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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

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Old 6th Dec 2018, 15:43
  #941 (permalink)  
 
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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
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Old 6th Dec 2018, 16:04
  #942 (permalink)  
 
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Originally Posted by [email protected] View Post
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.......
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.

OAP

Last edited by Onceapilot; 6th Dec 2018 at 16:09. Reason: Castleated nut spun against the pin-carrier
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Old 6th Dec 2018, 16:06
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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.

Last edited by ShyTorque; 6th Dec 2018 at 17:55. Reason: To hopefully clarify.
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Old 6th Dec 2018, 16:17
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A balance-spring bias on the servo unit towards left pedal( power input) would have been useful...
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Old 6th Dec 2018, 16:48
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I despair with Learmonts comments.
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Old 6th Dec 2018, 17:10
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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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Old 6th Dec 2018, 17:15
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Originally Posted by Echo Romeo View Post
I despair with Learmonts comments.
At least he stopped short of saying “The cable that broke is simply secured a by solderlesss nipple. Just like a 1963 BSA” From a mechanical engineering perspective, he was getting there.
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Old 6th Dec 2018, 17:34
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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..
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Old 6th Dec 2018, 17:35
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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.
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Old 6th Dec 2018, 17:38
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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.
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Old 6th Dec 2018, 18:02
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Originally Posted by [email protected] View Post
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.
Hence my earlier comment about the requirement for careful installation with this type of bearing.
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Old 6th Dec 2018, 18:06
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Originally Posted by [email protected] View Post
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.
Hang on, doesn't the report suggest the overtightening is from the spining shaft...?
ie Wind-up in the shaft has pulled the spider nut down and unwound the servo end?
Page 7.
Am I miss-reading this?

Sufficient force and torque had been applied to the castellated nut on the actuator end ofthe control shaft to friction weld it to the pin carrier and to shear the installed split pin. Theobserved condition of the duplex bearing and the increased torque load on the castellatednut that remained on the spider end of the shaft is consistent with rotation of the tail rotoractuator control shaft. Whilst the shaft was rotating and a yaw control input was applied, theshaft “unscrewed” from the nut, disconnecting the shaft from the actuator lever mechanism,and causing the nut to become welded to the pin carrier.


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Old 6th Dec 2018, 18:11
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Originally Posted by [email protected] View Post
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.
As I understand it the over tightening of the nut at the spider end of the shaft was likely caused by rotation of the shaft as the duplex bearing got tighter, not necessarily by over torquing during assembly. In other words the over tightening was a result, rather than a cause of the duplex bearing failure
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Old 6th Dec 2018, 18:14
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There was no grease, in it's original form, in the bearing. Was that likely to be cause or effect?
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Old 6th Dec 2018, 18:17
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Having looked again at the diagrams, it appears to me that the castellated nut assembly should be locked on the threaded end at the end of the shaft and it should be able to freely rotate with it. The duplex bearing should allow this. However, the duplex bearing seized, or partially seized and produced an extreme overheat. This resulted in the locking nut welding to the carrier and it then it unscrewed as the shaft continued to rotate. The carrier, which is vital for the pilot to input yaw demands, was then no longer connected.
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Old 6th Dec 2018, 18:28
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Originally Posted by gevans35 View Post
There was no grease, in it's original form, in the bearing. Was that likely to be cause or effect?
if that is the case, then I suspect it might.
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Old 6th Dec 2018, 18:33
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Originally Posted by Echo Romeo View Post

if that is the case, then I suspect it might.
That's what it says in the report.

Either it was burnt off by an overheated bearing, or it was the cause of the over temperature and seizure?
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Old 6th Dec 2018, 18:38
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Shy, to my eye the control shaft is not designed to rotate at all. That it did so is without a doubt and the effect of the shaft rotating pushed the rest of the mechanical system beyond design limits.
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Old 6th Dec 2018, 18:53
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Forgive me - not an engineer, but an observer of the scene. And I have read Bulletin S2. It's quite hard to understand...

However, I think what is confusing is that the bearing seized up at one end, and the nut came off at the other. Anybody else think that's the case?

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Old 6th Dec 2018, 18:55
  #960 (permalink)  

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Originally Posted by Just This Once... View Post
Shy, to my eye the control shaft is not designed to rotate at all. That it did so is without a doubt and the effect of the shaft rotating pushed the rest of the mechanical system beyond design limits.
You might be correct - that was my initial thought, as above. I do find the report difficult to understand without looking at the actual parts in question.
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