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 11th Dec 2018, 07:31
  #1061 (permalink)  
 
Join Date: Mar 2007
Location: UK
Posts: 321
It isn't possible to even move the flying controls on an AW169 without at least one hydraulic system pressurised. That's why I made the point of saying earlier that the MR/ TR pitch control is hydraulically operated and NOT hydraulically assisted. There is a major flying control design difference. The pilot input isn't direct to the rotor pitch control, but to the hydraulic servo input valves. If the pressurised servos aren't commanded to move, nothing is moving. That's why there are two systems, for redundancy. How much clearer does it have to be?
nodrama is offline  
Old 11th Dec 2018, 10:12
  #1062 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,894
Originally Posted by nodrama View Post
Both hydraulic system pumps are main transmission driven, so as long as the main rotor is turning there will be workable system pressure. Both hydraulic systems are completely separate.
The hydraulic systems can be turned off individually (this requires electrical power, as the solenoids fail ‘open’), but cannot be turned off at the same time due to an automatic logic protection system. Nor can a hydraulic system be manually turned off if the logic protection system detects that the other hydraulic system has low oil contents, low pressure or high temperature.
It seems to me that the manufacturer has gone to lengths to ensure that at least one hydraulic system is available for the tail rotor control at all times.
My honest answer to your question about manual control with a dual hydraulic failure, is that I don’t know.
Originally Posted by nodrama
It isn't possible to even move the flying controls on an AW169 without at least one hydraulic system pressurised. That's why I made the point of saying earlier that the MR/ TR pitch control is hydraulically operated and NOT hydraulically assisted. There is a major flying control design difference. The pilot input isn't direct to the rotor pitch control, but to the hydraulic servo input valves. If the pressurised servos aren't commanded to move, nothing is moving. That's why there are two systems, for redundancy. How much clearer does it have to be?
I think we all understand.
ShyTorque is offline  
Old 11th Dec 2018, 11:26
  #1063 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,894
Originally Posted by m25 View Post
ShyTorque, thanks for your explanation and that's how I initially read the report as well. However if you pick up a bolt and a couple of nuts it would seem that in order to loosen the bolt which came off the torque you need to apply via the inside race of a bearing would tend to loosen the nut holding that bearing on also. I am lost as to how this can happen the other way around. Do the actual directions make sense to you? If so I must be misunderstanding the way it is assembled.
M25,

The tail rotor hub and blades on a 169 are on the right hand side of the tail boom and rotate from nose to tail at the top (i.e anti-clockwise as you look at them from the right hand side of the aircraft). The control shaft sits inside the outer drive shaft and hub. When the duplex bearing began to seize, the control shaft would have also tended to rotate in the same direction as the hub, i.e. anti-clockwise. *Any drag on the nut on that end would tend to tighten it because it has a right hand thread; it would be the same as tightening any normal right hand threaded nut and bolt.

Edit: *As the hub rotates in its entirety, this cannot actually have been the case, my error!

If you now move to the left side of the aircraft, the tail rotor and "errant" control shaft shaft appear to be moving in a relatively clockwise direction. The castellated nut on that side also has a right hand thread. With the shaft rotating clockwise, any drag on the nut (i.e. from contact with the stationary pin carrier to which it was bolted) would tend to cause the two to be unscrewed.

If instead the control shaft and its nut on that end had a left handed thread, the relative motion would have tended to tighten them up, as is the case on the right hand side of the assembly. Whether that would have helped prevent the catastrophic failure, I really don't know. If the design of the pin carrier had allowed the control shaft and nut to spin freely together, they would have presumably stayed together, even in rotation.

Last edited by ShyTorque; 11th Dec 2018 at 23:35.
ShyTorque is offline  
Old 11th Dec 2018, 15:19
  #1064 (permalink)  
 
Join Date: May 2016
Location: Norway
Posts: 33
Regarding the TR control, I think there are several slightly different angles at play here: 1) In the control system on the 169 physically connected so that it's possible, given enough force to change the pitch (apparently no). 2) Would it be possible, considering the forces at play, to design a control system with a manual fallback.

If 2) is true, I guess one could discuss if 1) was a smart design choice. More than the TR control system in isolation should probably be considered if such, if the aircraft won't fly without hydraulics for other reasons, manual fallback for the TR might be completely pointless.

Generally I always prefer manual fallbacks, but they even make cars where the steering wheel and brakes has no such fallback these days. I guess it's part of a trend of over-confidence in system designs that leads to arrogance. They can probably also save some money designing a system without such fallback in many cases. I have no idea what considerations are behind the 169 control system were though, so this isn't meant as a speculation for their reasoning.
Nadar is offline  
Old 11th Dec 2018, 17:03
  #1065 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,894
I suspect that as has been stated here before, the forces required to move the tail rotor blades on such a big aircraft would be too large for the pilot to manage without assistance from a hydraulic servo. I know that from personal "feet on" experience on rather smaller aircraft where the hydraulics become no longer available (as we know, many smaller types of helicopter such as the Squirrel, A109 etc can revert to manual flight control, but even then it's very hard work to control them).

What would be better would be a design arrangement that "self centres" the pitch angle of the blades to a pre-determined, neutral setting if the pilot's normal yaw control system is lost. This would allow flight to be continued, to some sort of a controlled running landing, as on other helicopters. Pilots can be trained how to handle this lesser, although still serious, type of emergency.
ShyTorque is offline  
Old 11th Dec 2018, 18:32
  #1066 (permalink)  
 
Join Date: Mar 2007
Location: UK
Posts: 321
Interestingly, all the 'bearings' in the AW169 tail rotor (flap/ pitch/ lead-lag) are elastomeric. I'm wondering whether, with no hydraulic pressure to the TR control servo, the TR blades would return to their 'neutral' position by themselves under the force of the elastomeric's wanting to return to their normal unloaded position? I could try it with a hydraulic rig sometime, but it wouldn't take into consideration any dynamic loads that would be there in flight.
nodrama is offline  
Old 11th Dec 2018, 18:45
  #1067 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,894
Where they naturally revert to depends on their CTM / ATM ratio... but the aerodynamic loads are very large and would easily overcome any tendency to "self centre" at rest via the elastomerics.
ShyTorque is offline  
Old 11th Dec 2018, 18:58
  #1068 (permalink)  
 
Join Date: Nov 2008
Location: UK
Age: 62
Posts: 919
When the shaft unwound itself from the nut, presumably the shaft was then free to float in and out, so it was the buoyancy of the blades that pulled it out. These blade grips could have had bob weights fitted to keep them neutral if control went limp.
chopjock is offline  
Old 11th Dec 2018, 20:01
  #1069 (permalink)  
 
Join Date: Jun 2002
Location: east ESSEX
Posts: 3,738
Might have been different if he had done a yaw check after initial lift-off...
Todays helicopters are designed with much shorter and more blades giving a higher `solidity ratio`,and shorter moment arms to the tail rotor,with almost as much `area` in front of the rotor mast as behind,than `classics like the WX,S-K,etc,and giving reduced directional stability...
In that case in the event of a t/r failure,it should be possible to design the CTM/ATM to position to a net thrust against power bias,for control failures,and `fuses` in the hydraulic system in the t/r system in the case of a t/r hyd leak/failure there.
sycamore is online now  
Old 11th Dec 2018, 20:51
  #1070 (permalink)  
 
Join Date: Jul 2000
Location: Used to be God's own County
Posts: 1,588
Sycamore - I was thinking the same sort of thing - not in the pilot's AFTO actions but in the fail-safe design of modern helicopters used to low failure rates. After all, it only has to happen the once!
EESDL is offline  
Old 11th Dec 2018, 21:42
  #1071 (permalink)  
 
Join Date: Oct 2007
Location: Here
Posts: 789
Regarding duplex bearing side nut.

Originally Posted by ShyTorque View Post
the control shaft would have also tended to rotate in the same direction as the hub, i.e. anti-clockwise. Any drag on the nut on that end would tend to tighten it because it has a right hand thread
Yes - as long as God was flying alongside holding the nut:-)

Thing is that the nut is not held by anything. Instead the shaft is prevented from rotating because it is clamped to the feedback arm on the far side. If the inner race began to turn on the shaft because of forces transmitted through the failing bearing the inner race would rotate anti-clockwise relative to the shaft. This might tend to UN-screw the nut on the shaft.

Since the split pin on that nut remained intact the nut was not rotated very much if at all. The AAIB say that the release torque on the nut was higher than expected - however no cause or explanation for this is actually stated. Tightening was not caused by the nut being rotated by forces transmitted through the failed duplex bearing.

Two earlier posts mentioned this aspect of the geometry also, I apologise that I have not credited them but at present I don't want to take the time to look for the posts.
jimjim1 is offline  
Old 11th Dec 2018, 21:44
  #1072 (permalink)  
 
Join Date: May 2010
Location: Boston
Age: 69
Posts: 442
Originally Posted by chopjock View Post
When the shaft unwound itself from the nut, presumably the shaft was then free to float in and out, so it was the buoyancy of the blades that pulled it out. These blade grips could have had bob weights fitted to keep them neutral if control went limp.
The part that disconnected was the feedback path to the servo valve.
The shaft was still attached (well actually part of if I read the diagram correctly) to the hydraulic actuator that was hard over due to lack of feedback.

BTW: As another example of very bad outcome from failed feedback the preliminary report on the Lawrence MA gas explosions reveals that the overpressure (75psi in a low pressure 1/2psi line ) was caused by switching to a newly installed pipe section with out moving the pressure sensing lines from the decommissioned ancient iron pipe. This caused the regulators to go full open, why there was no local override path on the regulators is a different question.

Last edited by MurphyWasRight; 11th Dec 2018 at 21:46. Reason: typos
MurphyWasRight is offline  
Old 11th Dec 2018, 22:34
  #1073 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,894
Originally Posted by jimjim1 View Post
Regarding duplex bearing side nut.



Yes - as long as God was flying alongside holding the nut:-)

Thing is that the nut is not held by anything. Instead the shaft is prevented from rotating because it is clamped to the feedback arm on the far side. If the inner race began to turn on the shaft because of forces transmitted through the failing bearing the inner race would rotate anti-clockwise relative to the shaft. This might tend to UN-screw the nut on the shaft.

Since the split pin on that nut remained intact the nut was not rotated very much if at all. The AAIB say that the release torque on the nut was higher than expected - however no cause or explanation for this is actually stated. Tightening was not caused by the nut being rotated by forces transmitted through the failed duplex bearing.

Two earlier posts mentioned this aspect of the geometry also, I apologise that I have not credited them but at present I don't want to take the time to look for the posts.
Jimjim, I see what you are saying. But if the higher than expected torque on the starboard nut wasn't caused by the bearing failure, where did it come from?
ShyTorque is offline  
Old 11th Dec 2018, 23:38
  #1074 (permalink)  
 
Join Date: Oct 2007
Location: Here
Posts: 789
Originally Posted by ShyTorque View Post
Jimjim, ... if the higher than expected torque on the starboard nut wasn't caused by the bearing failure, where did it come from?
I don't know and AAIB is not commenting at present either.

I too assumed your (and others) proposed mechanism of tightening as I read the report. I then noticed that the mechanism was wrong and re-read the report carefully. The AAIB do not make any statement as to the mechanism of excessive tightening. Perhaps it would have been better if they had stated that explicitly as quite a few people here (and presumably other readers too) have made that incorrect (I believe) connection in the context presented.

Maybe it was tightened too much last time it was assembled? How about some fretting during the failure process creating rough surfaces or loose material within the joints? I just made the last one up, I have no idea if such a thing might actually occur.

I can't see that it is important since I doubt very, very much that the tight nut had anything to do with the crash. Well unless the bearing was designed to have it's preload set by the nut torque. Excessive preload would certainly explain the failure of the bearing. My experience of such bearing configurations (not aviation) is that they are always done up hard and the dual race bearing has a small clearance within it.

We'll likely find our next year.
jimjim1 is offline  
Old 12th Dec 2018, 07:30
  #1075 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 8,193
When the shaft unwound itself from the nut, presumably the shaft was then free to float in and out, so it was the buoyancy of the blades that pulled it out.
Oh dear Chopjock, your understanding of this subject is getting worse............I'm surprised TC hasn't jumped on this one!...... buoyancy of the blades OMG!

Try googling servo hardover.....
crab@SAAvn.co.uk is offline  
Old 12th Dec 2018, 10:07
  #1076 (permalink)  
 
Join Date: Nov 2018
Location: Manchester
Posts: 4
Am with chop on this one instead of looking down on him with your expert opinions would someone please kindly explain how that control shaft was still connected to the actuator when It was disconnected from the lever mechanism I can only see two fixing points on that shaft top and bottom?????
W.u.W is offline  
Old 12th Dec 2018, 11:57
  #1077 (permalink)  
 
Join Date: Oct 2012
Location: UK
Posts: 1,785
For those confused by the reported "significantly higher" disassembly torque of the nut at the duplex bearing end. One would hope that the final report will explain this observation. However, I believe that the initial report referred to here makes no observation about the split pin locking of that nut and so, presumably, the split pin and that nut were in their position, as fitted.

OAP
Onceapilot is offline  
Old 12th Dec 2018, 12:04
  #1078 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,894
I can understand what Chopjock is asking.

However, the hydraulic servo is designed to control the pitch angle of the blades, not the other way round. The self centreing effect of the blades (if any) isn't enough to overcome the power of the servo. The servo is obviously designed to be more than powerful enough to push / pull them into the required position. If the servo control valve remains open, because there is nothing to close it, the servo will drive the blade pitch angle to full travel, in this case it must have gone to full negative pitch, or close to it.
ShyTorque is offline  
Old 12th Dec 2018, 12:06
  #1079 (permalink)  
 
Join Date: Jan 2011
Location: on the cusp
Age: 48
Posts: 217
Originally Posted by W.u.W View Post
Am with chop on this one instead of looking down on him with your expert opinions would someone please kindly explain how that control shaft was still connected to the actuator when It was disconnected from the lever mechanism I can only see two fixing points on that shaft top and bottom?????
The servo acts on the control rod, visualise that as fixed. The feedback arm creates an angle between the control rod end position and the input control position. This acts on the servo control. The servo moves until the geometry reaches the "neutral" again. With the control rod end not fixed, the geometry is not fixed and so there is no shutoff.
dClbydalpha is offline  
Old 12th Dec 2018, 12:08
  #1080 (permalink)  
 
Join Date: Apr 2008
Location: Europe
Posts: 75
jimjim1 You are right regarding castellated nut holding duplex bearing. It is obvious that if failing bearing caused inner race to start rotating on shaft, this action would put UNSCREWING force to the nut if it has a normal, right tread - I believe it is. But nut stayed in place and only increased torque was reported. Will come back to that later.
Hovewer, torque transmitted to control shaft was strong enough to overcome the clamping force of castellated nut on the other side-at control lever and its pin carrier. Control shaft and nut (plus still intact but strained cotter pin) started rotating in pin carrier. I would speculate that nut immediatelly moved a little in UNSCREWING direction, thus slightly decreasing clamping force, (due to slack and/or initial deformation of cotter pin) . Enormous heat and clamping pressure then caused the nut to friction-weld to carrier pin, thus resisting rotation. Now cotter pin could not withstand the torque between control shaft and firmly welded nut and it snapped. Control shaft started unscrewing out of nut. Once shaft threads disengaged completelly, (it was very fast-just the length of the threads) control was lost. I suspect that while unthreading, rod moved (quickly) to the right for the length of the tread and changed pitch of blades, causing initial right yaw (?)

It remains unclear to me what exactly happened then , as in S2-2018 report there is not enough information how exactly hydraulic power cylinder engages with control rod and I do not have type knowledge-wether control shaft was hydraulically moved to the physical limit of actuator, or it was just free to move, thus leaving blades to rotate in pitch axis until equilibrium of all forces was established or pushed by runaway actuator.
Report says that T/R actuator continued changing the pitch of the blades until they reached physical limit. Sentence above says that rod was disconnected from lever.

Duplex bearing - if its castellated nut would unscrew or at least allow rotation of inner race on control shaft, the whole arrangement would still function until heating would weld inner race to the shaft. (with some lubricant, it might not happen very fast) Hovewer, evidence suggest that OUTER race rotated in its place. Wether inner race also rotated, report doesn`t say. I hope AAIB CT scanned also (duplex bearing side) castellated nut before they removed the cotter pin and found (abnormally high) nut torque. Any tell-tale deformation of the cotter pin would be altered-destroyed with removal.

I am not that surprised that nut torque on duplex bearing nut was found abnormally high, considering significant overheating, possible movement between to-be-clamped parts.. .
It often happens that correctly torqued assembly needs significantly higher unscrewing torque when disassembled again after a while, especially if exposed to elements, heat and dynamic loading.
Of course, if it is left-treaded, than torque increase is somewhat expected.

Bearing seizure obviously caused the accident, but what caused bearing failure, remains to be explained. Hovewer, I feel a bit uneasy that such failure dooms the helicopter.
I am trying to figure out if a feature, allowing rotation and thus integrity of flight controls, could (should?) be designed into the system, like alternative way of rotation between slider and control rod if bearing seizes-up and providing tell-tale sign (vibration) that would give time to pilot to recognize the problem and act before everything goes South. And I have a very personal interest in that.

hoistop

P.S. Could anyone provide a diagram of AW169 T/R ctrl sys that explains this description from S2/2018 report:

The lever pivots around the connection at the control shaft end and creates a demand on the hydraulic system vie the solenoid valve, which moves the hydraulic piston and control shaft of the actuator.
hoistop is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

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