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 Nov 2018, 15:18
  #641 (permalink)  
 
Join Date: Nov 1998
Location: south coast
Posts: 388
Trouble is JJ , the SB isnt really relevant to a servo runaway. Its relevant to a structural failure of the parts concerned or even more explicit of a missing split pin / nut/ wirelocking procedure. I hope they are also looking at security at Fairoaks.
Barcli is offline  
Old 6th Nov 2018, 15:30
  #642 (permalink)  
 
Join Date: Feb 2009
Location: England
Posts: 242
So, what maintenance procedure would necessitate the removal or loosening of that nut? Excepting the replacement of another component such as the servo.
Echo Romeo is offline  
Old 6th Nov 2018, 15:56
  #643 (permalink)  
 
Join Date: Nov 2008
Location: UK
Posts: 56
Excellent post JJ

Looking at the diagram of how the pitch change shaft is locked to the feedback lever bracket, if the nut was missing something would have had to initiate the extension or retraction of the pitch change shaft for the feedback lever bracket to disconnect from the pitch change shaft. (Namely a commanded yaw action)

It never happened on the ground, nor in the climb. It looks to have happened as the pilot turns the aircraft to transition to forward flight. This would make sense, the pitch change shaft extends fully, lever bracket slides off the end of the pitch control shaft and control is lost. As it happened during a yaw movement, possible full extension, the onset was a rapid rotation. Even if the servo did react to a neutral position, due to the momentum of the spin and the points you raised about pitch OGE hover not being enough to counteract main rotor then it wouldn’t have mattered if it corrected itself to neutral, the momentum caused by the violent initial rotation kept it on that path. The lever bracket coming off during this phase and at full pitch change shaft extension surely would have been the worst possible scenario? Servo neutral correction not really applicable if the violent onset is already underway.

Thoughts?


Last edited by Mitchaa; 6th Nov 2018 at 16:25.
Mitchaa is offline  
Old 6th Nov 2018, 16:05
  #644 (permalink)  
 
Join Date: Nov 2008
Location: UK
Posts: 56
Also, would the pilot have carried out a full left and right pedal check as part of start up and before lifting? Perhaps this is the point where the nut has detached. The very first yaw movement thereafter in flight disconnecting the bracket from the shaft and causing loss of control.
Mitchaa is offline  
Old 6th Nov 2018, 16:08
  #645 (permalink)  
 
Join Date: Apr 2009
Location: Hotel Gypsy
Posts: 2,830
Thoughts? A $2 nut (or thereabouts before being associated with an EASA Form 1), mitigated by a piece of locking wire, presents the potential of a catastrophic single point of failure.
Cows getting bigger is offline  
Old 6th Nov 2018, 16:20
  #646 (permalink)  
 
Join Date: Apr 2010
Location: U.K.
Posts: 31
Originally Posted by Cows getting bigger View Post
Thoughts? A $2 nut (or thereabouts before being associated with an EASA Form 1), mitigated by a piece of locking wire, presents the potential of a catastrophic single point of failure.
unfortunately yes, although it is split pinned and wire locked!

This is the case for many components, hence two locking methods, duplicate inspections and daily visuals.
FlightSpanner is offline  
Old 6th Nov 2018, 16:23
  #647 (permalink)  
 
Join Date: Sep 2004
Location: daworld
Posts: 605
Originally Posted by Cows getting bigger View Post
Thoughts? A $2 nut (or thereabouts before being associated with an EASA Form 1), mitigated by a piece of locking wire, presents the potential of a catastrophic single point of failure.
You are aware that helicopters are littered with single point failures aren't you.

Any MR Servo Mount bolt. Any MR Blade Bolt. Any TR Blade Bolt. The "Jesus" nut, to name but a few. There are certification limits for the probability of failure of these critical items, which has been mentioned here already.

This nut is double locked. A dirty fat Split/Cotter Pin and lockwire. No single locking method failure should induce a failure of the control. Both locking systems have to fail. And with the nut being upstream of the servo assist it does not have flight loads on it only the same loading that the pilot feels on his/her feet.

This "nut on the end of the servo" system has flown for years and years on the 139 and 189. Although the 139 nut is slightly different. The 189 also has an SB out for inspection of the nut (out this morning).

I anxiously await the next update from the AAIB.
noooby is offline  
Old 6th Nov 2018, 16:28
  #648 (permalink)  
 
Join Date: May 2010
Location: Boston
Age: 68
Posts: 411
Originally Posted by Barcli View Post
Trouble is JJ , the SB isnt really relevant to a servo runaway. Its relevant to a structural failure of the parts concerned or even more explicit of a missing split pin / nut/ wirelocking procedure
Not clear that this is true, looking at the SB wording and diagram only (and with no knowledge of the system) it refers to the 'Servo feedback link". If it is indeed inside the control loop disconnecting it could very much result in a servo runaway. That said it would also seem to be a critical path and might well have a redundant path of some sort beyond what is shown.
Hopefully someone with knowledge of the system can comment on this.
MurphyWasRight is online now  
Old 6th Nov 2018, 16:38
  #649 (permalink)  
 
Join Date: Sep 2004
Location: daworld
Posts: 605
Originally Posted by MurphyWasRight View Post
Not clear that this is true, looking at the SB wording and diagram only (and with no knowledge of the system) it refers to the 'Servo feedback link". If it is indeed inside the control loop disconnecting it could very much result in a servo runaway. That said it would also seem to be a critical path and might well have a redundant path of some sort beyond what is shown.
Hopefully someone with knowledge of the system can comment on this.
The lever has three attachment points. The far end is where the control rod from the pedals connects. In the middle is the linkage that operates the pilot valve that then moves the servo. The end that is being inspected is connected to the servo itself. When this moves it resets the pilot valve to stop servo movement.

Standard method of controlling servos.
noooby is offline  
Old 6th Nov 2018, 17:00
  #650 (permalink)  
 
Join Date: May 2010
Location: Boston
Age: 68
Posts: 411
Originally Posted by noooby View Post
The lever has three attachment points. The far end is where the control rod from the pedals connects. In the middle is the linkage that operates the pilot valve that then moves the servo. The end that is being inspected is connected to the servo itself. When this moves it resets the pilot valve to stop servo movement.

Standard method of controlling servos.
Thanks, that is roughly what I guessed from the diagram, works by balancing the control input and servo output to a "zero'' at the pilot valve, in electronics same as an op-amp.

So Is this roughly correct?
If the control path disconnects the system could be rigged to settle on the 'aerodynamic neutral' mentioned above.
If the feedback path fails then the servo will likely continue to be driven to a stop since there is no (or at least incorrect) input at the pilot valve.

Last edited by MurphyWasRight; 6th Nov 2018 at 17:02. Reason: typo
MurphyWasRight is online now  
Old 6th Nov 2018, 17:26
  #651 (permalink)  
 
Join Date: Mar 2008
Location: England... what's left of it...
Posts: 161
"3.2 Check the connection elements of the input lever (110) of the TR servo-actuator
taking particular care on the lever feedback link."



Was that photograph issued with the SB? If so, was there another photo? The position referred to in the above text is not shown, the way I'm interpreting (or trying to). Also, should the word after "(110)" be to the servo-actuator, not of? They also call component #110 both "Input lever" and "Lever feedback link". I'm thinking the intention of SB item 3.2 is to order a check of the connection to the servo from part 110, which can't be seen in the posted photo.
Overdrive is offline  
Old 6th Nov 2018, 17:29
  #652 (permalink)  
 
Join Date: Apr 2009
Location: Hotel Gypsy
Posts: 2,830
Originally Posted by noooby View Post
You are aware that helicopters are littered with single point failures aren't you.
Silly old me, I thought we were in the business of reducing/mitigating risk and not just accepting the status quo. Is a split pin and a locking wire really the best way? Clearly someone has recognised the importance of this nut. Perhaps the whole linkage could be redesigned.
Cows getting bigger is offline  
Old 6th Nov 2018, 17:42
  #653 (permalink)  
 
Join Date: Mar 2008
Location: England... what's left of it...
Posts: 161
At some point, Mr. Cows, an assembly or artefact has to terminate. There are only so many ways to connect things together in a manner that can be dissembled and rebuilt. There is also only so much material and weight that can be used and included too. I think all methods have been tried by now!

If a split pin and a lockwired nut on an assembly of high quality design and material fail, on the back of proper maintenance and inspection, then one really is subject to the worst of probability.
Overdrive is offline  
Old 6th Nov 2018, 17:43
  #654 (permalink)  
 
Join Date: Nov 2008
Location: UK
Posts: 56
Originally Posted by Cows getting bigger View Post
Thoughts? A $2 nut (or thereabouts before being associated with an EASA Form 1), mitigated by a piece of locking wire, presents the potential of a catastrophic single point of failure.
I would think it would be nigh on impossible to have a failure of the pin and the wire locking considering as someone else has mentioned, there’s no loading there, they are simply to lock the nut to the shaft.

The release of the SB most probably points to neither the pin or the locking wire being present. The big question, were they ever fitted and locked and if so, how did 2 sets of eyes during a duplicate inspection miss it? Not impossible, but certainly unusual.
Mitchaa is offline  
Old 6th Nov 2018, 17:46
  #655 (permalink)  
 
Join Date: Nov 2008
Location: UK
Posts: 56
Originally Posted by Overdrive View Post
"3.2 Check the connection elements of the input lever (110) of the TR servo-actuator
taking particular care on the lever feedback link."



Was that photograph issued with the SB? If so, was there another photo? The position referred to in the above text is not shown, the way I'm interpreting (or trying to). Also, should the word after "(110)" be to the servo-actuator, not of? They also call component #110 both "Input lever" and "Lever feedback link". I'm thinking the intention of SB item 3.2 is to order a check of the connection to the servo from part 110, which can't be seen in the posted photo.
Yes, the picture and the parts diagram taken directly from the SB. No other pictures presented.
Mitchaa is offline  
Old 6th Nov 2018, 17:49
  #656 (permalink)  
 
Join Date: Mar 2008
Location: England... what's left of it...
Posts: 161
Mitchaa - thanks.
Overdrive is offline  
Old 6th Nov 2018, 17:57
  #657 (permalink)  
 
Join Date: Nov 2008
Location: UK
Posts: 56
Originally Posted by Cows getting bigger View Post
Silly old me, I thought we were in the business of reducing/mitigating risk and not just accepting the status quo. Is a split pin and a locking wire really the best way? Clearly someone has recognised the importance of this nut. Perhaps the whole linkage could be redesigned.
I would think a possible modification would be to introduce a locking bracket so that if the nut ever did unwind then it would hit against the bracket and not come loose off the shaft. I guess with all the hours on the 139/189 and the double locking measure and duplicate inspection they probably didn’t think this extra fail safe was necessary in design. Saying that, it’s not hard to line up the holes in the Swiss cheese looking at it afterwards.








Last edited by Mitchaa; 6th Nov 2018 at 18:18. Reason: Typo
Mitchaa is offline  
Old 6th Nov 2018, 18:05
  #658 (permalink)  
 
Join Date: May 2010
Location: Boston
Age: 68
Posts: 411
Originally Posted by Mitchaa View Post


I would think it would be nigh on impossible to have a failure of the pin and the wire locking considering as someone else has mentioned, there’s no loading there, they are simply to lock the nut to the shaft.

The release of the SB most probably points to neither the pin or the locking wire being present. The big question, were they ever fitted and locked and if so, how did 2 sets of eyes during a duplicate inspection miss it? Not impossible, but certainly unusual.
It is also possible that the SB resulted from an observation of condition of the recovered assembly. By itself does not 'prove' that this is the cause of the accident.
In other words they saw something unexpected that warranted inspections such as incorrect assembly order but not by itself the root cause.
Would a hard over servo runaway result in the reported (by some only) grinding noises?
MurphyWasRight is online now  
Old 6th Nov 2018, 18:29
  #659 (permalink)  
 
Join Date: Nov 2009
Location: The sky mainly
Posts: 240
It is also possible that the SB resulted from an observation of condition of the recovered assembly. By itself does not 'prove' that this is the cause of the accident.
In other words they saw something unexpected that warranted inspections such as incorrect assembly order but not by itself the root cause.
It is also a possibility that the SB resulted from an observation on an aircraft on the other side of the world. Quite often, maintenance engineers, when they get a sense or a hunch of what caused an accident, will in the following days, take a stroll over to a similar machine in the hangar and just take another good look at the suspect system. Its sometimes at that point that things like anti-locking on components - that have been there since manufacture - suddenly become apparent.
Sky Sports is offline  
Old 6th Nov 2018, 18:32
  #660 (permalink)  
 
Join Date: May 2007
Location: Merriott, Somerset, UK
Age: 73
Posts: 222
Originally Posted by Barcli View Post
Trouble is JJ , the SB isnt really relevant to a servo runaway. Its relevant to a structural failure of the parts concerned or even more explicit of a missing split pin / nut/ wirelocking procedure. I hope they are also looking at security at Fairoaks.
'The security at Fairoaks'.................... as a retired LAE, I am wondering about the possibility of sabotage, locking pins and wire locking do not fail simultaneously.
Tony
Tony Mabelis is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.