Helicopter down outside Leicester City Football Club
AAIB Special Bulletin here ; https://assets.publishing.service.go...018_G-VSKP.pdf .
Already reported on the BBC .
Already reported on the BBC .
AAIB Special Bulletin here ; https://assets.publishing.service.go...018_G-VSKP.pdf .
Already reported on the BBC .
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?
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“Sufficient force and torque had been applied to the castellated nut on the actuator end of the control shaft to friction weld it to the pin carrier and to shear the installed split pin. 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. Whilst the shaft was rotating and a yaw control input was applied, the shaft “unscrewed” from the nut, disconnecting the shaft from the actuator lever mechanism, and causing the nut to become welded to the pin carrier.”
So... what provided the force and torque which welded the actuator-end nut and broke split pin? Failure of the duplex bearing locking the inner shaft to the outer shaft?
So... what provided the force and torque which welded the actuator-end nut and broke split pin? Failure of the duplex bearing locking the inner shaft to the outer shaft?
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.......
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.......
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GHH,
If you look back through the message thread there are a few pics of the arrangement. The locking in the pic implies a RH threaded nut and the directions of rotation would imply your theory could be correct.
Normally the servo piston is keyed in some way to stop this happening but you can't tell from the IPC drawing. Having a split/cotter pin and locking kind of implies that this is the only locking. If it was keyed you wouldn't go to that length.
Similar things have happened before but not through design issues.
Once the feedback link is disconnected the servo would invariably motor to the end of full travel and stay there.
Pic at #682 and IPC at #630
If you look back through the message thread there are a few pics of the arrangement. The locking in the pic implies a RH threaded nut and the directions of rotation would imply your theory could be correct.
Normally the servo piston is keyed in some way to stop this happening but you can't tell from the IPC drawing. Having a split/cotter pin and locking kind of implies that this is the only locking. If it was keyed you wouldn't go to that length.
Similar things have happened before but not through design issues.
Once the feedback link is disconnected the servo would invariably motor to the end of full travel and stay there.
Pic at #682 and IPC at #630
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The rent-an-idiot David Learmount has just been on the BBC News saying that the report reveals that the control cable between the pedals and the tail rotor had snapped!
Really?
Why would somebody agree to an interview about a very short bulletin without taking five minutes to read and understand it?
Looking at the photos and CAT scans of the bearing and carrier, I tend to agree that the bearing failed for some reason and the actuator linkage became disconnected as a result of this failure.
Really?
Why would somebody agree to an interview about a very short bulletin without taking five minutes to read and understand it?
Looking at the photos and CAT scans of the bearing and carrier, I tend to agree that the bearing failed for some reason and the actuator linkage became disconnected as a result of this failure.
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Originally Posted by [email protected]
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.......
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.......
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 ??
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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
Originally Posted by [email protected]
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.......
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.......
OAP
Last edited by Onceapilot; 6th Dec 2018 at 15:09. Reason: Castleated nut spun against the pin-carrier
Avoid imitations
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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.
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.
Last edited by ShyTorque; 6th Dec 2018 at 16:55. Reason: To hopefully clarify.
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
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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.
OAP - the nut at the duplex bearing end was clearly overtightened - hence my comment about assembly problems. It may be that the TR pitch change mechanism has to be taken apart and greased regularly which ,might make the over-tightening a servicing error instead.
If the nut is over-tightened and impinges on the duplex bearing, you have a probable cause for the excess heat and seizing of the bearing.
If the nut is over-tightened and impinges on the duplex bearing, you have a probable cause for the excess heat and seizing of the bearing.