helo, then I cut engines and safe landing!.
level-d sim and so... but a good computer game anyway, I wouldn't try that again in real life in a hover below at least 2000' agl.
I put 30º nose down, lowered collective and I got airspeed to streamline the helo, then I cut engines and safe landing!.level-d sim and so... but a good computer game anyway, I wouldn't try that again in real life in a hover below at least 2000' agl.
Remember this one.http://www.pprune.org/forums/showthr...ma+crash+video I agree with Blender... overhead throttles should be banned.... it's like having the ignition switch of your car in the trunk..just stupid. 
But you could be smart and fly Bell
But you could be smart and fly Bell
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Great video, and some interesting comments. Re the Wessex that went in, the captain of that aircraft was an instructor of mine at shawbury and had some interesting thoughts on tail rotor drive failures at low level. As for me I'd love some more info on the S61 test pilot and how he coped with the failure from the 100ft hvr so I can try it out in the sim. With ref to the sim, they changed the model for this particular failure with the c of g making a much greater impact than before and after looking at the video it would seem that these changes have made the sim more realistic. As for walking away from the tailrotor failure in the S61 sim, the lowest I could be in transit and then have an 'acceptable' ditching was 500', hence the fact that I now fly at a min of 500' unless required to be lower! fly safe :@)
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Overhead Throttles
One benefit of overhead throttles in an emergency is crew coordination. Nothing worse than calling for rollback on no.1 one then getting no. 2 rolled back by the CP. Had that happen in the 430 sim, where you have different collective throttle orientation between pilot and CP sticks.
Thanks for the info crab, Aser and ISB.
HP, I had forgotten about that thread. It was worth reading again.
Check out this thread for another TR incident: http://www.pprune.org/forums/showthread.php?t=247997
HP, I had forgotten about that thread. It was worth reading again.
Check out this thread for another TR incident: http://www.pprune.org/forums/showthread.php?t=247997
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