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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 6th Dec 2018, 18:19
  #961 (permalink)  
 
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Nige 321 and Grumanniser - yes, having re-read it, that is what the report says so the question is why did the bearing fail?

Airsound, as I understand the system, you have two shafts, one inside the other - one just goes in and out (control rod for pitch change) which is the inner one and the other spins around it to transmit the drive from the TR driveshaft to spin the TR.

The duplex bearing allows the inner (control) shaft to move longitudinally in the outer (driving) shaft so that you can superimpose pitch change inputs onto the spinning TR.

If the bearing seizes, the driving shaft starts to turn the control shaft and that is what broke the split pin and allowed the castellated nut to undo.

Presumably the extra drag on the driving shaft is what caused the spider end nut to tighten too much.
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Old 6th Dec 2018, 18:20
  #962 (permalink)  
 
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I agree with JTO...The shaft was able to rotate with the rotor head enabling the nut to be compromised and shear the spilt pin to twist off. The issue with the bearing appears to be a lack of grease which was either burnt off or just not there when installed - that is not clear in the report.

In my experience similar TR pitch shafts have been mounted in splined guides to stop rotation.

I understand why the Actuator went to full travel and I'm unsure if a neutral default/detent setting is possible for an input failure mode.
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Old 6th Dec 2018, 18:22
  #963 (permalink)  
 
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I was always the sort of pilot that was likely, at any one time, to earn an “E” for Aircraft Knowledge. Reading the latest Special, I can see what the author is saying, but I lost track early on and am happy to take his word for it. That perhaps explains my immediate and intense admiration of the inspection team as I read the first paragraph of “Findings from the technical investigation”. The aircraft is presumably one they were not familiar with. By definition, what they get from speed-reading the appropriate manuals is not what has actually happened. Add to that the muck-and-corruption in which the wreckage has settled. Just a few of the many components in front of them went wrong before the crash, while almost everything else was ruined after it. This work is not as brave as bomb-disposal work, but by heavens these experts have to be very intelligent, alert and cool. The hangar and laboratory work that follows is as impressive in its own way.

I have read many NTSB, several USAF and a couple of BEA reports (including the Orly Concorde). All of these tend to lose concentration or to have a clear angle. One or two of the USAF reports are clearly politically driven and miss the point they should be driving for. Year by year, the AAIB reports get to the point with diligence and logic whether the object of the investigation is a dinged gyrocopter or something very much more significant. How long shall we be provided with such a service ?
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Old 6th Dec 2018, 18:27
  #964 (permalink)  
 
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Originally Posted by Rigga View Post
The issue with the bearing appears to be a lack of grease which was either burnt off or just not there when installed - that is not clear in the report.
They're still investigating the "initial cause and exact sequence of the failure".

I have heard of bearings being stripped of grease by over aggressive cleaning, pressure washer?
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Old 6th Dec 2018, 18:37
  #965 (permalink)  
 
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Originally Posted by airsound View Post
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?

airsound
I can imagine one end tightening up as far as it can, then shearing the weakest point at the other end (the split pin).
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Old 6th Dec 2018, 18:55
  #966 (permalink)  
 
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it makes sense to me how things transpired once familiar with the parts and terminology.
what happened to the grease. Was the grease non existent or minimalat manufacture, leading to a failure 300hrs later? or did the bearing start failing and from the heat burn away the grease?
if the system doesn't get a redesign, whats the resultant inspection and remedy to prevent this from happening again?
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Old 6th Dec 2018, 18:56
  #967 (permalink)  
 
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Gentlemen, in view of the report limitations, the future identification of the cause of the failure of the duplex bearing will be very important. Without specific experience of this type, I would venture the following thoughts. After the duplex bearing became effectively locked and the shaft rotated at T/R speed, I would guess that (with an input axial load) the castleated nut would friction weld to the pin-carrier in a short time (probably measured in seconds) then, followed almost immediately by the unscrewing of the nut and T/R runaway. The question must be, how to mitigate this process in the event of a duplex bearing failure.

OAP
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Old 6th Dec 2018, 18:58
  #968 (permalink)  
 
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Sounds like a 'MOD', fitting a grease nipple might not be a bad idea, if it already has one, then a maintenance schedule revision to lube it more often.
Tony
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Old 6th Dec 2018, 19:10
  #969 (permalink)  
 
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Sorry to be pedantic. The report refers to the "castleated nut" that unscrewed from the input end of the shaft and, the "locking nut" on the duplex bearing end (that remained in place). The illustration also includes a depiction of the "locking nut" at the duplex bearing end as, "castleated nut". Obviously, these possible confusions should be reviewed, IMO.

OAP
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Old 6th Dec 2018, 19:30
  #970 (permalink)  
 
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Originally Posted by nodrama View Post

What the report is saying is that the duplex bearing failed, for whatever reason, and stopped doing its job of allowing the spider to rotate freely around a stationary rod. The rod then started rotating with the spider. Because the rod would rotate anti-clock wise, it tightened the big nut up as far as it’s cotter pin would allow, and then the big nut would continue to rotate with the rod. At the smaller nut end, the effect would be the opposite. The locking wire and cotter pin would prevent the nut from undoing, and the smaller nut would turn with the rod. This caused friction against the carrier, eventually welding the smaller nut to it. The nut is now part of the fixed servo link. The rod would be turning against a cotter pin and locking wire, which it seems, from the report, that it sheared, and wound itself out of the nut.

The pitch change rod would now be no longer attached to the servo link to provide feedback of its position, and the servo would continue to move in the direction of its last input.
Agree, been here since my #944, the words used in the report just make it tricky. As far as the direction of movement of the servo input, it will depend upon the forces acting after the disconnection of the input nut and it's contribution (now removed).

OAP
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Old 6th Dec 2018, 19:52
  #971 (permalink)  

 
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nodrama
Maybe I can help clear some of the confusion.....the duplex bearing failed, for whatever reason, and stopped doing its job of allowing the spider to rotate freely around a stationary rod. The rod then started rotating with the spider. Because the rod would rotate anti-clock wise, it tightened the big nut up as far as itís cotter pin would allow, and then the big nut would continue to rotate with the rod. At the smaller nut end, the effect would be the opposite. The locking wire and cotter pin would prevent the nut from undoing, and the smaller nut would turn with the rod. This caused friction against the carrier, eventually welding the smaller nut to it. The nut is now part of the fixed servo link. The rod would be turning against a cotter pin and locking wire, which it seems, from the report, that it sheared, and wound itself out of the nut.
You did. (clear up confusion). Thank you!

airsound
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Old 6th Dec 2018, 20:28
  #972 (permalink)  
 
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I don't care how many times you practice in the sim, that one is pretty unrecoverable from that height and speed.
I'm curious how this could possibly be recovered given any amount of height/speed. As I understand, even entering autorotation wouldn't help - the TR is in full right pitch and you'll just rotate all the way down. I suppose with an infinite amount of good luck it MIGHT be possible to flare and touch the ground with this going on and survive, if the cabin integrity remained. But who could realistically fly an auto while rotating at what, once every couple of seconds?
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Old 6th Dec 2018, 20:34
  #973 (permalink)  
 
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I can't see clearly from the diagrams in the SB, but I think that if you have the actuator rod able to rotate on the duplex bearing then setting the correct torque on the castellated nuts is going to be difficult during assembly or maintenance.

Is there a method to do this or is it a matter of getting the nut to a position where the split pin can be fitted. That does not guarantee the final position of the nut, so any torque value would be at best vague.
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Old 6th Dec 2018, 20:41
  #974 (permalink)  
 
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I am not an engineer or a pilot, but involved in healthcare risk management.
In summary, there is no pilot error, he was faced with an impossible situation.
Now, is the remaining situation a design error, a maintenance error, a build error or a combination of all three?
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Old 6th Dec 2018, 20:53
  #975 (permalink)  
 
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Originally Posted by mercurydancer View Post
I am not an engineer or a pilot, but involved in healthcare risk management.
In summary, there is no pilot error, he was faced with an impossible situation.
Now, is the remaining situation a design error, a maintenance error, a build error or a combination of all three?
You could add manufacturing and materials quality.
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Old 6th Dec 2018, 21:22
  #976 (permalink)  
 
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Originally Posted by Feathers McGraw View Post
I can't see clearly from the diagrams in the SB, but I think that if you have the actuator rod able to rotate on the duplex bearing then setting the correct torque on the castellated nuts is going to be difficult during assembly or maintenance.
Figure 5 in the SB seems to show a square-section end to the actuator rod which could be used to prevent the rod from rotating while the nut is torqued.
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Old 6th Dec 2018, 21:28
  #977 (permalink)  
 
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....and a surprise conclusion after the conjecture in this thread. This must have quite a relevance to many other Rotaries in operation not just the same model.
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Old 6th Dec 2018, 21:33
  #978 (permalink)  
 
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Originally Posted by Speed of Sound View Post
Figure 5 in the SB seems to show a square-section end to the actuator rod which could be used to prevent the rod from rotating while the nut is torqued.
I had just prepared the screenshot -

https://assets.publishing.service.go...018_G-VSKP.pdf
Fig 5.
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Old 6th Dec 2018, 21:44
  #979 (permalink)  
 
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Originally Posted by jimjim1 View Post
I had just prepared the screenshot -

https://assets.publishing.service.go...018_G-VSKP.pdf
Fig 5.
Well spotted. And now you've mentioned it, Figure 2 also appears to show flats at the other end, slightly obscured behind the lock wire.
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Old 6th Dec 2018, 22:24
  #980 (permalink)  
 
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The torque on the castellated nut retaining the shaft in the duplex bearing has no effect on the function of the bearing. It's an opposed angular contact set-up, with solid inner races. The torque applied to the nut will have no effect whatsoever on the internal bearing clearances. Any over-torque of the nut would serve only to stress the shaft, most likely in the waisted section immediately below the thread, and that doesn't seem to have occurred give the photos above.
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