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5 minutes ago, urban.spaceman said:

Now they need to figure out how the **** that happened in the 10 minutes it spent sitting on the pitch after a dozen safe flights earlier in the day. 

Yep. Haven't yet read the official report, perhaps that suggests anything. 

 

Here if anyone is interested: https://assets.publishing.service.gov.uk/media/5c090ab1e5274a0b64c8a2f4/S2-2018_G-VSKP.pdf

 

Edit: report suggests they are still investigating why the part failed.

Edited by Kopic
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Sky News BreakingVerified account @SkyNewsBreak
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Air Accidents Investigation Branch has issued an update on its investigation into the Leicester City helicopter crash which killed five people in October and says a pin had become disconnected in the control mechanism of the tail rotor which made the helicopter "uncontrollable"

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47 minutes ago, urban.spaceman said:

Now they need to figure out how the **** that happened in the 10 minutes it spent sitting on the pitch after a dozen safe flights earlier in the day. 

It must have happened once in the air surely, as he would not have been able to keep his heading while taking off.

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8 minutes ago, Jon the Hat said:

It must have happened once in the air surely, as he would not have been able to keep his heading while taking off.

Correct. This is a mechanical failure that had been developing for some time (the reason for which they are still investigating). When the tail rotor came under load during the take off, the nut suddenly popped off, rendering it immediately uncontrollable.

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4 hours ago, Saxondale said:

Correct. This is a mechanical failure that had been developing for some time (the reason for which they are still investigating). When the tail rotor came under load during the take off, the nut suddenly popped off, rendering it immediately uncontrollable.

Scary when you think something so that could appear to be so inconsequential could have such a dramatic impact when it failed. 

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31 minutes ago, joachim1965 said:

That is the reason for the pin, it physically stops the nut working loose, the big question is , why was this pin missing.

 

I went to Rotorheads to read the thread on the crash. I think they are saying the bearing failed, and then the outer spinning shaft turned the inner shaft, which is not meant to happen at all. If I understand what is being said correctly!

 

Here is a post with a reasonably coherent description, as an example of what they are discussing. Even they seem to be taking a while to understand quite what the reports are saying, but they do seem to be focusing on the bearing failure as the start of the problems, with the nut failure as a consequence rather than the cause. Maybe your question now could be, where's the grease gone?

 

 

 

Note that TR is tail rotor.

 

 

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.
 
 
 
crab@SAAvn.co.uk is offline  
Edited by Vardinio'sCat
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Here is part of another post from Rotorheads, my bold.

 

 

Maybe I can help clear some of the confusion, which I think comes from the report referring to both nuts involved as ‘the castellated nut’ and part of the servo link as ‘the carrier’.

I will refer to the nut at the spider/ duplex bearing end as ‘the big nut’, and the nut at the servo end as ‘the smaller nut’. The carrier is not to be confused with a bearing carrier. It is actually a hinged block of metal arrangement with a hole in it that the pitch control rod passes through, 'a pin carrier', and forms the attachment of the pitch link rod to the servo feedback link.

The tail rotor rotates anti-clockwise (looking from the righthand side of the aircraft) at about 5000 rpm. It is directly driven (no clutch) by two engines that can provide in the region of 1000 shp each.

The big nut is at the spider/ duplex bearing end of the pitch rod, the smaller nut at the servo link end. Both are cotter pinned, and the smaller nut is also locked with locking wire. They are both conventional RH threads.
The pitch rod doesn’t rotate, as has been explained in previous posts, but the tail rotor and the spider are rotating around it. The duplex bearing in the spider (the pitch rod/ spider interface if you like) enables this to happen.
The tightness of the two nuts at each end is what prevents the pitch rod from rotating freely under normal operation. There is no key, or other method to prevent it turning if there was enough force.

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 flying control input.

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18 hours ago, joachim1965 said:

Makes a lot more sense after reading that thanks. Lack of grease would seem to point to poor maintenance, that is of course if the bearing that failed should have been greased regularly,  I am sure we will find out in due course.

The bearing would be a sealed unit and would be greased for life. When the bearing overheats the grease escapes because the seals get destroyed. The bearing would be a serviceable item which would need to be replaced as laid down by the manufacturer. It would have failed due to being either out of it's usable life, an incorrectly specified bearing during the design or vibration from another source. 

 Also the nut undoing and consequently shearing the cotter pin due to the centre of the bearing friction welding itself against the nut is irrelevant in a catastrophic bearing failure because the pin is just there to stop the nut turning due to normal forces. It may have benefited from having a left hand thread because it would have prevented  the nut undoing and possibly cause the inner race of the bearing to spin on the control shaft and given the pilot more time to react but, it's all if's and buts.

 

 Edit. I've just read they were duplex bearings, which are two roller bearings that are matched together so the two contact faces fit together perfectly, If they are installed the wrong way round, so they are no longer matched, there is an excessive axial pressure, which causes them to wear prematurely and seize. (a bit like car front wheel bearings that are tightened to a certain torque setting) 

Edited by yorkie1999
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7 hours ago, yorkie1999 said:

The bearing would be a sealed unit and would be greased for life. When the bearing overheats the grease escapes because the seals get destroyed. The bearing would be a serviceable item which would need to be replaced as laid down by the manufacturer. It would have failed due to being either out of it's usable life, an incorrectly specified bearing during the design or vibration from another source. 

 Also the nut undoing and consequently shearing the cotter pin due to the centre of the bearing friction welding itself against the nut is irrelevant in a catastrophic bearing failure because the pin is just there to stop the nut turning due to normal forces. It may have benefited from having a left hand thread because it would have prevented  the nut undoing and possibly cause the inner race of the bearing to spin on the control shaft and given the pilot more time to react but, it's all if's and buts.

 

 Edit. I've just read they were duplex bearings, which are two roller bearings that are matched together so the two contact faces fit together perfectly, If they are installed the wrong way round, so they are no longer matched, there is an excessive axial pressure, which causes them to wear prematurely and seize. (a bit like car front wheel bearings that are tightened to a certain torque setting) 

 

So where was the grease isn't quite right. Thanks.

 

The bit about the outer shaft doing 5000 revs kind of blew my mind really, but then I'm very much at the limits of my understanding, engineering wise.

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19 hours ago, yorkie1999 said:

The bearing would be a sealed unit and would be greased for life. When the bearing overheats the grease escapes because the seals get destroyed. The bearing would be a serviceable item which would need to be replaced as laid down by the manufacturer. It would have failed due to being either out of it's usable life, an incorrectly specified bearing during the design or vibration from another source. 

 Also the nut undoing and consequently shearing the cotter pin due to the centre of the bearing friction welding itself against the nut is irrelevant in a catastrophic bearing failure because the pin is just there to stop the nut turning due to normal forces. It may have benefited from having a left hand thread because it would have prevented  the nut undoing and possibly cause the inner race of the bearing to spin on the control shaft and given the pilot more time to react but, it's all if's and buts.

 

 Edit. I've just read they were duplex bearings, which are two roller bearings that are matched together so the two contact faces fit together perfectly, If they are installed the wrong way round, so they are no longer matched, there is an excessive axial pressure, which causes them to wear prematurely and seize. (a bit like car front wheel bearings that are tightened to a certain torque setting) 

I've always known duplex bearings as ball bearings not roller, also they are usually 1 unit so it doesn't matter which way round they are fitted, where have you seen the information on this?

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2 hours ago, red5 said:

I've always known duplex bearings as ball bearings not roller, also they are usually 1 unit so it doesn't matter which way round they are fitted, where have you seen the information on this?

I've known about their use for years because of my job, but here's some info if you're interested (not many people are).  Basically the load on the bearing needs to  supported by the stationary object, because the spinning object doesn't offer any support. The helicopter would have had back to back duplex bearings and the nut would have been to pre-load them. 

 

 

https://www.brighthubengineering.com/machine-design/19757-duplex-bearings-back-to-back-arrangement/

face.jpg

back.jpg

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