Go Back  PPRuNe Forums > Aircrew Forums > Rotorheads
Reload this Page >

Helicopter down outside Leicester City Football Club

Rotorheads A haven for helicopter professionals to discuss the things that affect them

Helicopter down outside Leicester City Football Club

Old 6th Dec 2018, 13:04
  #901 (permalink)  
 
Join Date: May 2008
Location: LEIC
Posts: 96
Received 5 Likes on 2 Posts
Further info on AAIB website released today at 1400
ROC man is offline  
Old 6th Dec 2018, 13:15
  #902 (permalink)  
 
Join Date: Jun 2012
Location: Teesside
Posts: 260
Received 12 Likes on 4 Posts
Originally Posted by ROC man
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 .
David Thompson is offline  
Old 6th Dec 2018, 13:33
  #903 (permalink)  
 
Join Date: Oct 1999
Location: Den Haag
Age: 57
Posts: 6,380
Received 384 Likes on 221 Posts
Originally Posted by David Thompson
AAIB Special Bulletin here ; https://assets.publishing.service.go...018_G-VSKP.pdf .

Already reported on the BBC .
so clearly the EADs were not based on speculation
212man is offline  
Old 6th Dec 2018, 13:34
  #904 (permalink)  
 
Join Date: Oct 2018
Location: South West
Posts: 311
Received 22 Likes on 18 Posts
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?
gipsymagpie is offline  
Old 6th Dec 2018, 13:55
  #905 (permalink)  
 
Join Date: Oct 2018
Location: UK
Posts: 9
Likes: 0
Received 0 Likes on 0 Posts
“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?
Slowclimb is offline  
Old 6th Dec 2018, 14:10
  #906 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 10,386
Received 734 Likes on 331 Posts
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.......
crab@SAAvn.co.uk is offline  
Old 6th Dec 2018, 14:18
  #907 (permalink)  
 
Join Date: Oct 2018
Location: UK
Posts: 9
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by RVDT
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
Looks like you guys were right.
Slowclimb is offline  
Old 6th Dec 2018, 14:23
  #908 (permalink)  
 
Join Date: Jul 2002
Location: Ireland
Posts: 596
Likes: 0
Received 0 Likes on 0 Posts
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.
Speed of Sound is offline  
Old 6th Dec 2018, 14:39
  #909 (permalink)  
 
Join Date: Jan 2010
Location: Edinburgh
Age: 85
Posts: 74
Likes: 0
Received 22 Likes on 13 Posts
Looks like it should have been a left hand thread, although that might just have postponed disaster.
DType is offline  
Old 6th Dec 2018, 14:42
  #910 (permalink)  
 
Join Date: Jan 2018
Location: Leatherhead
Posts: 12
Likes: 0
Received 0 Likes on 0 Posts
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.......
​​​​​​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 ??
GAPU is offline  
Old 6th Dec 2018, 14:43
  #911 (permalink)  
 
Join Date: Nov 2001
Location: 1000ft above you, giving you the bird!
Posts: 579
Received 0 Likes on 0 Posts
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
Jetscream 32 is offline  
Old 6th Dec 2018, 15:04
  #912 (permalink)  
 
Join Date: Oct 2012
Location: UK
Posts: 1,785
Likes: 0
Received 1 Like on 1 Post
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.......
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 15:09. Reason: Castleated nut spun against the pin-carrier
Onceapilot is offline  
Old 6th Dec 2018, 15:06
  #913 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,643
Received 517 Likes on 275 Posts
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 16:55. Reason: To hopefully clarify.
ShyTorque is offline  
Old 6th Dec 2018, 15:17
  #914 (permalink)  
 
Join Date: Jun 2002
Location: east ESSEX
Posts: 4,710
Received 84 Likes on 51 Posts
A balance-spring bias on the servo unit towards left pedal( power input) would have been useful...
sycamore is offline  
Old 6th Dec 2018, 15:48
  #915 (permalink)  
 
Join Date: Feb 2009
Location: England
Posts: 245
Likes: 0
Received 0 Likes on 0 Posts
I despair with Learmonts comments.
Echo Romeo is offline  
Old 6th Dec 2018, 16:10
  #916 (permalink)  
 
Join Date: Jan 2009
Location: Corpus Christi, Texas
Age: 69
Posts: 71
Received 2 Likes on 2 Posts
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
Otterotor is offline  
Old 6th Dec 2018, 16:15
  #917 (permalink)  
 
Join Date: Apr 2009
Location: Scotland
Age: 70
Posts: 9
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Echo Romeo
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.
bumpthump is offline  
Old 6th Dec 2018, 16:34
  #918 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 10,386
Received 734 Likes on 331 Posts
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..
crab@SAAvn.co.uk is offline  
Old 6th Dec 2018, 16:35
  #919 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 10,386
Received 734 Likes on 331 Posts
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.
crab@SAAvn.co.uk is offline  
Old 6th Dec 2018, 16:38
  #920 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 10,386
Received 734 Likes on 331 Posts
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.
crab@SAAvn.co.uk is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.