Helicopter down outside Leicester City Football Club
Avoid imitations
Originally Posted by [email protected]
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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Originally Posted by [email protected]
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.
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.
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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Originally Posted by [email protected]
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.
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.
Avoid imitations
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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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.
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
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
Avoid imitations
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.
If the control shaft had been keyed rather than a plain rounded shaft the outcome with binding at the TR end would have been different. Of course, this could have moved the failure point elsewhere if still not detectable to vib sensors or feet.
The design of the locking nut and carrier assembly looks totally appropriate for a rod that will only ever act in a push/pull manner. What the designer appears to have missed during failure analysis is that binding at the TR end would transmit the rotational force of the tail rotor onto the linear control rod inside the powered shaft. The rod was neither designed to freely rotate in this failure mode or mechanically prevented from spinning. It does not appear to be monitored by sensors either, so this control rod may have been spinning for a while.
The design of the locking nut and carrier assembly looks totally appropriate for a rod that will only ever act in a push/pull manner. What the designer appears to have missed during failure analysis is that binding at the TR end would transmit the rotational force of the tail rotor onto the linear control rod inside the powered shaft. The rod was neither designed to freely rotate in this failure mode or mechanically prevented from spinning. It does not appear to be monitored by sensors either, so this control rod may have been spinning for a while.
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.
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.
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.
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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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 ?
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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I have heard of bearings being stripped of grease by over aggressive cleaning, pressure washer?
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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?
airsound
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
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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?
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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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.
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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
Tony
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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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