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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 11th Dec 2018, 02:24
  #1061 (permalink)  
 
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I don't know if 2-bladed TR systems take less control input force than three bladed, I'm just expecting some correlation between weight of helicopter and the tail rotor thrust, or control input force. AW169 gross (increased with a kit) is 10582 lbs. Bell mediums with a single hydraulic to the tail-rotor were the 430 - 9300, 205 - 11,200, 212 - 11,200, 412 - 11,900. All these are flyable and landable with no hydraulics to the tail-rotor servos. I had to do it in training for those types and I trained others to do it later.

I'm hesitating on the AW169 because I'm unsure of the mechanics of the Teleflex system and whether the leverage and control strength is there. Agusta/Leonardo, like Bell, can build some pretty stout pedals though, judging from the abuse they can take on the AW139 just setting the parking brake. It would be a major surprise to me if the 169 tail rotor wasn't controllable without hydraulics Gotta eat crow and agree with crab on this one, given no drama's patient explanation, but it has two systems and the main rotor isn't controllable without at least one, so a somewhat moot point that only serves to pad the thread while we wait for answers.

Not thread drift, but just to correct any misunderstandings, the 212 main rotor is controlled through a stab bar and can be flown with both hydraulics shut off, of course, otherwise Bell could not have certified it with that switch logic. Flight Check Procedures require shutting both off in flight at 70 knots - look it up. In my early days in the industry we expected pilots to be able to land it with both systems off, and we all did, and when we became instructors we taught it. Even spaghetti-armed Brit pilots that found it impossible at BHL, magically found the strength on this side of the Atlantic where failure meant no job.

Last edited by malabo; 11th Dec 2018 at 16:41.
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Old 11th Dec 2018, 07:31
  #1062 (permalink)  
 
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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?
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Old 11th Dec 2018, 10:12
  #1063 (permalink)  

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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.
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Old 11th Dec 2018, 11:26
  #1064 (permalink)  

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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.
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Old 11th Dec 2018, 15:19
  #1065 (permalink)  
 
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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.
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Old 11th Dec 2018, 17:03
  #1066 (permalink)  

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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.
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Old 11th Dec 2018, 18:32
  #1067 (permalink)  
 
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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.
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Old 11th Dec 2018, 18:45
  #1068 (permalink)  

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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.
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Old 11th Dec 2018, 18:58
  #1069 (permalink)  
 
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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.
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Old 11th Dec 2018, 20:01
  #1070 (permalink)  
 
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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.
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Old 11th Dec 2018, 20:51
  #1071 (permalink)  
 
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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!
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Old 11th Dec 2018, 21:42
  #1072 (permalink)  
 
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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.
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Old 11th Dec 2018, 21:44
  #1073 (permalink)  
 
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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
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Old 11th Dec 2018, 22:34
  #1074 (permalink)  

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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?
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Old 11th Dec 2018, 23:38
  #1075 (permalink)  
 
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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.
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Old 12th Dec 2018, 07:30
  #1076 (permalink)  
 
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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.....
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Old 12th Dec 2018, 10:07
  #1077 (permalink)  
 
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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?????
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Old 12th Dec 2018, 11:57
  #1078 (permalink)  
 
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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
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Old 12th Dec 2018, 12:04
  #1079 (permalink)  

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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.
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Old 12th Dec 2018, 12:06
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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.
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