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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 8th Dec 2018, 11:37
  #1021 (permalink)  
 
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Originally Posted by ShyTorque View Post
...The answer is to mandate that by design that all helicopter tail rotor control systems are equipped with a centreing device for the servo valve mechanism, to prevent a servo runaway in the event of the pilot's flight yaw controls becoming disconnected, such as now appears to have occurred here. With such a device fitted, the servo is automatically biased to move to a fixed, central position enabling controlled flight to still be available and a safe landing of some sort to be carried out...
This wouldn't help if the failure was beyond the servo, i.e. the shaft to the tail rotor. There is also the down side of adding weight and complexity.
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Old 8th Dec 2018, 11:39
  #1022 (permalink)  
 
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Perhaps a way of removing hydraulic power to the actuator in an emergency as well?

The TR pitch control linkage of this aircraft looks insanely complicated and convoluted to my untrained eye - so many linkages, pivots, rose joints, cranks and other components: they must be a nightmare to maintain and keep working smoothly and check during the pre-flight inspection?

Seems bizarre to have only a single yaw control system, which if it fails will guarantee a crash.





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Old 8th Dec 2018, 11:48
  #1023 (permalink)  
 
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NOTAR technology was meant to solve this failure mode, wasn't it? Having flown an MD-600 and a Jet Ranger I couldn't 'feel' any difference.

Is there a reason why the design hasn't found wider adoption?
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Old 8th Dec 2018, 12:57
  #1024 (permalink)  
 
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Originally Posted by ShyTorque View Post
Actually, neither of those are new ideas. The late Ray Prouty discussed both in his books, years ago.
Probably yes but not in conjunction with computerised control .
Just for fun - kind of amazing - 2.20
Anyway i just read that fly by wire for helicopters is being researched in the military so if it's capable of coping with TR loss we will find someday .
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Old 8th Dec 2018, 13:58
  #1025 (permalink)  

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Originally Posted by Pozidrive View Post
This wouldn't help if the failure was beyond the servo, i.e. the shaft to the tail rotor. There is also the down side of adding weight and complexity.
A) Maybe not but what is the point of that statement?

B) The size and weight of such a mechanism (they do exist!) are very small, possibly adding not much more than couple of kilos, if as much as that. All we're talking about here is a device to spring bias the hydraulic servo control valve to a central position, i.e. to replicate the pilot's normal pedal input.

Can I suggest you read the following publication? Section 3 is of relevance to this accident:

https://publicapps.caa.co.uk/docs/33/CAPAP2003_01.PDF
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Old 8th Dec 2018, 14:12
  #1026 (permalink)  
 
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The answer is to mandate that by design that all helicopter tail rotor control systems are equipped with a centreing device for the servo valve mechanism, to prevent a servo runaway in the event of the pilot's flight yaw controls becoming disconnected, such as now appears to have occurred here.
Really? Go back and read the report.
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Old 8th Dec 2018, 14:35
  #1027 (permalink)  

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RVDT, Your point isn't clear. I've read the report, thanks, more than once. It would help if you would explain what your disagreement is based on, rather than asking a very vague question and giving an order..
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Old 8th Dec 2018, 14:55
  #1028 (permalink)  
 
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I think what RVDT is pointing out is that it wasn't 'a servo runaway'. That would be when a servo malfunctions and moves uncommanded. The hydraulic systems and servo were operating correctly, as far as has been revealed, and the servo was just doing what it had been commanded to do before the feedback link stopped working.
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Old 8th Dec 2018, 15:10
  #1029 (permalink)  
 
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Shy take a look at how the thing works. It was not the input that failed but lets say the "output" to cancel the input via the feedback mechanism.

It's pretty simple - when all 3 points attached to the lever line up with the servo valve in the neutral position nothing happens as the control shaft moves and cancels the input request.

As the feed back end became detached - nothing to cancel the input - servo motors to the stop and in this case full right pedal?

BTW - Sikorsky has been fitting centreing as the result of accidents or incidents not unlike this one - not pro-actively.

The design and acceptance of the AW models may be under review shortly. If not I will be very surprised.
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Old 8th Dec 2018, 15:25
  #1030 (permalink)  
 
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Shy,

At the risk of telling you how to suck eggs -



Setpoint = TR pedal control

Load = TR Pitch change or duplex bearing

Feedback lever disconnected at the hydraulic cylinder (servo)

Disconnect the feedback lever after the hydraulic valve has moved and there is nothing for the input side to work against to shift it again. Moving the pedals would more than likely just move the free end of the feedback lever not the pilot valve.
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Old 8th Dec 2018, 15:42
  #1031 (permalink)  
 
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Rainy cold day here....second cup of Coffee in hand and some time on my hands while I anticipate the Army-Navy Football game this afternoon at which the World pauses for a few hours (for those like me anyway)!

After reading all of the posts I have garnered some thoughts on this tragedy....mostly relating to the design of the Tail Rotor Control system on the Mishap Aircraft.

I have only the knowledge gathered by means of the posts here and some articles in the media.

I assume the 169 and 189 share a common design ( or identical systems common to the two types) thus the AD's apply only to the 169/189.

The design must not be as "simple" as suggested due to the various discussions upon what role all the various components played in that event.

The conversation about which Nut loosened/tighten and so forth....was indicative of that.

Some questions:

Why did the 169/189 wind up with this particular design of Tail Rotor Control system?

Is the design "new" and "different" to all previous such systems used by AW?

If so....if a new and unique design for the 169/189.....why?

Did the Design Team ever consider such a failure mode during the design and testing process?

If so....how did they resolve any issues that arose?

Is this design "failure tolerant" to the minimum degree necessary assuming inspection and servicing intervals and procedures were properly carried out at the factory and by the operator ?

Is this design an actual improvement over past designs or is it too complicated/complex in design?

Are there design flaws that set the stage for just such a failure as this to occur?

Is there a method to shut off the hydraulic pressure to the Tail Rotor Servo(s) and if so....would the Tail Rotor assume a somewhat neutral position?

(As violent and rapidly as the aircraft reacted to this actual failure.....I am thinking there would have been scant time for analyzing the problem by any Pilot.)

Anyone else have these or similar questions about the AW-169/189 Tail Rotor Control System?
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Old 8th Dec 2018, 15:51
  #1032 (permalink)  
 
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Originally Posted by SASless View Post
Is there a method to shut off the hydraulic pressure to the Tail Rotor Servo(s) and if so....would the Tail Rotor assume a somewhat neutral position?
The TR pitch control is hydraulically operated (dual system), not hydraulically assisted. There is no manually 'selectable' TR hydraulic system shut off or manual reversion. This isn't a unique design particular to this helicopter type.

Last edited by nodrama; 8th Dec 2018 at 16:04. Reason: accuracy of wording
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Old 8th Dec 2018, 15:56
  #1033 (permalink)  

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Originally Posted by RVDT View Post
Shy take a look at how the thing works. It was not the input that failed but lets say the "output" to cancel the input via the feedback mechanism.

It's pretty simple - when all 3 points attached to the lever line up with the servo valve in the neutral position nothing happens as the control shaft moves and cancels the input request.

As the feed back end became detached - nothing to cancel the input - servo motors to the stop and in this case full right pedal?

BTW - Sikorsky has been fitting centreing as the result of accidents or incidents not unlike this one - not pro-actively.

The design and acceptance of the AW models may be under review shortly. If not I will be very surprised.
Yes, I have done. If you can be bothered to read back my previous posts on this topic (they date back some 17 years), you will see that I don't disagree with you.The servo ran away to full travel because the servo valve remained open; there was nothing to centralise it once a common part became detached. The common factor / weak point in this design is that the pilot control inputs route via the same lever that nulls the servo; unfortunately that's what came adrift because a single nut disconnected. On other designs, this isn't the case.

BTW - Sikorsky has been fitting centreing as the result of accidents or incidents not unlike this one - not pro-actively.
But surely the purpose of airworthiness legislation is to ensure that everyone designing aircraft learns from past history; irrespective of which type first had an accident....

Last edited by ShyTorque; 8th Dec 2018 at 16:07.
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Old 8th Dec 2018, 16:20
  #1034 (permalink)  
 
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Am I correct in assuming the Tail Rotor cannot be controlled manually in the event of a dual hydraulic failure?
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Old 8th Dec 2018, 21:50
  #1035 (permalink)  
 
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Loss of feedback.
Remember as a little boy seeing the remains of a mill boiler house after the steam engine flywheel exploded -because the speed governor was BELT driven from the power output shaft, and the belt had broken!!!!
No loss of life on that occasion, but more by luck than judgement.
Lessons were learned, not least by this budding engineer.
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Old 9th Dec 2018, 10:50
  #1036 (permalink)  
 
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Originally Posted by Onceapilot View Post
There will be a reason for the bearing failure that probably is the main cause of the subsequent accident. Questions about the overall design of the input assembly might come into it. However, there will be great effort made to identify why the bearing failed and, the mitigation of that process. It might be anything from: a major redesign, an improved bearing, improved QC of the bearing, revised servicing or an improved inspection.

OAP
Undoubtedly but the main focus should be as to why a relatively common and foreseeable problem - a bearing binding, would allow a primary control to disconnect itself and be driven to the pitch limits.

It is a single point of failure and, worryingly, one that is designed to fail at some point.
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Old 9th Dec 2018, 10:51
  #1037 (permalink)  
 
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Originally Posted by SASless View Post
Am I correct in assuming the Tail Rotor cannot be controlled manually in the event of a dual hydraulic failure?
Hi SASless,
Unfortunately, I cannot tell if the T/R system offers any level of T/R manual-only control or, the response of the T/R blade pitch to total loss of Hyd power. I will have to take issue with the writers of the report here. Under the paragraph "Tail rotor control operation" there is scant detail. Particularly, the "servo actuator" component is arrowed twice on the illustrations but not referred to in the para. Additionally, this part of the para "The lever pivots around the connection at the control shaft end and creates a demand on the hydraulic system via the SOLENOID VALVE, which moves the hydraulic piston and control shaft of the actuator" seems to be in error as a solenoid valve is an electrically operated valve. Should one read this to understand that movement of the lever is switching a solenoid valve? Hmmmm

OAP
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Old 9th Dec 2018, 11:22
  #1038 (permalink)  
 
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OAP,part of the yaw channel AFCS...?
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Old 9th Dec 2018, 11:34
  #1039 (permalink)  
 
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Hi sycamore,
No idea I am afraid. My reference to the para "The lever pivots around the connection at the control shaft end and creates a demand on the hydraulic system via the solenoid valve, which moves the hydraulic piston and control shaft of the actuator" is to highlight words that seem to me to be either; badly written, incorrect or confusing.

OAP
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Old 9th Dec 2018, 12:32
  #1040 (permalink)  
 
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Among my Laundry List of questions was this one......

The design must not be as "simple" as suggested due to the various discussions upon what role all the various components played in that event.
Are we seeing in the various posts before and after my question showing up....confirmation that understanding how this 168/189 Tail Rotor Control system operates is far more difficult than at first glance?

I appreciate all the posts as they come from knowledgeable people and that quality of discussion is very informative.

This is where having a Maintenance Manual for the 169 would be very useful....to read what the Manufacturer has to say about it all.


Oh....and by the way....Army BEAT Navy yesterday! Go Army-Beat Navy!
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