Helicopter down outside Leicester City Football Club [Archive] - Page 5

SASless

8th Dec 2018, 14:42

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?

nodrama

8th Dec 2018, 14:51

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.

ShyTorque

8th Dec 2018, 14:56

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....

SASless

8th Dec 2018, 15:20

Am I correct in assuming the Tail Rotor cannot be controlled manually in the event of a dual hydraulic failure?

DType

8th Dec 2018, 20:50

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.

Just This Once...

9th Dec 2018, 09:50

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.

Onceapilot

9th Dec 2018, 09:51

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

sycamore

9th Dec 2018, 10:22

OAP,part of the yaw channel AFCS...?

Onceapilot

9th Dec 2018, 10:34

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

SASless

9th Dec 2018, 11:32

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! :D:ok:

Old Farang

9th Dec 2018, 11:55

OAP,part of the yaw channel AFCS...?

Not current on any of this, far too old! But proportional solenoid valves are available, so it is possible but not shown on the drawing posted. Maybe a dual operated valve?

nodrama

9th Dec 2018, 14:13

badly written, incorrect or confusing.
OAP

Forget the fact that I have type knowledge, from a maintenance engineer's point of view I found the report poorly written, with poor terminology, and confusing. This has been proven by the majority of the posts and questions in this thread since the report was published.
The use of the word 'solenoid', as quoted above, was incorrect and misleading. As has been pointed out, a solenoid is an electrical-mechanical component. I would refer to the item that the report is referring to as a servo valve, or spool valve. These are standard hydraulic servo component terms.
The AW169 tail rotor servo and pitch control system really is pretty straight forward and not particularly unique, compared to some I've seen over the years. It's not always easy though to visualise these things unless you have one in front of you, or a series of detailed photos. I still think that the report could have done a better job of clearly relaying the technical information to the industry and public.

sycamore

9th Dec 2018, 19:17

nodrama, perhaps you could elaborate on the `cable` arrangement to the tail ,from the pedals..? ..Is this a new terminology for a `teleflex` control,as cables are usually `pullers,not pushers`..?Why not carbon fibre rods ?
If one had a hyd failure,does the servo have a `bypass`loop,..?to allow manual inputs from the pedals..?
Can the controls be checked on the ground for `full,free and correct` operation prior to start,or only with an ext,hyd rig...?
Could you also confirm the spider pitch links go to the rear of the t/r blades,ie behind the feathering axis....?
thanks in advance...

megan

10th Dec 2018, 02:00

Oh....and by the way....Army BEAT Navy yesterday! Go Army-Beat Navy!Having served in both I come out a winner either way SAS.

nodrama

10th Dec 2018, 04:47

nodrama, perhaps you could elaborate on the `cable` arrangement to the tail ,from the pedals..? ..Is this a new terminology for a `teleflex` control,as cables are usually `pullers,not pushers`..?

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/975x682/flexiball_6e59cbde3f5f810e11957daba1280f143d872388.png

A ‘flexiball cable’ is what it is called. They’ve been used successfully by several helicopter manufacturers for the last 20+ years (e.g Eurocopter/ Airbus, MD, Agusta), though a flexiball control was attributed to the cause of an EC135 crash in Japan in 2007. The last statement with reference to no maintenance and lubrication isn’t strictly true. They are subject to periodic friction force checks and the eye-end sliders sometimes get greased (depends on aircraft type).

nodrama

10th Dec 2018, 06:32

If one had a hyd failure,does the servo have a `bypass`loop,..?to allow manual inputs from the pedals..?
Can the controls be checked on the ground for `full,free and correct` operation prior to start,or only with an ext,hyd rig...?
Could you also confirm the spider pitch links go to the rear of the t/r blades,ie behind the feathering axis....?
thanks in advance...
1. See previous posts
2. With one engine running in 'Acc Drive'
3. Yes

Triskelle

10th Dec 2018, 09:55

nodrama. I would be interested in your thoughts on lubrication of the failed bearing, with the burned 'black grease' on the rod nearby. In my experience with classic cars, black graphite grease can 'dry out' if not used regularly - and this a/c was apparently not used all that frequently?

sycamore

10th Dec 2018, 10:01

Nodrama,thanks for that; assume `acc drive` is similar to WX,S-K,Lynx...
Leonardo (the Engineer),said `Friction keeps the World together,Lubrication (grease,oil, money ), allows it to spin``...pity his namesake didn`t take note......

hoistop

10th Dec 2018, 11:28

With my 28 yrs of aircraft maintenance involvement I can only agree with nodrama, that report is surprisingly ambiguous - AAIB was always a first-class source of information. Using term "solenoid valve" for a hydraulic system component is softly said, misleading. (except in case there is truly an electromagnetic valve involved - in that case I will have to bury myself )

nodrama, can you comment or confirm that torque applied to castellated nut, that holds duplex bearing in place on control rod, has no effect on the bearing itself i.e. that overtorquing it will not "squeeze" balls between inner and outer race, but "only" overload the rod-threads?
Is duplex bearing actually made of two completelly separate bearings, turned one against the other, or is it one component? From the Fig. 4 in AAIB S2/2018 it seems there are two inner races, but one outer race. Such design implies that nut torque could have an effect on bearing friction? (if there is a gap between inner races-depends on design) Please help clarifying this, if you can!

hoistop

m25

10th Dec 2018, 11:57

[email protected]

10th Dec 2018, 14:22

If one had a hyd failure,does the servo have a `bypass`loop,..?to allow manual inputs from the pedals..? on an aircraft the size of a 169 there is no way you are going to move the TR without hydraulic assistance.

Aircraft like the Squirrel or Gazelle are enough of a handful with hyds switched off in yaw.

malabo

10th Dec 2018, 16:07

Quote:
Originally Posted by [email protected]
on an aircraft the size of a 169 there is no way you are going to move the TR without hydraulic assistance.

Lots of Bell mediums, bigger than the 169, only had a single hydraulic on the tailrotor, and if you shut Hyd1 off, yes you could still move it slowly with a lot of foot pressure.

169 I don’t know, and there is no way for the average pilot to test without bypassing the lockout logic. Leonardo flight test pilots would know.

Dave B

10th Dec 2018, 16:32

In the nineties, at BHL Alister Gordon, introduced an amendment to the 212 flight test schedule to include a double hydraulic failure. His reasoning was that if an aircraft had a single failure on a rig, it could still be flown back, on the assumption that a second failure would be controllable. It wasn't, the aircraft each time went into a right hand dive and turn, and the pilot urgently said turn it back on, or words to that effect.
The amendment was withdrawn after Bell said ,"stop being so stupid, your going to kill yourselves".
With regard to the S76 tail control aft quadrant spring centring system, this was fitted to put the system to neutral in the event of a cable brake. It would have no effect on a servo runaway.

ShyTorque

10th Dec 2018, 18:27

My experience of flying various types from three major manufacturers is that helicopters with a single hydraulic system have the facility for the pilot to revert to manual control. Those with two separate hydraulic systems do not have manual reversion and the design is such that both systems cannot be simultaneously switched off.

I have been told by an engineer working on the type that there is no manual reversion on the 169 and it is not possible to switch off both systems at the same time.

ShyTorque

10th Dec 2018, 19:03

hoistop, I think this is a good question. The AAIB report on page 5 states that this nut was "found to have a torque load significantly higher than the required assembly value".

In addition, on page 7, paragraph 2 "the increased torque load on the castellated nut that remained on the spider end of the shaft is consistent with rotation of the tail rotor actuator control shaft"

Are they saying the jammed bearing would tighten the nut? If the rotor is turning anti-clockwise, the bearing jams and the thread is clockwise it does not seem to make sense. Or does it mean it could have tightened on impact?

I think it's likely that as the right hand duplex bearing began to seize, the nut on the left side came under a loosening torque but was initially held firm by the resistance of the split pin, as per the design intention and it began to rotate clockwise in the pin carrier. No doubt the locking wire would break at this point. The continued rotation of the shaft simultaneously caused a tightening torque on the starboard nut. When the starboard nut could be tightened no more, the port nut, still rapidly rotating as it was not designed to do, friction welded to the carrier, the split pin then came under sufficient torque to be sheared off, allowing the nut to instantly be unwound off the thread on the end of its shaft, at which point the controls no longer responded to the pilot's input.

[email protected]

10th Dec 2018, 19:28

Lots of Bell mediums, bigger than the 169, only had a single hydraulic on the tailrotor, and if you shut Hyd1 off, yes you could still move it slowly with a lot of foot pressure.
malabo - which models? Ones with 2 bladed TRs? I think TR power in these modern aircraft is considerably higher - the 139 for example can deal with a 40 Kts crosswind - I stand by my assertion that manual control isn't possible.

m25

10th Dec 2018, 19:36

I think it's likely that as the right hand duplex bearing began to seize, the nut on the left side came under a loosening torque but was initially held firm by the resistance of the split pin, as per the design intention and it began to rotate clockwise in the pin carrier. No doubt the locking wire would break at this point. The continued rotation of the shaft simultaneously caused a tightening torque on the starboard nut. When the starboard nut could be tightened no more, the port nut, still rapidly rotating as it was not designed to do, friction welded to the carrier, the split pin then came under sufficient torque to be sheared off, allowing the nut to instantly be unwound off the thread on the end of its shaft, at which point the controls no longer responded to the pilot's input.

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.

ShyTorque

10th Dec 2018, 21:36

:ugh::ugh::ugh:

It wasn't you. :rolleyes:

malabo

11th Dec 2018, 01:24

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.

nodrama

11th Dec 2018, 06:31

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?

ShyTorque

11th Dec 2018, 09:12

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.

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

11th Dec 2018, 10:26

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.

Nadar

11th Dec 2018, 14:19

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.

ShyTorque

11th Dec 2018, 16:03

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.

nodrama

11th Dec 2018, 17:32

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.

ShyTorque

11th Dec 2018, 17:45

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.

chopjock

11th Dec 2018, 17:58

sycamore

11th Dec 2018, 19:01

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.

EESDL

11th Dec 2018, 19:51

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!

jimjim1

11th Dec 2018, 20:42

Regarding duplex bearing side nut.

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.

MurphyWasRight

11th Dec 2018, 20:44

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.

ShyTorque

11th Dec 2018, 21:34

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?

jimjim1

11th Dec 2018, 22:38

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.

[email protected]

12th Dec 2018, 06:30

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.....

W.u.W

12th Dec 2018, 09:07

Onceapilot

12th Dec 2018, 10:57

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

ShyTorque

12th Dec 2018, 11:04

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.

dClbydalpha

12th Dec 2018, 11:06

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.

hoistop

12th Dec 2018, 11:08

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.

W.u.W

12th Dec 2018, 12:05

Still don't understand how a servo hardover could full pitch them blades all the way when the control shaft (inner) was found inside the drive shaft (outer) some engineering that for a hydraulic system to still be able to push/pull on that shaft why its inside there surely it was aerodynamics that pitched the blades after the failures. Someone please make me eat humble pie

nodrama

12th Dec 2018, 12:06

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/2000x1504/img_1463_1a0bcff1a323facbe0fcc87da45810ce465699c6.jpg

ShyTorque

12th Dec 2018, 12:35

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.

As a helicopter pilot and instructor previously teaching others how to deal with tail rotor malfunctions, and knowing the irrecoverable situation this pilot and his passengers was put into, I'm horrified, rather than just a bit uneasy!

If the pin carrier had been designed to contain a simple bearing allowing free shaft rotation of the control end of the shaft, instead of clamping it, the nut would not have come undone.

GrayHorizonsHeli

12th Dec 2018, 12:38

A little pop of color would have been nice NoDrama...sheesh.

nodrama

12th Dec 2018, 12:45

I thought my crayons might ruin the clarity.....

hoistop

12th Dec 2018, 13:45

Many thanks nodrama for your effort!
I understand this system is a classic follow-up mechanism.
eee... I am still confused a bit. Would that mean that control shaft itself is actually a hydraulic piston with attachments on both sides?? It means when it started rotating O rings/packings/seals became runners for fast spinning control shaft??? I really wonder how this control shaft looks like when it`s put on the table...
ShyTorque my slight uneasiness on this design feature is with pun intended... I agree with you totally.
Hovewer, there are many many individual parts in each helicopter, that will make it stop flying immediatelly in case of failure/breakage. For instance, there are over 40 bolts in Bell 412 flight control system that are replaced every 1000 hours, b`couse - any of these breaks - bye, bye.... plus many other things. Truly, these are simple items and thus very reliable, (until someone fails to put in a cotter pin. and no one notices...) A failed bearing is another story and is not so uncommon.
hoistop

nodrama

12th Dec 2018, 14:09

Would that mean that control shaft itself is actually a hydraulic piston with attachments on both sides?
I think so. Never seen the inside of a servo that’s been stripped down. I do know that the pitch control rod is one part though, from input end to output end (where the nuts attach).

m25

12th Dec 2018, 14:56

n5296s

12th Dec 2018, 16:00

If the pin carrier had been designed to contain a simple bearing allowing rotation of the at the control end of the shaft, instead of clamping it, the nut would not have come undone.
Well yes, but the idea was that the duplex bearing took care of this. So a second bearing would only be relevant when the main duplex bearing had failed - I guess it never occurred to anyone this could happen. That second bearing would have to be pretty beefy given what it is handling, and could itself be a source of failure. Also, as I understand it, once the duplex bearing failed, the insides of the servo are spinning in a way they were never designed for, so even with the second bearing something was going to go very wrong very quickly.

Many thanks to nodrama for the explanations here - without the pictures it's very hard to understand what is going on here. Have been lurking here since the beginning, with increasing horror.

Pozidrive

12th Dec 2018, 16:33

Many thanks nodrama for your effort!
I understand this system is a classic follow-up mechanism.
eee... I am still confused a bit. Would that mean that control shaft itself is actually a hydraulic piston with attachments on both sides??...

No, Fig 3 in the bulletin shows the control shaft (in red) as a separate component that passes through a hollow piston. Description in the text also describes this. No detail given on how the piston acts on the control shaft.

To muddy the waters, the same figure also indicates a mechanical link from the control cable to the spider, but a number of posts have said it can't move without the hydraulics.

[email protected]

12th Dec 2018, 16:42

[email protected]

12th Dec 2018, 17:01

Overdrive

12th Dec 2018, 17:26

I saw a picture today of a duplex bearing from another helicopter TR pitch change mechanism where the inner race and the ball bearings had separated longitudinally from the outer race which is an interference fit within the housing - manufacturing fault??????

Maybe but rare. Or the application of a large load in the axial direction, in which bearings have a relatively low structural strength, if not specific end-load bearing. Other alternatives are a large misalignment of the shaft combined with a large axial load (not really possible in normal service), jumping/climbing out due to partial seizing from insufficient lubrication, or an enormous amount of wear in the bearing - not something you'd expect to go unchecked on an aircraft.

sycamore

12th Dec 2018, 17:37

Which company makes the g/box ,and associated attachments,and is it the same on the 189/139??/

Onceapilot

12th Dec 2018, 18:15

One thing that I note is that this type of duplex bearing might not be suitable to this application. Obviously, failure of the bearing in this installation can lead to loss of the aircraft. The predicted life / failure rate of the bearing might appear very good but, is it satisfactory in such a critical component? Packed and sealed bearings will fail when the lubricant leaks out and / or water or other contamination gets in. The internal structure of the bearing may also promote early failure if the grease fill from both of the races can all leak out or, if the contamination or debris can migrate internally from one race to the other. This accident bearing is described as packed with debris and, in general terms, this is an issue with sealed lubrication. A similar bearing with oil circulation lubrication may avoid debris packing and, any debris particles from a failing bearing may be detected early with monitoring of the lubricating oil.

OAP

Onceapilot

12th Dec 2018, 18:24

Further to some questions about the "significantly greater torque" to remove the nut. jimjim1 showed a nice close-up in his post #979. I believe that the carbonised remains of leaked lubricant or other contamination can be seen burnt into the threads of the shaft.

OAP

Overdrive

12th Dec 2018, 18:42

It might be that what is described as "Debris" within the bearing is actually the debris of lubricant and bearing components, following overheating and being carbonized due to seizure. The description so far has not been as precise as it might be.

W.u.W

12th Dec 2018, 18:45

WuW - chopjock has a reputation for asking 'interesting' questions on this forum - he is a helicopter pilot so I assume some level of technical knowledge, like how a servo works.

I saw a picture today of a duplex bearing from another helicopter TR pitch change mechanism where the inner race and the ball bearings had separated longitudinally from the outer race which is an interference fit within the housing - manufacturing fault??????
Crab absolutely no disrespect aimed at nodrama at all massive respect for him if any man on here is going to make me eat humble pie it's likely going to be him. From what I understand the end of the inner shaft was found inside the outer shaft with what remained of the split pin. All am trying to understand is how the servo actuator could apply any push/pull force to it in that situation

[email protected]

12th Dec 2018, 18:50

What I'm not clear about is the exact layout of the two bearings that make up the 'duplex' bearing.

It seems the outer (nearest the spider) bearing is a roller bearing that takes the axial loads between the driving outer shaft that is spinning the TR and the inner control rod.

Is the inner (furthest from the spider) bearing attached by its inner race to the control rod and allowed to move linearly within the driving shaft or is it also an interference fit with the driving shaft but allows the control rod to pass through its inner race and move linearly that way?

ShyTorque

12th Dec 2018, 18:53

W.u.W

12th Dec 2018, 19:14

Pozidrive - The control rod is a hydraulic piston with attachments as hoistop and nodrama say.

The input arm acts on the servo pilot valve to direct hyd fluid to one side or other of that piston, thus moving the control rod in and out.

The reason for having the other end of the input arm connected to the control rod is to provide the follow-up action and recentralise the servo valve when the TR pitch has been driven to the required position.
Crab no disrespect aimed at u or anyone else on here. This is the bit I am struggling to understand and why am asking the experts if it is a hydraulic piston with attachments on both ends how did it end up inside the outer shaft wouldn't it have to break/destroy the servo actuator?

[email protected]

12th Dec 2018, 19:23

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/852x643/139_tr_abca25505d9396787acae2c7df45ab60c534a7d3.jpg
Having looked closer at my 139 tech notes - the duplex bearing can be clearly seen - what isn't so obvious is the slider assembly (to which the spider is attached) is splined into the TR drive shaft so the whole assembly slides in and out on the splines whilst being supported at the outboard end by the duplex bearing.
I am assuming that the 169 has a very similar setup.
The text talks about the outboard nut and washer setting the preload for the outer race and the stepped end of the control rod providing the preload for the inner race - if the preloads are too high, can that damage the bearing?

GrayHorizonsHeli

12th Dec 2018, 19:45

can it damage the bearing?
of course, duplex bearings by design are meant for axial load capabilities as well as rigidity.
if it's not preloaded right, you lose that. The bearing could move some, there would be obvious friction issues and subsequent grease breakdown and or loss.

ShyTorque

12th Dec 2018, 20:23

Crab, your diagram helps clarify something for me. I know it's from a 139 but I believe the design principle to be similar to a 169 (although the pin carrier at the control end is different).

If the duplex bearing seizes and the outer race spins in its housing, rather than the inner one (the latter being pre-loaded against the stepped end of the control shaft), there would be no dragging force tending to undo the nut. Is the outer race pre-loaded in it's housing by torquing up the screws on the outer, dome shaped cover?

nodrama

12th Dec 2018, 20:33

Duplex bearings are ball, not roller. To cope with radial and axle loads they work like this
https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/600x600/f1692e14_ad7f_4e35_91c7_f1eaabc45acd_07ee579a6ce57f0c2a69a76 d6542b8d4872537bf.jpeg

sycamore

12th Dec 2018, 20:47

And the next `Q` is..if they were assembled in reverse order,or even one,what would be the outcome...?

[email protected]

12th Dec 2018, 20:52

No drama - sorry, I realise I typed roller when I meant ball - I was very clear they were both ball bearing races in the duplex setup but my fingers didn't seem to notice:ok:

ShyTorque

12th Dec 2018, 20:53

Duplex bearings, such as those in the diagram give the same support in both directions.

[email protected]

12th Dec 2018, 20:59

If the duplex bearing seizes and the outer race spins in its housing, rather than the inner one (the latter being pre-loaded against the stepped end of the control shaft), there would be no dragging force tending to undo the nut. Is the outer race pre-loaded in it's housing by torquing up the screws on the outer, dome shaped cover? Shy, yes according to my tech notes.

I am reliably informed by my engineers that the bearings are interference fit and have to be cooled with liquid nitrogen for installation so that they expand in place once fitted so the outer race rotating shouldn't be possible.

ShyTorque

12th Dec 2018, 21:05

Shy, yes according to my tech notes.

I am reliably informed by my engineers that the bearings are interference fit and have to be cooled with liquid nitrogen for installation so that they expand in place once fitted so the outer race rotating shouldn't be possible.

But when the bearing seizes, something mechanically abnormal will happen; it's a matter of what gives first.

Pozidrive

12th Dec 2018, 23:05

Pozidrive - The control rod is a hydraulic piston with attachments as hoistop and nodrama say.

The input arm acts on the servo pilot valve to direct hyd fluid to one side or other of that piston, thus moving the control rod in and out.

The reason for having the other end of the input arm connected to the control rod is to provide the follow-up action and recentralise the servo valve when the TR pitch has been driven to the required position.

The control shaft (not rod - the bulletin calls another part "control rod") is not the piston. The bulletin says "the control shaft passes through an outer shaft which forms part of the tail rotor hydraulic actuator piston". So the outer shaft is the piston.

Still no explanation of how the hydraulics act on the control shaft. Sectional drawing anyone?

Pozidrive

12th Dec 2018, 23:10

Pozidrive

12th Dec 2018, 23:16

Crab no disrespect aimed at u or anyone else on here. This is the bit I am struggling to understand and why am asking the experts if it is a hydraulic piston with attachments on both ends how did it end up inside the outer shaft wouldn't it have to break/destroy the servo actuator?

Same here, according to the bulletin the control shaft passes through a hollow piston. There is a direct mechanical connection, with the castellated nuts at each end. The detached nut allowed the control shaft to move beyond its normal travel. So how do the hydraulics act on this shaft?

Pozidrive

12th Dec 2018, 23:21

And the next `Q` is..if they were assembled in reverse order,or even one,what would be the outcome...?

Fig 4 appears to show one outer race, two separate inner races. If so, the bearing is in one piece and not possible to fit with one part reversed.

[email protected]

13th Dec 2018, 04:45

Pozidrive - picture it as a MR servojack, the moving part of the jack pushes upwards and pulls downwards on the swashplate and is a piston acted on directly by hydraulic pressure - the direction of travel is determined by which side of that piston the pilot valve/servo valve directs the pressure fluid.

It is a one piece element, fescalised to reduce wear and the TR servo jack (the control rod) is exactly the same.

nodrama

13th Dec 2018, 06:06

[email protected]

13th Dec 2018, 06:09

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/838x544/trservojack_988a8912ad0f4edc542ac2ed5192ed92340b7138.jpg

Tail Rotor Servo Jack
https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/750x560/mrservojack_0b0d6c12ccf5ae6f8966afc706640c2a7c16a8c9.jpg

Main Rotor Servo Jack
https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/806x580/servovalveandpiston_4971e24b115961946a2bfbba96156f324d959f20 .jpg

Servo valve and piston

Onceapilot

13th Dec 2018, 08:17

Duplex bearings, such as those in the diagram give the same support in both directions.

I think if you look very carefully, you will see in that diagram that the outer races are made with a deeper track on their inside faces so that they can carry a greater axial load in one direction. The diagram does show that it is designed to carry equal load in each direction but, offset. Obviously, there are a huge variety of options that suit different applications. I believe that the "preload" term may be something of a misnomer. The manufacturing tolerances are designed so that as the inner races are clamped, and move together, the clearances of the balls in their races are positioned where they are designed to run so, this part of the process is one of correctly positioning the bearing components. Also worth noting that, unless the parts are Murphy proof, the component bearings might be assembled in wrong order(s), with their characteristics corrupted. Depending upon the method of clamping and holding the inner races, the required torque load on the retaining nut will also be defined and this will be part of the overall design.

OAP

nodrama

13th Dec 2018, 08:31

For those who might be interested, I've compiled a 'tutorial' from another source that hopefully explains duplex bearing pre-load. The duplex bearing that is the subject of this thread is a 'back-to-back' paired bearing and it is the inner race that is pre-loaded. As Crab mentioned, pre-installation, the spider case is heated and the bearing frozen to enable an interference fit on the outer race. The inner race is then pre-loaded with the nut.

From another source- "One of the final steps in the bearing manufacturing process is the assembly of the individual bearing components: the outer ring, inner ring, balls and retainer (or ball separator). When the bearings are assembled, it is necessary to have a controlled amount of internal clearance, or looseness between the rings and balls. This is referred to as radial play in most bearing catalogs.

In certain applications, this internal clearance must be removed for a pair of bearings to operate properly. The application of an axial load across a pair of bearings – for the purpose of removing free internal clearances – is called preload.

Benefits of preloading ball bearings include:

Rotational accuracy and precise shaft positioning
Elimination or reduction of ball skidding
Control and reduction of axial and radial deflection under applied load
Noise reduction
Load sharing between bearings

Preload is critical in most high-precision and high-speed applications, particularly where rotational and positional accuracy is required.

If, under operating conditions, a bearing has radial play this means that one bearing race can be moved radially and axially relative to the other. With rotation, this looseness translates into wobble or non-repetitive runout. This motion is unacceptable in applications such as machine tool spindles, electric motors, optical encoders, flow meters, and high-speed hand tools.

The application of axial preload forces the balls into contact with raceways, establishing a contact angle which causes the ball set to rotate in a uniform circumferential plane.

Duplex bearings are matched pairs of bearings with “built-in” preload. The inner or outer ring faces have been ground to a precise dimension known as the preload offset. This offset corresponds to the rings axial movement when a specific axial preload is applied. When the bearings are clamped together at assembly the offset faces abut, establishing a permanent, rigid preload in the bearing set.

Duplex bearings have increased radial and axial rigidity. There are three common preload configurations. DB (back-to-back) and DF (face-to-face) can handle bi-directional thrust loads. DT (tandem) can handle very heavy unidirectional thrust loads. At higher speeds, these bearings can run hotter due to the rigid preload. These bearings are commonly used in machine tool and other spindle applications, due to their low deflection rate, minimal runout and ease of assembly.

How much preload should be applied?

In general, provided the design requirements are met, the least amount of preload is desired

What are the benefits of preload?

Rotational accuracy and precise shaft positioning, elimination or reduction of ball skidding, control and reduction of axial and radial deflection under applied load, noise reduction, load sharing between bearings

Does preload have any effect on a bearings operational life?

Bearing life decreases as preload is increased

What other side-effects of preload should I know about?

With increased or excessive preload, stresses are higher and excess heat is generated "

sycamore

13th Dec 2018, 09:25

nodrama ,PM for you

Pozidrive

13th Dec 2018, 10:10

Pozidrive - "The control shaft passes through an outer shaft, which forms part of the tail rotor hydraulic actuator piston" is referring to the part of the servo (actuator) that is boxed in Fig 3, the RH part of that boxed area. They are referring to the internal workings of the servo (actuator).

'Control shaft' is the term that the AAIB have chosen to use. Shaft, rod, whatever - it's the same part.

In Crabs diagrams below, your hollow, or outer shaft is the cylinder. In my earlier sketch drawing, it is the dotted line box in the servo.

Sorry, nodrama and crab, I still don't get it.

"The control shaft passes through an outer shaft which forms part of the hydraulic actuator piston" If that description is correct there must be more than one component, and the control shaft is not the piston.

[email protected]

13th Dec 2018, 10:18

hoistop

13th Dec 2018, 10:21

It would be interesting to know the maintenance requirements for and intended service life of the duplex bearing. From what I've discovered already by asking around the bazaars, I understand there is no provision for re-greasing the bearing once installed.

Well, I would not like to see that - this is a critical bearing (like swashplate bearing) and is quite exposed, eventhough protected with boot.
For instance: EC-135 has practically no grease lubricating requirement anywhere on airframe, as it was a goal of designer to make it low maintenance as much as possible. Swashplate bearing is still regularly grease lubricated with grease gun (comes about once a year/400FH)
Just a side note: A109Power had originally T/R duplex bearing grease lubrication schedule every 600hours or 24 months. To put grease in, the whole assembly had to be removed and disassembled to access the bearing (including removal of Tail rotor, as duplex bearing is "around" T/R driveshaft, connected with set of levers and scissors - entirelly different design than AW169)
Then in 2009 a mandatory SB came, requiring change of bearing housing to one with grease nipple and lubrication interval shortened to 100 hours / 6 months.

nodrama, many thanks for clarifying duplex bearing to everyone - nice piece of machinery engineering knowledge.

I still do not know, how exactly the control shaft and hydraulic power are interacting on T/R servo of AW169 :(

nodrama

13th Dec 2018, 10:21

Pozidrive - I can't explain it any better than Crabs last diagram.... the one with the pressure switch in red. Also, replace 'swashplate' for 'spider' in that diagram.

sycamore

13th Dec 2018, 10:26

nodrama-email for you

Onceapilot

13th Dec 2018, 10:30

nodrama,

"Duplex bearings are matched pairs of bearings with “built-in” preload. The inner or outer ring faces have been ground to a precise dimension known as the preload offset. This offset corresponds to the rings axial movement when a specific axial preload is applied. When the bearings are clamped together at assembly the offset faces abut, establishing a permanent, rigid preload in the bearing set"

I think that this quote from your tutorial is probably the important bit. IMO, the "preload" in this case is the achievement of the design running clearance and this is mainly done by the machining process. Where your tutorial refers to preload, I believe that it means precisely this, the amount of bearing clearance (or interference) when the assembly is clamped together, achieved through the machined dimensions of the components. Yes, the assembly will come with a design clamping load or torque setting to achieve and maintain the integrity of the assembly in use.

Where your tutorial goes on to say these comments:

"How much preload should be applied?

In general, provided the design requirements are met, the least amount of preload is desired

What are the benefits of preload?

Rotational accuracy and precise shaft positioning, elimination or reduction of ball skidding, control and reduction of axial and radial deflection under applied load, noise reduction, load sharing between bearings

Does preload have any effect on a bearings operational life?

Bearing life decreases as preload is increased

What other side-effects of preload should I know about?

With increased or excessive preload, stresses are higher and excess heat is generated "

Again, these refer to the machined-in preload, not the amount that the retaining nut is tightened. Obviously, the installation / assembly instructions for the components will be specific.

OAP

[email protected]

13th Dec 2018, 11:08

I still do not know, how exactly the control shaft and hydraulic power are interacting on T/R servo of AW169 https://www.pprune.org/images/smilies/sowee.gif Hoistop - if the bottom diagram in my last but one post doesn't explain it then I have run out of ideas to show you.

The input lever from the yaw controls moves the pilot/spool valve which allows hyd pressure to one side or other of the piston - that piston is part of the control rod.

The feedback link stops the spool valve directing fluid once the piston has moved in the appropriate direction.

It is that feedback link that came undone on the 189 meaning that the follow up action was removed and the piston did what the spool valve asked it to do - and it kept motoring until it reached full travel.

nodrama

13th Dec 2018, 11:36

OAP - you may be correct? It is still possible to relate this sentence ......

"The inner or outer ring faces have been ground to a precise dimension known as the preload offset. This offset corresponds to the rings axial movement when a specific axial preload is applied. When the bearings are clamped together at assembly the offset faces abut, establishing a permanent, rigid preload in the bearing set." ...

..... to the image I posted at #1105 to understand how torquing the nut effects the Duplex bearing.

And maybe this sentence.....

"Duplex bearings are matched pairs of bearings with “built-in” preload"

......should finish "built-in preload offset".

ShyTorque

13th Dec 2018, 11:41

I think if you look very carefully, you will see in that diagram that the outer races are made with a deeper track on their inside faces so that they can carry a greater axial load in one direction. The diagram does show that it is designed to carry equal load in each direction but, offset. Obviously, there are a huge variety of options that suit different applications. I believe that the "preload" term may be something of a misnomer. The manufacturing tolerances are designed so that as the inner races are clamped, and move together, the clearances of the balls in their races are positioned where they are designed to run so, this part of the process is one of correctly positioning the bearing components. Also worth noting that, unless the parts are Murphy proof, the component bearings might be assembled in wrong order(s), with their characteristics corrupted. Depending upon the method of clamping and holding the inner races, the required torque load on the retaining nut will also be defined and this will be part of the overall design.

OAP

I had already looked carefully and saw that the "load lines" on the diagram show equal angles once the pre-load is applied. I'm familiar with Duplex bearings to some extent (I needed to research the subject in the past before buying some for my various vehicle projects).

W.u.W

13th Dec 2018, 11:46

Pozidrive - "The control shaft passes through an outer shaft, which forms part of the tail rotor hydraulic actuator piston" is referring to the part of the servo (actuator) that is boxed in Fig 3, the RH part of that boxed area. They are referring to the internal workings of the servo (actuator).

'Control shaft' is the term that the AAIB have chosen to use. Shaft, rod, whatever - it's the same part.

In Crabs diagrams below, your hollow, or outer shaft is the cylinder. In my earlier sketch drawing, it is the dotted line box in the servo.
And now it makes sense :ugh:, many thanks nodrama for your professionalism and to crabs for the diagrams :ok:

Pozidrive

13th Dec 2018, 12:08

Nodrama - great post, very informative :ok: Thank you.

Pozidrive - I think you are getting too hung up on the wording in the report. My diagrams are of a 139 TR setup which is near identical.

if you replace the word 'which' with the word 'and' then it might make it clearer.

Just trying to clarify an earlier post which said the control shaft was the hydraulic piston. Also to understand why manual control isn't possible - in spite of a direct link from lever to spider via the control shaft.

[email protected]

13th Dec 2018, 12:36

Pozidrive - The input lever is attached to the end of the control shaft (with the nut that did come undone) - then the shaft runs through the TR servo assembly with the piston section as an integral part of the shaft - the shaft then runs out the other side, through the TR drive tube, through the duplex bearing and is attached to the spider with the nut that didn't come undone.

A TR control input moves the input lever, but only enough to displace the pilot/spool valve in the servo - this directs the hyd fluid to move the piston (which is part of the control shaft) which moves the spider and changes the TR pitch.

The feedback/followup movement of the control shaft recentralises the spool/pilot valve in the servo, fluid stops being directed and the TR pitch stops changing.

The whole system is designed to have hydraulic assistance and that allows the autopilot to create inputs to the spool/pilot valve without the TR pedals needing to be moved.

I don't think the input lever has enough mechanical advantage/leverage to move the control shaft in a manual mode, even if you and the control runs were strong enough to move the pedals.

hoistop

13th Dec 2018, 12:57

Hoistop - if the bottom diagram in my last but one post doesn't explain it then I have run out of ideas to show you.

The input lever from the yaw controls moves the pilot/spool valve which allows hyd pressure to one side or other of the piston - that piston is part of the control rod.

Thanks, crab - I understand well, how follow-up hydraulic servo control mechanism works, believe me.
I am (still) wondering if piston is trully part of control shaft, or control shaft passes thru the hydraulic cylinder and is clamped/attached... to (hollow)piston somehow - in other words, did rotation of (undone) control shaft strain seals of hydraulic cylinder in a way they are not designed to or not? It is an important question.
If answer is yes, it means that in case this failure occured in cruise and pilot managed to somehow maintain (some) control, he might be soon faced with massive hydraulic leak - loss of hyd. system as a consequence. If this servoactuator is powered by both hyd systems, (two pistons one on top of other and dual spool valves as in standard dual servoactuators) both systems would probably soon leak hyd. fluid, rendering helicopter uncontrollable.

[email protected]

13th Dec 2018, 13:37

Hoistop - as I mentioned earlier, look at a MR servo jack - the piston is part of the control rod that moves the swash plate - why would you make it more complex by having to clamp a piston to a control rod when you can just machine it in one piece?

If there had been evidence of a hydraulic leak, it would have been mentioned in the initial report.

nodrama

13th Dec 2018, 14:51

Hoistop - As I've said before, I've never seen the inside workings of one of these servo actuators.
The diagram is a schematic of what I think the TR actuator looks like judging by the shape of the one fitted to an AW169. I've also provided an AW169 'technical' description that I have found. I am assuming that 'connected by a flange' is describing how the cylinders are connected together.

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/300x153/10_9_small_791efff54b7e827668c257b2d27c6edabbc1eda4.jpg
"A Linear Tandem Hydraulic Actuator, consisting of a single jack with two pistons housed in two different cylinders mutually connected by means of a flange; the two cylinders receive the control flows from the two control valves and provide the force necessary to drive the pitch control mechanism of the tail rotor blades."

[email protected]

13th Dec 2018, 14:53

Overdrive

13th Dec 2018, 18:11

"Elimination or reduction of ball skidding"

These people should be producing underpants.

DownWest

14th Dec 2018, 06:36

After all the excellent information about duplex bearings and preload offsets , we are still left to find a reason for the bearing failure and the excess torque on the spider end nut.

Unless the split pin at the spider end had come adrift, the control rod can't have rotated relative to the nut to tighten it - is the initial report wrong when it says that the torque is a result of that rotation and if so, how?

Unless there is a clear explanation of that process you come to the conclusion that the nut had been overtightened either on assembly or during servicing. Whether that excess torque directly affected the operation of the duplex bearing or just exacerbated an existing problem with it is one of the questions the AAIB will hopefully answer.

Surely the bearing failed first?, so the inner race turned on the shaft, tightening the nut and shearing the pin? A bit of extra torque on the nut would not make any difference to the bearing clearance unless it deformed the inner races, which is unlikely. As above, the clearance is set by the machining process, the nut only has to apply enough pressure to hold the inner races firmly together.

Aucky

14th Dec 2018, 08:20

... in other words, did rotation of (undone) control shaft strain seals of hydraulic cylinder in a way they are not designed to or not? It is an important question. If answer is yes, it means that in case this failure occured in cruise and pilot managed to somehow maintain (some) control, he might be soon faced with massive hydraulic leak - loss of hyd. system as a consequence. If this servoactuator is powered by both hyd systems, (two pistons one on top of other and dual spool valves as in standard dual servoactuators) both systems would probably soon leak hyd. fluid, rendering helicopter uncontrollable.

I don't know the inner workings of exactly how the inner shaft is coupled to (and therefore pushed/pulled by) the hydraulic actuator and whether rotating the inner shaft could cause a leak, however the Hydraulic system #1 has an automatic TR shut off valve (TRSOV in the diagram). If pressure drops at the TRA pressure switch, or the system level drops to a pre-determied level the TRSOV closes to maintain the remaining system #1 hydraulic fluid within the main rotor actuators to maintain main rotor control.

I.e. a catastrophic loss of the TR hydraulic servo would not in itself lead to a loss of main rotor hydraulics. Such a leak would however lead to a loss of system #2 pressure as there is no TRSOV on that system, so you would lose MR hydraulic redundancy.

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1222x1018/screen_shot_2018_12_14_at_09_14_41_bd68577febf63b192649e1dc4 5c0ccabafe42b6f.png

[email protected]

14th Dec 2018, 09:48

Surely the bearing failed first?, so the inner race turned on the shaft, tightening the nut and shearing the pin? A bit of extra torque on the nut would not make any difference to the bearing clearance unless it deformed the inner races, which is unlikely. As above, the clearance is set by the machining process, the nut only has to apply enough pressure to hold the inner races firmly together. It was the pin and nut on the other end - not the spider/duplex bearing end - that failed and the excessive Tq was on the spider end nut which didn't fail and the split pin was intact. How did the excess Tq happen?

Aucky - just like the 139.

RVDT

14th Dec 2018, 11:19

Crab,

The excess torque could be for a number of reasons and it is only the effort required to remove the nut.

It is easily conceivable that the clamp up was correct.

At first glance in the picture in the report, the TR end of the control shaft there appears to be burnt grease that has migrated along the shaft.

It woudnt surprise me if it migrated to the nut end as well. As you can see it can get in to very small places as there are traces between the outer part of the inner race and the shaft.

The thread seems to be full of it as well. You dont need much friction in the thread to change the torque requirement in the nut. Ever used Loctite? It doesn't have much inherent strength but spread over the area of the thread?

sycamore

14th Dec 2018, 11:34

Nodrama,did you get my email??

nodrama

14th Dec 2018, 12:00

Yes. I replied by email yesterday?

nodrama

14th Dec 2018, 12:13

[email protected]

14th Dec 2018, 12:14

RVDT - I see what you are saying.

Onceapilot

14th Dec 2018, 13:21

Onceapilot

14th Dec 2018, 13:42

I'm not really one for speculating, so the picture below is inferring nothing (except that I drew on it with my foot!). The diagram of the spider assembly to the pitch change rod (shaft) in the report isn't entirely accurate. It doesn't show the metal spacer that is between the nut and the duplex bearing. My picture shows that- nut, washer, spacer, duplex bearing. Note the step, where the spacer couldn't move any further despite what torque was applied to the nut. Maybe this is purposely designed to prevent crushing of the inner race?

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/828x671/rod_end_4063b7c8e2af38f0268b644c13cf8b2f33802d48.png

nodrama,
The step is probably not designed as a stop, it is far too narrow. If you look, there also is a similar step undercut at the inboard end of the bearing seating. This is probably designed so that the inner races very slightly overhang the seating at each end and so, there is no chance of the inner race being wedged up on a radius at the inner end and at the outer end the spacer can also fit against the whole outer end face of the inner race. These bearing races are very hard but quite brittle. Any unequal or point loading can cause fracture.

OAP

nodrama

14th Dec 2018, 13:50

Thanks OAP. That makes sense to me.

[email protected]

14th Dec 2018, 16:11

So what caused the failure?

Since these duplex bearings are widely used in TR assemblies, their failure rate should be very low unless they are not installed properly.

jimjim1

14th Dec 2018, 18:14

Since these duplex bearings are widely used in TR assemblies, their failure rate should be very low unless they are not installed properly.

They are used in vast numbers everywhere in engineering.

I don't really see much of a functional difference between the duplex bearing and double or even single ball bearings which expands the installed base even further.

Onceapilot

14th Dec 2018, 19:27

So what caused the failure?

Since these duplex bearings are widely used in TR assemblies, their failure rate should be very low unless they are not installed properly.

Well, all bearings have some sort of life / failure profile. Off the cuff, I would venture that this application was quite arduous for a relatively small, sealed lubricant bearing so, I would presume that this bearing could fail quite quickly in use after loss of lubricant, or other causes of failure. Can anyone with type knowledge comment if any method of monitoring is used on this bearing?

OAP

Old Farang

15th Dec 2018, 02:29

nodrama,
The step is probably not designed as a stop, it is far too narrow. If you look, there also is a similar step undercut at the inboard end of the bearing seating. This is probably designed so that the inner races very slightly overhang the seating at each end and so, there is no chance of the inner race being wedged up on a radius at the inner end and at the outer end the spacer can also fit against the whole outer end face of the inner race. These bearing races are very hard but quite brittle. Any unequal or point loading can cause fracture.

OAP
This is basically correct. But the "grooves" are stress relieving grooves ground or cut on a radius to prevent a sharp right angle on the shaft material. The outer edge of the inner race of the bearing also has a radius for a similar reason, in addition to ensuring that the pre-load clamping covers the flat face of the bearing without putting any radial force to it which could lead to misalignment.

Onceapilot

15th Dec 2018, 11:44

Basically, thanks for corrupting my correct post Old Farang. ;) But, the groove and the and the step are there specifically to accommodate the engineering needs of the bearing inner races, as I described. Corner radii are used as standard practice and are incorporated as required. If the bearing did not need the inner races to be precisely seated in this way, the shaft would not be waisted by the groove or the step.

OAP

hoistop

17th Dec 2018, 08:58

Onceapilot you are right. Groove at the right end of bearing surface is engineering need for manufacturing (free space for grinding wheel/tool for close tolerance work on shaft) and a stress relief, as sharp corners are not desired there - stress/cracking promotion. On the left side - same thing - manufacturing process requirement - transition to close tolerance surface-not a step for spacer. Anyway, spacer should contact inner race freely, to transmit clamping force from nut. Am surprised sketch on S2/2018 report is not correct, as Nodrama suggests.
From photo it is obvious there is a standard right hand thread provided for bearing nut.

Would be of interest to hear what maintenance/servicing manufacturer put in Instructions for Continuing Airworthiness? As mentioned in my post#1123 A-109E had duplex bearing design and lubrication requirements changed considerably, Bell 212 and 412 requires lubrication of this bearing every 50 hours with warning - two shots of grease only.

Onceapilot

17th Dec 2018, 09:05

Thanks for your input hoistop. Between yourself, nodrama, Old Farang and myself, I think we got this covered!:) The next important thing will probably be some clarity from the investigators on why this failure occured. :sad:

OAP

nodrama

17th Dec 2018, 09:58

Am surprised sketch on S2/2018 report is not correct, as Nodrama suggests.

Not a suggestion, fact.

The picture is from the Training School notes, not from an approved Technical Publication.

[email protected]

17th Dec 2018, 12:52

The next important thing will probably be some clarity from the investigators on why this failure occured which will surely come down to installation issues or maintenance issues unless there is a bad batch of bearings out there...then the lawsuits will start flying!

GrayHorizonsHeli

17th Dec 2018, 18:11

Between yourself, nodrama, Old Farang and myself, I think we got this covered!:)

OAP

damn, I am so glad you all got this...where would we be without the A team.

Pozidrive

18th Dec 2018, 12:17

Thanks, crab - I understand well, how follow-up hydraulic servo control mechanism works, believe me.
I am (still) wondering if piston is trully part of control shaft, or control shaft passes thru the hydraulic cylinder and is clamped/attached... to (hollow)piston somehow - in other words, did rotation of (undone) control shaft strain seals of hydraulic cylinder in a way they are not designed to or not? It is an important question.
If answer is yes, it means that in case this failure occured in cruise and pilot managed to somehow maintain (some) control, he might be soon faced with massive hydraulic leak - loss of hyd. system as a consequence. If this servoactuator is powered by both hyd systems, (two pistons one on top of other and dual spool valves as in standard dual servoactuators) both systems would probably soon leak hyd. fluid, rendering helicopter uncontrollable.

That was my original point, which still hasn't been answered. The bulletin clearly says what you say - "the control shaft passes through an outer shaft, which forms part of the … piston"

Pozidrive

18th Dec 2018, 12:24

It was the pin and nut on the other end - not the spider/duplex bearing end - that failed and the excessive Tq was on the spider end nut which didn't fail and the split pin was intact. How did the excess Tq happen?...

Is it possible a few degrees of movement occurred, enough to slightly deform but not shear the split pin?

sycamore

18th Dec 2018, 14:47

Pozi, I`ll try to clarify a bit, p56 #1117; if you undo the nut at the `spider end’, and the gearbox back-plate, you should be able to remove the hyd. unit complete, leaving the t/r supported on the rotor mast, which has internal splines at its top-end #1102. The spider/slider have splines matching the mast-splines, but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well. The pitch control rod must run through the piston to the feedback link, and the piston limits its travel at each end of the jack... That`s how I see it, unless anyone can produce a full cutaway drawing.

Pozidrive

18th Dec 2018, 17:26

Pozi,I`ll try to clarify a bit,p56 #1117; if you undo the nut at the `spider end`,and the gearbox back-plate,you`should be able to remove the hyd. unit complete,leaving the t/r supported on the rotor mast,which has internal splines at it`s `top-end #1102.The spider/slider have splines matching the mast-splines,but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well.The pitch control rod must run through the piston to the feedback link,and the piston limits it`s travel at each end of the jack...That`s how I see it,unless anyone can produce a full `cutaway drawing.

Thank you, you seem to be agreeing with me!
A sectional drawing of this particular unit would indeed be helpful, some of the examples posted have been "schematic" and don't show the arrangement of the control shaft and outer (hollow) shaft.

Understand how the TR will stay where it is, supported by its own bearings. But wouldn't removing and replacing the hydraulic unit be complicated by the fit of the duplex bearing? However, this isn't really relevant to the incident.

nodrama

18th Dec 2018, 19:21

Understand how the TR will stay where it is, supported by its own bearings. But wouldn't removing and replacing the hydraulic unit be complicated by the fit of the duplex bearing? However, this isn't really relevant to the incident.

No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. There’s the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, and some electrical plugs. That’s it. Very straight forward.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox.

Sir Korsky

18th Dec 2018, 19:42

What percentage on the #1 Hydro system will auto-close the TRSOV after fluid loss/leakage?

nodrama

18th Dec 2018, 19:43

The spider/slider have splines matching the mast-splines, but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well.

The spider slider isn’t splined on this aircraft type. The scissor links transfer the rotation of the tail rotor hub to the spider.
Yes, there is a ‘nut’ holding the tail rotor hub on (there is always a ‘nut’), though it doesn’t look like a nut in the conventional sense.

nodrama

18th Dec 2018, 19:59

What percentage on the #1 Hydro system will auto-close the TRSOV after fluid loss/leakage?

55% reservoir quantity

Pozidrive

18th Dec 2018, 20:55

No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. There’s the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, a some electrical plugs. That’s it. Very straight forward.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox.

Perfectly explained. So the control shaft is an easy sliding fit through the inner race of the duplex bearing? And it's the castellated nut (and spacer) that clamp the inner race to the step at the outer end of the control shaft?

Only question left is the detail of the control shaft/outer shaft/hydraulic piston, which isn't really relevant.

nodrama

18th Dec 2018, 21:12

So the control shaft is an easy sliding fit through the inner race of the duplex bearing? And it's the castellated nut (and spacer) that clamp the inner race to the step at the outer end of the control shaft?
.

Yes.
I can’t help you with a detailed diagram of the servo actuator system. I’m not involved with component strip and overhaul.

weemonkey

18th Dec 2018, 21:23

No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. There’s the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, and some electrical plugs. That’s it. Very straight forward.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox.

I appreciate your contribution, however there is something in your delivery that sets the weemonkey "hang on" alarm going...

nodrama

18th Dec 2018, 21:27

No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. There’s the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, and some electrical plugs. That’s it. Very straight forward.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox.

Part of the above is incorrect (I was watching the football, pathetic penalty shoot-out). The nut on the servo end of the pitch control rod doesn’t get touched during an actuator replacement. It is part of the hyd actuator component and assembled/ locked at manufacture of the component.

Sir Korsky

18th Dec 2018, 21:44

55% reservoir quantity
Thanks ND - also, do you know if hot fluid is the only time an EP permits closing the main 1(2) SOV ?

nodrama

19th Dec 2018, 07:55

Thanks ND - also, do you know if hot fluid is the only time an EP permits closing the main 1(2) SOV ?

OPC ? 🙂

EP instructs main SOV closure for high fluid temperature and low fluid quantity (providing other system is serviceable)

Pozidrive

19th Dec 2018, 12:34

Part of the above is incorrect (I was watching the football, pathetic penalty shoot-out). The nut on the servo end of the pitch control rod doesn’t get touched during an actuator replacement. It is part of the hyd actuator component and assembled/ locked at manufacture of the component.

Yes, understood. That bit is clearly shown in the bulletin Figures.

In spite of the football, it was still a good explanation!

Sir Korsky

19th Dec 2018, 16:16

OPC ? 🙂

Not quite, but thanks again for your answer. I'm just interested in evolution/progression of Augusta design logic.

MurphyWasRight

18th Jun 2020, 16:54

Have not seen any updates on investigation. Wondering if I missed something posted elsewhere.
Not sure when updates would be expected.
thanks

W u W

19th Jun 2020, 06:16

Have not seen any updates on investigation. Wondering if I missed something posted elsewhere.
Not sure when updates would be expected.
thanks

Still under investigation no new updates from the aaib since October, not even been changed to finalizing, they must be struggling with something in my opinion.

WuW

Bravo73

19th Jun 2020, 16:22

I suspect that Covid 19 hasn’t helped with the speed of their investigations.

chopjock

20th Jun 2020, 10:12

Still under investigation no new updates from the aaib since October, not even been changed to finalizing, they must be struggling with something in my opinion.

WuW

Perhaps they are just baffled by how a heavily regulated supposedly ultra safe Cat A take off profile with all those engines can still manage to crash and burn...

W u W

20th Jun 2020, 13:42

Perhaps they are just baffled by how a heavily regulated supposedly ultra safe Cat A take off profile with all those engines can still manage to crash and burn...

Perhaps you are right chop, I have followed you for awhile, I can't help but think this is one rare instance that hindsight quick thinking thought processes couldn't of turned this pilot into a hero :(, I try to stay in the background on this forum, this thread gets to me, RIP to all involved.

WuW

jimjim1

21st Jun 2020, 00:14

Is this not the one where the tail rotor bearing failed resulting in the tail rotor not rotating anymore? What went wrong is known. The causal chain is not (yet public).

etudiant

21st Jun 2020, 00:25

Perhaps they are just baffled by how a heavily regulated supposedly ultra safe Cat A take off profile with all those engines can still manage to crash and burn...

Exactly. Having a well supported private aircraft kill everyone on board during a routine takeoff does not inspire confidence.
Evidently there was no obvious design defect, so the investigators are now clutching at straws.

Monty Niveau

21st Jun 2020, 08:02

Evidently there was no obvious design defect, so the investigators are now clutching at straws.

Your ‘Evidently’ interests me. What evidence are you referring to?

W u W

21st Jun 2020, 08:08

Is this not the one where the tail rotor bearing failed resulting in the tail rotor not rotating anymore? What went wrong is known. The causal chain is not (yet public).

I believe TR did not lose rpm but went full pitch. The chain of how that happened is all in this thread.

They could be clutching at straws struggling to try work out how a simple component (duplex bearing) can fail in away that's not been seen before and lead to such a catastrophic loss of control and what could be done to prevent it again. Just a opinion :ok:

WuW

ShyTorque

21st Jun 2020, 08:58

Is this not the one where the tail rotor bearing failed resulting in the tail rotor not rotating anymore? What went wrong is known. The causal chain is not (yet public).

No, it was a tail rotor control failure. The tail rotor continued turning but the blades ran to full negative pitch, which is worse than a failed tail rotor drive.

[email protected]

21st Jun 2020, 09:38

Perhaps they are just baffled by how a heavily regulated supposedly ultra safe Cat A take off profile with all those engines can still manage to crash and burn... poor maintenance procedures will get you whatever other protections you put in place.

212man

21st Jun 2020, 11:05

poor maintenance procedures will get you whatever other protections you put in place.
let’s see if this component was ever touched after it left the factory - pretty low hours I think.

Monty Niveau

21st Jun 2020, 20:50

Well, none of the above should take anything like as long as the enormously expensive AAIB are taking. And I invite those who think, unhesitatingly, to defend the organisation, to pause for once and consider the facts.

[email protected]

21st Jun 2020, 21:49

let’s see if this component was ever touched after it left the factory - pretty low hours I think.That leaves you with poor design or incorrect installation - nothing you can do as a pilot will affect those.

John R81

23rd Jun 2020, 07:59

Monty N

Slightly naive comment, as it is not just the technical members of the AAIB who determine when a report is released. I can think of delayed reports in the past where it has not been the AAIB or the investigation that were the problem, it was the lawyers for the party who appears to be "at fault" and thereby facing financial and reputational consequences, who then tried every avenue to "water down" the report findings before publication.

This particular airframe was one of the first in production, and it had reasonable use. One might therefore expect any issues in design, original build or maintenance scheduling to become apparent on an airframe like this. The information released so far indicates that a duplex bearing in the tail rotor control control mechanism failed and AW issued a service bulletin requiring this part to be checked in all 169 & 189 back in 2018. I guess that both AW and the AAIB know the result of that check, but to my knowledge that result has not been published. It would be interesting to hear from any owner or mechanic who can say what this check found.

etudiant

23rd Jun 2020, 09:06

Your ‘Evidently’ interests me. What evidence are you referring to?

The type was not grounded, nor was there any immediate service bulletin, as would have been expected had there been an obvious issue.
The cause, a duplex bearing failure, did get a service bulletin as noted by Jahn R81, as a result of the investigation.

ericferret

23rd Jun 2020, 20:15

The type was not grounded, nor was there any immediate service bulletin, as would have been expected had there been an obvious issue.
The cause, a duplex bearing failure, did get a service bulletin as noted by Jahn R81, as a result of the investigation.

The current situation is that the actuator has been modified by replacing the right hand thread at the input end of the control shaft with a left hand thread.
This removes the repetetive inspection on the nut for loss of torque.
The 10 hour inspection on the duplex bearing remains. As does the rotational force check at 100(?) hours.

A number of duplex bearings have been replaced on a precautionary basis, I believe so far they have all been found to be serviceable.

My guess is a one off failure, either a manufacturing fault on the bearing or maybe it just didn't get greased.

John R81

25th Jun 2020, 13:59

Ericferret - Thanks for the input. The input control shaft redesign makes sense. Has there been any redesign of the duplex bearing (I know some were swapped-out under the SB, I don't refer to that)?

ShyTorque

25th Jun 2020, 15:37

I hope the design issue was also a one-off failure.

ericferret

25th Jun 2020, 17:38

Ericferret - Thanks for the input. The input control shaft redesign makes sense. Has there been any redesign of the duplex bearing (I know some were swapped-out under the SB, I don't refer to that)?
Not as far as I am aware. We have carried out repetitive inspections probably running into well into three figures with no issues.

ericferret

25th Jun 2020, 17:42

Which design issue?

ShyTorque

25th Jun 2020, 18:20

Which design issue?

The design issue where a seized bearing resulted in the single nut holding the tail rotor control mechanism in place breaking an ineffective locking pin and unwinding. As stated above, the design has since
been altered so that the nut does not undo; simply by making the thread left handed.

MightyGem

25th Jun 2020, 19:59

As stated above, the design has since
been altered so that the nut does not undo; simply by making the thread left handed.
I'm surprised that it wasn't designed like that in the first place.

ShyTorque

25th Jun 2020, 20:18

Precisely! I'd have thought an even safer design would be one where the nut on the control shaft isn't fixed but can rotate on its own bearing.

W u W

26th Jun 2020, 06:53

Precisely! I'd have thought an even safer design would be one where the nut on the control shaft isn't fixed but can rotate on its own bearing.

I believe this would be the safest configuration, because my question is if there was enough force and heat at play to weld the washer to the nut and snap the locking pin when the control shaft started to rotate as not designed is that not significant force to cause just as bad damage on a left-handed thread configuration if the same fault occurred?

WuW

[email protected]

26th Jun 2020, 08:47

Yes, the binding and subsequent overheating of the bearing must be the main issue.

ericferret

27th Jun 2020, 10:09

All helicopters are a compromise, weight,complexity, cost. The 169 has clearly been built down to a weight. Hence the hundreds of titanium panel screws. Around 30 euro per screw.
The basic design philosophy for the duplex bearing is sound. The 139 hit 2 million flying hours, two and a half years ago with no undetected failures. So it is not unreasonable to use this system.

All helicopters have multiple areas where a single failure will cause the loss of the aircraft, if you design them all out the aircraft goes nowhere due to weight constraints bought on by duplication.

At the moment we have the tail servo mod, however, I am sure that a redesign is on the cards. This is not a quick fix and will require re-certification.
The idea of a second bearing at the input end of the control shaft seems a good one.
Helicopters tend to evolve and few have entered service without at least one major defect.

Worth remembering that more than one S76 was lost due to tail rotor control failure, main rotor head failure, not to mention engines chucking out turbine wheels. In comparison so far the 169 has seen a better introduction to service.

[email protected]

27th Jun 2020, 11:40

But the S-76 was designed over 40 years ago. We are supposed to learn from our mistakes, not keep repeating them.

John R81

29th Jun 2020, 19:09

A fellow pilot referred me to CAA Paper 2003/1 (available here https://publicapps.caa.co.uk/docs/33/CAPAP2003_01.PDF). Makes interesting reading, especially compared to the current climate 17 years later

skridlov

30th Jun 2020, 15:05

ShyTorque

30th Jun 2020, 20:51

The part that rotated, the tail rotor pitch control shaft, wasn't supposed to rotate at all. Although it runs through and parallel with the (rotating) main tail rotor shaft it's only supposed to push/pull. Problem was, when one of the bearings that separated the two shafts seized, both locked and rotated together and this spun off the stationary retaining nut, breaking straight through its locking pin.

On a critically important system such as the tail rotor pitch control mechanism, the seizure of a relatively small bearing shouldn't have been allowed to cause a completely irretrievable situation without any prior warning. The design just didn't cater for the control shaft being spun up like it did.

Once the tail rotor pitch went to full negative, the crew had no chance of recovery.

[email protected]

30th Jun 2020, 21:21

Once the tail rotor pitch went to full negative, the crew had no chance of recovery. yup, no-one practices for that one - it's usually stuck pedals or a pitch control failure that allows the TR to go to min pitch not full negative.

Echo Romeo

12th Dec 2020, 17:56

Have the AAIB published the final accident report on this? I can find no reference at all on the Gov AAIB search facility.

Bravo73

12th Dec 2020, 18:16

No, not yet.

No, not yet (for the benefit of the min character count).

srobarts

12th Dec 2020, 18:18

Showing as Consultation Stage on AAIB website.

"Consultation stage - when an investigation is largely complete and a confidential draft report has been sent out for formal consultation in accordance with the regulations. The consultation process includes the time taken to consider representations and amend the draft report prior to publication."

John R81

22nd Feb 2021, 13:59

Sort of embarrassing to be still stuck here. Crash in October 2018. Rapid progress to find the initial facts (SB1/2018 Nov 2018) and to find out what actually failed (SB2/18 Dec 2018), then two years later nothing published. The report is in consultation for some time now, which I take to mean that the lawyers for someone don't like the potential consequences of what it says.

Requiring the swap to LH thread on the rod seems like a "and in any event..." fix to me, as the bearing separating the control rod from the tail rotor drive shaft should not have failed in the first place. No doubt the report says why that bearing failed, as this is the information needed to ensure that t does not happen again. .

JulieAndrews

23rd Feb 2021, 09:07

OvertHawk

23rd Feb 2021, 10:30

agree - very embarrassing - not to mention other (more straight-forward?) reports which appear to be taking an inordinate amount of time to be published (S-92). Kind of begs the question "is our renowned AAIB still truly independent"?

Which S-92 are you referring to? If it's the Irish SAR machine (report significantly delayed) then I would remind you that that accident is being investigated by the Irish AAIU, not UK AAIB.

jimf671

23rd Feb 2021, 15:26

It's been a few years since I had the temerity to post on PPRUNE but something I just read caused a double-take:
"The current situation is that the actuator has been modified by replacing the right hand thread at the input end of the control shaft with a left hand thread.
This removes the repetetive inspection on the nut for loss of torque."
I used to operate a couple of 1940s ex-US Army 6x6 Studebakers. All the wheel nuts on the L/H side wheels were left-hand thread. That's 1940s: wheels: on a truck...
These days I work on vintage watches, dating back to WW1 in some cases. Almost without exception the crown-wheels, which rotate counter-clockwise, have left-hand threads. There are some esoteric exceptions, but...
It boggles my easily boggled ageing mind to think that this strategy could have been ignored on a HELICOPTER! Blimey.

That was a popular solution on truck wheels and also a few vans and high performance cars in the 1960s. This method has died out now that truck manufacturers have found out how to do proper fastener design, tighten them properly, and use reliable tightening methods. Truck wheels are generally no longer user serviceable because of the high torques and specialist tools.

Specialist fastener design is troublesome whenever the format of the threaded components prevent the male threaded element taking the role of a spring. Every standard bolt is designed to act as a very stiff spring and effective reliable tightening requires that spring to be taken to near its limit. If the format of the threaded components, or the low compressive strength of the components being retained by the threaded components, prevents taking the spring to its limit then there is a serious problem. This serious problem is extremely common, even in automotive and aviation sectors where one might think things are all sorted! This explains all the split pins, locking wire, adhesives, and so on that we are familiar with in those industries. It also explains why in modern designs we can find fasteners abandoned and they just glue it if they can!

Aviation also has the Titanium problem. This is because Ti behaviour makes it probably the worst material for threaded fasteners that is in regular use. (S-92 oil housing, Cougar 91!!!)

Echo Romeo

11th Jul 2021, 10:53

Find it incredible that the AAIB still haven't published this report. What is going on, or not ?!

Arnie Madsen

12th Jul 2021, 01:19

The mind of a bureaucrat is different than ours.
The bureaucrat will never have a component failure while sitting at his desk investigating component failures in the world of aviation. Unless his coffee machine breaks down or something.

That is why I always say pprune discussions like this serve a useful function .... within days we will speculate on all the possibilities and it eventually gets narrowed down to just a few ..... pilots and mechanics will pay attention and maybe even check their own machines.

In the case of the Leicester crash we pretty well know what happened ... we just dont know where the finger points as to the cause .... was it people ... metal ... or design .... that is the tedious job of the investigators to sort out.

Same as the Cougar S-92 that went down in Newfoundland ..... within a few hours this forum discussion had pretty much narrowed it down to the titanium stud failure on the MTX oil filter ..... 3 years later investigators confirmed it.

For that reason I do not agree with folks who tell us not to speculate , but tell us to wait years for the official report .

Such folks have the mindset of a bureaucrat.

GrayHorizonsHeli

30th Oct 2021, 17:22

https://www.leicestermercury.co.uk/news/leicester-news/families-pilots-sue-helicopter-part-6118515

gmrwiz

9th Sep 2022, 15:22

Any news about the final report of this accident?

FloaterNorthWest

28th Oct 2022, 16:08

https://www.gov.uk/government/news/aw169-helicopter-g-vskp-anniversary-statement
The AAIB has now concluded its investigation into the accident and expects to publish a detailed final report in early 2023.

Mike Flynn

28th Oct 2022, 18:47

212man

28th Oct 2022, 22:12

That took long enough.
One recommendation should be that vanity flights in to stadiums should be banned. It was lucky that just the crew and passengers died in this incident.

My deepest sympathy goes to the crew who like the incident at Beccles were servants to those in the back.
When you say Beccles I assume you mean the AW139 accident with Lord Ballyedmond? I fail to see the connection. It is clear the AW169 accident was due to a mechanical fault and the outcome exacerbated by an unfortunate impact with a small wall (that ruptured the fuel tanks). There is no suggestion of pressure on the crew or crew competence, unlike the 139 accident. So what’s your point?

gsa

29th Oct 2022, 02:42

When you say Beccles I assume you mean the AW139 accident with Lord Ballyedmond? I fail to see the connection. It is clear the AW169 accident was due to a mechanical fault and the outcome exacerbated by an unfortunate impact with a small wall (that ruptured the fuel tanks). There is no suggestion of pressure on the crew or crew competence, unlike the 139 accident. So what’s your point?

i think that he hasn’t worked out yet the difference between saying no and a mechanical failure, Captaincy has the priority no matter who the talking and paying bagage Is.

Hot and Hi

29th Oct 2022, 08:55

That took long enough.
One recommendation should be that vanity flights in to stadiums should be banned. It was lucky that just the crew and passengers died in this incident.

My deepest sympathy goes to the crew who like the incident at Beccles were servants to those in the back.

There seem to be a number of people on this network who feel that flying is an inherently bad and asocial activity. And that it should be restricted to the pursuit of saving the lives of (worthy) people, or the pursuit of taking the lives of the enemy.

Not surprisingly, many of those who carry this grudge have themselves indulged in the joy of flying their entire career using no expenses spared government-funded hardware, were trained to the highest standards by tax payers money and were exempted from many of the inconvenient regulations that govern the world of civilian flight.

I think it's a bit rich.

JulieAndrews

29th Oct 2022, 11:17

Mike Flynn

29th Oct 2022, 19:15

'Vanity Flight' my ar5e. Football pitch-sized HLS for a Perf A departure in a highly capable machine, get a grip. Or are you suggesting that helicopters should only be used on the understanding that a new machine will suffer from a mechanical/servicing HF failure at any time? I admit that I always assume something will go wrong just to combat possible 'startle effect' and to be prepared but your mentality means choppers should only land and depart using clear areas - which kind of negates the whole point of a helicopter......#darkages

In two words risk assessment.

I spend a lot of my time in Asia where the owners of King Power have vast commercial monopolies.

The football match prior to the accident was to highlight their chief executive and promote their brand.They arrived near the stadium at the training ground 1.5 miles away but chose to leave from the stadium after the match as this departure was being screened live in Thailand.

The risk although slight of such a mishap is not worth publicity gained by such a stunt.Demands from very wealthy people often lead to the death of pilots put under pressure by their employers.

In this case it went wrong as did this accident at Beccles in Norfolk.https://www.theguardian.com/uk-news/2014/mar/14/norfolk-helicopter-crash-northern-ireland-lord-ballyedmond

SASless

29th Oct 2022, 20:00

Stunt?

Did any authority suggest there was a violation of any Rule or Regulation?

MightyGem

29th Oct 2022, 20:49

In two words risk assessment.
The risk although slight of such a mishap is not worth publicity gained by such a stunt.Demands from very wealthy people often lead to the death of pilots put under pressure by their employers.

The risk associated with that departure was no higher than a departure from any other site that required that type of departure profile. Given the cause of the accident, it would have happened on the next flight anyway, and who can say what the outcome would have been.

jeepys

29th Oct 2022, 21:06

You simply cannot compare this accident to the Norfolk one. This was mechanical failure, the other was pilot error/CFIT of a perfectly serviceable helicopter.
Yes, I agree, if this helicopter took off from a 2km runway and the bearing gave up soon after lifting, then maybe just maybe the outcome may have been different (but not vastly). If the bearing gave up on approach to a 2km runway it could have ended up in someone's front room.
We mitigate against risk as much as possible every time we fly, but if we continually fly a helicopter like a plane, we may as well get a plane.
If you want to mitigate risk to zero, then lock the hangar doors, eat Ryvita and drink tomato juice. 10 years later write a 1 page book on how interesting your life is!

SASless

29th Oct 2022, 22:59

Mike Flynn,

Perhaps you might provide us a copy of the Risk Assessment Protocol that you are using to predicate your evaluation of the Risks for the Takeoff performed that sad day so that we can all have a better understanding of exactly why you have the opinion you do re that event.

It might prove useful to be able to demonstrate the benefit of the Risk Analysis you do for your own operations and perhaps others might consider incorporating yours into their theirs.

Brutal

30th Oct 2022, 09:25

Oh c'mon Mike Flynn....I guess you better give up your car as the chances of an accident and someone being killed are 10,000 fold? Was there really a need for you to pop to the shops that night for a bottle of wine, or get that ever so important takeaway, every journey being a risk to someone? If you go out for a walk do you wear a crash helmet incase of falling debris from an airliner? There was nothing risky about that departure.... just bad luck on the crew. I suggest you light up some candles in your house, and take cold showers, so that the boys and girls don't have to risk their lives going to oil/gas/wind platforms/farms in the N.Sea for our energy needs, as this is way more risky than departing a stadium...just look at the terrible loss of life over the last 20 years..:ugh:

B.

Two's in

30th Oct 2022, 15:38

...My deepest sympathy goes to the crew who like the incident at Beccles were servants to those in the back.

If you don't understand the conduct of every flight is entirely down to Captaincy, you probably shouldn't be a Captain.

ShyTorque

30th Oct 2022, 20:31

I suggest we stay on topic….this accident was caused by mechanical failure, rather than a captaincy issue.

Pickuptruck

14th Nov 2022, 14:09

I suggest we stay on topic….this accident was caused by mechanical failure, rather than a captaincy issue.
Was the accident caused by mechanical failure or the pilots inability to cope with or action training to cope with the mechanical failure? I have no idea the recurrent training that UK licences go through but as a long retired FAA ATPL holder the failure of the tail rotor assembly, or all manner of associated failures was beaten to death in annual proficiency checks in North America. I'm at a loss as to why so many posters on here say in the UK you never train for this, I'd like to hear why.

gipsymagpie

14th Nov 2022, 14:17

Pickuptruck

14th Nov 2022, 14:49

It went to sub-minimum pitch which is below the lowest possible power setting conceived by the manufacturer. So even if you executed as much training you like it was beyond saving from a hover. And we do train for the scenario - a lot.
I think it was at 430 ft? You couldn't get the collective down and the power off and get some forward airspeed and airflow over the fuselage? What did you practice when you train for that scenario since I'm failing to see how it was unrecoverable.

hargreaves99

14th Nov 2022, 15:05

mmm. yeah, good luck with that, especially at night

I imagine most pilots with a tail rotor failure in a hover, at height, would not be able to deal with it. after two rotations it's game over

I think it was at 430 ft? You couldn't get the collective down and the power off and get some forward airspeed and airflow over the fuselage? What did you practice when you train for that scenario since I'm failing to see how it was unrecoverable.

gipsymagpie

14th Nov 2022, 15:44

The tail rotor wasn't failed, it was sub minimum pitch. So even if you snapped the collective down it would continue to spin left (the tail rotor is driving the thing left rather than the yaw being a function of the torque reaction). So even with the lever full down and cyclic stuffed forward, you wouldn't gain much airspeed as you're yawing hard left despite the lever being down.

Of note the minimum position of the tail rotor is determined in design by some flight condition where left yaw is required at very low power. On several types I've operated this was defined (and tested during a maintenance test flight) as the ability to yaw left at a reasonable rate (cannot recall the exact condition) whilst in a zero speed autorotation (great fun to do that flight test). In this incident the yaw setting was even less than this due to the mode of failure. That's a lot of yaw and even by day at several thousand feet I would suggest you would struggle. I would expect the required recovery action would be to lower the lever, pitch hard nose down and roll - I think roll to oppose the yaw (and also further reducing power/torque requirements) so you could accelerate but rolling into the yaw might offer the chance to align the fuselage with the airflow. Pure speculation as you could never test it.

Those guys had a awful roll of the dice that night.

212man

14th Nov 2022, 16:22

You couldn't get the collective down and the power off and get some forward airspeed and airflow over the fuselage? What did you practice when you train for that scenario since I'm failing to see how it was unrecoverable.

Because the scenario they had is not the one in your head. I think their bad luck was compounded by landing on a small wall which ruptured the fuel tanks. It may well have been survivable up until that point.

ShyTorque

14th Nov 2022, 17:48

Was the accident caused by mechanical failure or the pilots inability to cope with or action training to cope with the mechanical failure? I have no idea the recurrent training that UK licences go through but as a long retired FAA ATPL holder the failure of the tail rotor assembly, or all manner of associated failures was beaten to death in annual proficiency checks in North America. I'm at a loss as to why so many posters on here say in the UK you never train for this, I'd like to hear why.

I thought this had already been discussed at some length earlier on.

A tail rotor drive failure, believe it or not, is relatively straightforward to deal with, albeit needing altitude and time once the engines are shut down.

A tail rotor control failure is most definitely not the same kettle of fish and is likely to be far more difficult to deal with, unless the tail rotor blade pitch remains at a setting close to neutral. A control runaway to maximum positive pitch may require maximum collective pitch to slow the yaw (and then what, how do you get the aircraft down under control?).

A tail rotor which runs away to minimum pitch will very likely have enough authority to yaw the aircraft at a high rate even after the engines are shut down. If you think about this, a helicopter in full auto rotation needs enough tail rotor authority to enable full yaw control in both directions, so there is more negative pitch designed in than some might realise. For example, one aircraft I used to instruct (RAF Puma) had something like 35 degrees of positive pitch and 17 degrees of negative pitch. We regularly used a full motion simulator to practice tail rotor malfunctions of all types and it showed that runaways to negative pitch were usually impossible to deal with, especially if the failure occurred at high engine power settings, as in the Leicester accident being discussed here. The tail rotor control system failed and drove the tail rotor blades to full negative pitch. As recently stated by others here, shutting the engines down immediately the failure occurred would not have stopped the yawing.

A helicopter yawing out of control in this instance is unlikely to remain stable in the roll and pitch attitudes and may go completely out of control despite the best crew in the world sitting in the cockpit.

HeliComparator

14th Nov 2022, 21:46

I think it was at 430 ft? You couldn't get the collective down and the power off and get some forward airspeed and airflow over the fuselage? What did you practice when you train for that scenario since I'm failing to see how it was unrecoverable.

Seems likely that you are not a helicopter pilot. Or if you are, not one I’d like to fly with.

Pickuptruck

15th Nov 2022, 01:41

I thought this had already been discussed at some length earlier on.

A tail rotor drive failure, believe it or not, is relatively straightforward to deal with, albeit needing altitude and time once the engines are shut down.

A tail rotor control failure is most definitely not the same kettle of fish and is likely to be far more difficult to deal with, unless the tail rotor blade pitch remains at a setting close to neutral. A control runaway to maximum positive pitch may require maximum collective pitch to slow the yaw (and then what, how do you get the aircraft down under control?).

A tail rotor which runs away to minimum pitch will very likely have enough authority to yaw the aircraft at a high rate even after the engines are shut down. If you think about this, a helicopter in full auto rotation needs enough tail rotor authority to enable full yaw control in both directions, so there is more negative pitch designed in than some might realise. For example, one aircraft I used to instruct (RAF Puma) had something like 35 degrees of positive pitch and 17 degrees of negative pitch. We regularly used a full motion simulator to practice tail rotor malfunctions of all types and it showed that runaways to negative pitch were usually impossible to deal with, especially if the failure occurred at high engine power settings, as in the Leicester accident being discussed here. The tail rotor control system failed and drove the tail rotor blades to full negative pitch. As recently stated by others here, shutting the engines down immediately the failure occurred would not have stopped the yawing.

A helicopter yawing out of control in this instance is unlikely to remain stable in the roll and pitch attitudes and may go completely out of control despite the best crew in the world sitting in the cockpit.
Thank you for the explanation.

212man

15th Nov 2022, 12:19

Thank you for the explanation.
Out of interest, how exactly did you train in the machine for TR failure? What ever you did, I doubt it was an accurate reflection of reality.

Pickuptruck

15th Nov 2022, 12:33

Out of interest, how exactly did you train in the machine for TR failure? What ever you did, I doubt it was an accurate reflection of reality.
It was a few decades ago and you're probably right, pedals jammed full left or full right or locked centre. Check airman was retired USMC.

MightyGem

15th Nov 2022, 20:33

It was a few decades ago and you're probably right, pedals jammed full left or full right or locked centre. Check airman was retired USMC.
Yes, done it that way myself.

ShyTorque

15th Nov 2022, 22:21

Yes, done it that way myself.

So did the RAF. From what I experienced, for many years too little thought was put into the subject. When we first began using a capable simulator for tail rotor control malfunctions, rather than the “more simple” tail rotor drive failures, the learning curve was steep. It was a startling revelation.

MightyGem

16th Nov 2022, 20:19

So did the RAF. From what I experienced, for many years too little thought was put into the subject. When we first began using a capable simulator for tail rotor control malfunctions, rather than the “more simple” tail rotor drive failures, the learning curve was steep. It was a startling revelation.
Yes, in the sim you could save a Lynx from a TRDS failure in the cruise enough to choose when to enter into autorotation. Whether it would work in real life??

ShyTorque

16th Nov 2022, 22:08

Yes, in the sim you could save a Lynx from a TRDS failure in the cruise enough to choose when to enter into autorotation. Whether it would work in real life??

That would depend on how it had been programmed, of course.

The important lesson to learn is that there are different types of TR malfunction apart from driveshaft failure and they need some thought, preferably before they occur in the real aircraft - Hence the value of a simulator.

SASless

16th Nov 2022, 22:21

There are more ways to control yaw thrust than by moving the pedals....far too few helicopter pilots fully understand that.

Bell designs with the Throttle(s) on the Collective make it fairly easy.... but aircraft with conventional ECL's can also be made to work in a similar manner.

But....in the accident under discussion....there was a very unusual mechanical failure that certainly made a successful recovery very unlikely under the circumstances the Crew were confronted with.

KRviator

17th Nov 2022, 19:43

There are more ways to control yaw thrust than by moving the pedals....far too few helicopter pilots fully understand that.Could you expand a little on that for this non-rotary-winged pilot? I (think) I can understand the basics whereby pulling pitch increases the yaw tendency, but that's the extent of my understanding and I'd like to learn a bit more more.

SASless

17th Nov 2022, 20:11

Think of the tail rotor much like a constant speed Prop on an airplane....the amount of thrust produced can be varied by changing the pitch of the Prop....or if you leave the Prop Pitch constant....you can alter the thrust produced by changing the RPM.

Assuming the Tail Rotor is turning....and it is a fixed control problem causing the issue....one can vary the RPM by means of the Engine(s) throttle(s).

Collective setting or movement can also affect yaw in that situation by increasing or decreasing the amount of Torque of the Main Rotor produces.

Adding Collective produces more an increase in torque.....and the opposite when Collective is reduced.

Page 11-16 provides the FAA discussion of Tail Rotor Malfunctions.

https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/hfh_ch11.pdf

212man

17th Nov 2022, 21:03

ShyTorque

17th Nov 2022, 23:19

SAS,

That paper contains useful information on tail rotor drive failure and “stuck pedals” but it does not cover the very unusual situation suffered by the Leicester crew.

(I can’t imagine a situation where any helicopter in normal operational flight would need full negative tail rotor pitch).

During a normal Class A rearwards climb, such as this was, the pilot would have been applying a large amount of positive tail rotor pitch.

The aircraft mechanical controls failure suddenly gave him the maximum possible opposite tail rotor input. As I said before, I doubt very much that any pilot would have been able to recover from that situation. I’ve seen it in a simulator many times, both as a handling pilot and watching (and trying to assist and train from the instructor point of view).

This was far more severe than stuck pedals or even total loss of tail rotor power through drive shaft failure and even more so because the aircraft was in a low IAS climb, in accordance with that type of departure. The only hope would be to rapidly achieve full auto rotation and even if that was achievable (it probably wasn’t because of the rapid onset of a high yaw rate) the pilot would still be trying to deal with uncontrollable yaw at very low level - and in the dark!

For those pilots not familiar with helicopters, the closest analogy I can think of in fixed wing terms is for an aeroplane with a high torque propellor engine to be in a minimum IAS climb. The pilot is keeping the aircraft straight using ant-torque rudder. A mechanical failure causes sudden full opposite rudder to be applied, causing a high rotational spin to develop with full pro spin rudder jammed on.

SASless

17th Nov 2022, 23:32

Shy....also remember Simulators "simulate" not "replicate" actual aircraft flight characteristics.

Computers can do only so much despite being very useful bits of kit

We havre to remind folks not to hang their hats on a direct transfer from the real to the surreal in those pretty shiny boxes that oft times reek of someone's spilled lunch.

Plenty of very experienced Aviators have gotten Sim Sick who have never been Air Sick in their careers.

My. post was to offer an analogy that a fixed wing only pilot might be able to use to better understand the basic concepts.

KRviator

18th Nov 2022, 01:32

ShyTorque

18th Nov 2022, 06:38

Shy....also remember Simulators "simulate" not "replicate" actual aircraft flight characteristics

Yes, hence the first line of my post #1216.

There is no way that a manufacturer could obtain fully representative aerodynamic data for all stages of a helicopter going out of control and it’s subsequent flight path…...they would soon run out of test pilots and aircraft. I was lucky enough to be able to work alongside the chap who programmed the sim we worked on and he was always keen to remind us of that fact. In turn, we used to emphasise this to all the RAF pilots we trained. We used the simulator as a sophisticated procedures trainer, not an actual aircraft.

I’ve mentioned before that one of our crews subsequently suffered a tail rotor control failure over the sea and successfully ditched the aircraft in the water with no injuries. The pilot stated afterwards that his simulator training prevented him from doing what he said would previously have been his prior instinct of merely chopping the throttles and entering autorotation. Doing that would have made matters worse in his case, because the tail control mechanism had failed with some positive pitch. Instead he made a gentle yawing descent onto the surface. Despite not having floats the aircraft was subsequently recovered almost intact.

But unfortunately the Leicester accident was a far less benign failure.

megan

18th Nov 2022, 23:39

Plenty of very experienced Aviators have gotten Sim Sick who have never been Air Sick in their careers.I'm one of them SAS, first time in a sim (level D) was for a four hour session, I was first up and got through my two hours keeping stomach where it should, swapping seats I just couldn't take it anymore and bailed, leaving my partner to ride alone. Got better over the years but never at home. Sim instructor explained that very subtle clues can induce the reaction, one was light sources in the visuals not being realistic in that they don't get brighter as you get closer, such as runway edge lighting when flying an approach.

ShyTorque

18th Nov 2022, 23:58

Megan, I noticed that pilots with a lot of hours on the actual aircraft often tended to suffer the most. I put this down to a discrepancy between the “seat of the pants” learning already in memory and what the eyes were seeing. This is obviously a limitation of the motion of a simulator bolted to the floor, partly because the motion gently resets between inputs.

megan

19th Nov 2022, 00:39

Could be something in that Shy, we typically had a couple of thousand in type (76) prior to starting the sim work.

PlasticCabDriver

19th Nov 2022, 07:15

Certainly remember feeling a bit ropey at times in the Puma sim in Stavanger…..

ShyTorque

19th Nov 2022, 07:21

PCD, Quite a few did. Including one of my colleagues there who later went on to manage the project! :cool:

SASless

19th Nov 2022, 13:56

In my time teaching in the Sim....from the early 76 Simulator at American Airlines Training....to the very much improved Sims for the 212/412 at the FSI facility at the Bell factory.....I only experienced Sim sickness one time....and have never had any problem with motion sickness ever even in very rough water on boats and ships.

The lag between the visual and the Sim is one factor but the most important cause is the way the Sim System works to generate felt forces due to pitch and roll.

In the real aircraft the airframe moves relative to the horizon and once in a stabilized turn or change in pitch attitude that results in a constant relative angle....no other "motion" is felt.

In the Sim....once the "Box" moves per the flight control input....say for example a thirty degree bank.....the "Box" leans over.....and then despite the instruments showing that angle of bank....the Box begins to re-center so it can have a full range of motion available.

Our well tuned hind quarters feel the forces of gravity....and our eyes are seeing something else and the confusion caused by those inputs is what gets us.

The usual victim is the guy not doing much because if you are flying or running the Sim at the Control Panel....each are "busy" and are not free to focus upon the odd sensations.

SASless

20th Nov 2022, 16:45

I was asked if the Pilot had shut down or reduced engine power by means of the ECL's or throttles....and could not respond to that question.

Does anyone know of a reference to that in any official report or discussion?

PoloJamie

31st Aug 2023, 11:13

jimjim1

31st Aug 2023, 11:56

FYI - The AAIB have announced that the report is scheduled for publication 6 September 2023

https://www.gov.uk/government/publications/aaib-current-field-investigations/air-accidents-investigation-branch-current-field-investigations

Document lists another AW helicopter tail rotor duplex bearing failure. Found during post-flight inspection. Phew!

13 Jun 2022 G-CIMU AW139 Failure of tail rotor duplex bearing found during post-flight inspection, Norwich Airport

Leicester — Scheduled for publication 6 September 2023

If they keep to that schedule that will be four years and ten months after the accident. Guess the billionaire family didn't like the report?

Northernstar

31st Aug 2023, 13:08

Maybe the manufacturer has been the issue preventing it’s publication.

nomorehelosforme

6th Sep 2023, 01:16

AAIB Report here.

https://www.gov.uk/aaib-reports/aircraft-accident-report-aar-1-slash-2023-leonardo-aw169-g-vskpIntroduction:The Air Accidents Investigation Branch (AAIB) became aware of this accident during the evening of 27 October 2018. In exercise of his powers, the Chief Inspector of Air Accidents ordered an investigation to be carried out in accordance with the provisions of Regulation (EU) 996/2010 and the UK Civil Aviation (Investigation of Air Accidents and Incidents) Regulations 2018.

The sole objective of the investigation of an accident or incident under these regulations is the prevention of future accidents and incidents. It shall not be the purpose of such an investigation to apportion blame or liability.

In accordance with established international arrangements, the Agenzia Nazionale per la Sicurezza del Volo (ANSV) of Italy, representing the State of Design and Manufacture of the helicopter, appointed an Accredited Representative (Accrep) to participate in the investigation. The Transportation Safety Board (TSB) of Canada, representing the State of Design and Manufacture for the helicopter’s engines, the National Transportation Safety Board (NTSB) of the USA, representing the State of Design and Manufacture of the tail rotor actuator and the Bureau d’Enquêtes et d’Analyses pour la sécurité de l’aviation civile (BEA) of France representing the State of Design and Manufacture of the tail rotor duplex bearing, also appointed Accreps.

Experts were appointed by the Aircraft Accident Investigation Committee of Thailand and the State Commission on Aircraft Accidents Investigation of Poland.

The helicopter, bearing, tail rotor actuator and grease manufacturers, the operator, the European Union Aviation Safety Agency (EASA), and the UK Civil Aviation Authority (CAA) also assisted the AAIB investigation.

BBC already making a headline of it.

https://www.bbc.com/news/uk-england-leicestershire-66716572

Asturias56

6th Sep 2023, 07:41

also an interesting video reconstruction available on the BBC link - they never stood a chance...................

Bearing seized, drive shaft unscrewed = total loss of any yaw control at all

Genghis the Engineer

6th Sep 2023, 07:47

A sobering read, but a small comfort for those involved that it was one of those rare events - a true accident where the people there really couldn't have prevented it. It's assuring to see that Leonardo and EASA have published a raft of SBs, ADs and revisions to the airworthiness standards, as they should.

G

Non-Driver

6th Sep 2023, 08:12

One of those rare "old fashioned" accidents that we used to have quite regularly but learned from them to initiate better design & oversight criteria. In some ways it shows how far we have come over the last 50 years. Unfortunately helicopters, more than fixed wing, generally have catastrophic outcomes when it comes to failure of Critical parts. No solace to those lost that had no virtually no chance, but hopefully the industry will not see a category repeat.

ShyTorque

6th Sep 2023, 09:48

If the tail rotor control push/pull rod running through the outer drive shaft had been designed to "free float" in rotation, rather than held stationary by a single nut, this failure wouldn't have happened. Hopefully this will be taken into account in future designs.

happyjack

6th Sep 2023, 12:59

I knew Eric and had mutual friends with Izabella. She did have a PPLH and an ATPLA G550 rated so please do not try to taint the ghastly situation by making subline suggestions!
I along with many (read everyone) have occasionally flown aircraft we are not rated to fly under the watchful eye of the rated guy in the other seat on empty sectors. That is quite normal.
Izabella was along for the ride and had no input with the operation.

flight beyond sight

6th Sep 2023, 15:21

I do not post on here due to crass comments like yours
Happy Jack is quite correct to what he has posted
We could all have been in the terrible situation that Eric found himself in

hargreaves99

6th Sep 2023, 15:40

My apologies, my post was purely a comment on the 'two-crew' crewing practises that still go on in onshore VIP twin operations, that subject is probably best raised in a different thread. I have deleted the post.

DOUBLE BOGEY

7th Sep 2023, 12:57

also an interesting video reconstruction available on the BBC link - they never stood a chance...................

Bearing seized, drive shaft unscrewed = total loss of any yaw control at all

Maybe I misunderstood the report but I believed the TR Pitch went to FULL POWER. Which leaves no chance of a recovery as we saw in this event. No just loss of control . The worst scenario imaginable. Its odd that all other manufacturers fit a safety mechanism, or inherent blade aerodynamics, that would "center" the servo to a reasonable, hopefully survivable pitch setting. I also understand this latent failure event exists in 109/139/169 and 189. Are there any Leonardo techs on here that could confirm this.

noooby

7th Sep 2023, 15:39

DOUBLE BOGEY

7th Sep 2023, 15:49

Could you perhaps expand on the "all other manufacuters fit a safety mechanism" passage please? Specifically, which aircraft models have this safety mechanism and what exactly is it?

The Elastomerics on the 169 are pretty stiff and help with self centering a bit. I'm trying to think what other tail rotors do for self centering if the control linkage is broken and I can't really think of anything.

Remember, the servo got disconnected. No point in centering an item that isn't part of the control system anymore.

Hi Nooby, that’s the point, if the servo becomes disconnected either an inherent aerodynamic force or a spring will act on the control valve to return the servo to neutral. If the servo control arm disconnects from the spider, aerodynamic force or spring should return the blades to a neutral value. The idea is to prevent a hydraulic or aerodynamic hardcover one way or tother. Eg, in the S61 and 332 series, a force spring does the job. In 412 the pitch horns do it. Leonardo seem to have forgotten these simple safety features.

Unless I am wrong (happened once, not nice) all Leonard’s have this latent failure mode.

DB

212man

7th Sep 2023, 15:51

Could you perhaps expand on the "all other manufacuters fit a safety mechanism" passage please? Specifically, which aircraft models have this safety mechanism and what exactly is it?

The Elastomerics on the 169 are pretty stiff and help with self centering a bit. I'm trying to think what other tail rotors do for self centering if the control linkage is broken and I can't really think of anything.

Remember, the servo got disconnected. No point in centering an item that isn't part of the control system anymore.
Yes I believe you are correct, those types that have centering mechanisms (such as Super Puma family and S76) still require the TR Pitch change rod to be attached to the servo!

One area that puzzles me is there is no analysis of the different outcome in survivabilty if the landing gear had been down at impact, other than this statement:

Cockpit voice recording revealed that the pilot had asked the front seat passenger to select the landing gear up, indicating that he did not take his hand off the collective lever to do so himself. The call to raise the landing gear came after the pilot had committed to the CTO. The investigation did not consider raising the helicopter’s landing gear before reaching climb speed to be a contributory factor in the accident or in its survivability.

I know the gear would have stroked and the underside would still have contacted the wall, but I would think there would at least be some energy attenuation by the gear, which may have reduced the fuselage disruption?

DOUBLE BOGEY

7th Sep 2023, 15:52

ShyTorque

7th Sep 2023, 17:00

Kipper1985

7th Sep 2023, 17:43

See section 1.12.2 on Page 126, it’s covered there. Essentially the rate of descent exceeded the stroke rate of the landing gear so it’s ability to absorb impact forces was limits.
K

Aser

7th Sep 2023, 18:17

Maybe I misunderstood the report but I believed the TR Pitch went to FULL POWER. Which leaves no chance of a recovery as we saw in this event. No just loss of control . The worst scenario imaginable. Its odd that all other manufacturers fit a safety mechanism, or inherent blade aerodynamics, that would "center" the servo to a reasonable, hopefully survivable pitch setting. I also understand this latent failure event exists in 109/139/169 and 189. Are there any Leonardo techs on here that could confirm this.

In the report:
The AW139 was designed to have a left-hand thread on the actuator shaft and
locking feature, to prevent it unscrewing if the actuator shaft rotated following a
bearing failure. This suggests that the failure mode had been considered as part
of the AW139 development. An AW139 bearing failure in 2012 demonstrated
that this design successfully prevented the actuator shaft from unscrewing from
the pin holder.
2012 an operator based in Qatar suffered a loss of yaw control incident on
an AW139. This was found to have been caused by a failure of the duplex
bearing (p/n 3G6430V00151). Evidence of rotation of the tail rotor actuator
control shaft confirmed that the bearing had seized at some point. However,
as the control shaft had a left-hand thread, the pin carrier had tightened onto
the actuator shaft, rather than unscrewing. This transferred the torque load
back into the bearing, forcing rotation until the bearing components became so
heavily worn that the bearing failed completely, to the extent that it no longer
provided any resistance to the movement of the hydraulic actuator. Effectively
no longer attached to the control system, the tail rotor blades moved to, and
remained at, a positive blade pitch angle of approximately 10° with no means of
changing the blade position possible by pilot action. The helicopter started to
turn under the influence of the main rotor torque couple, but the loss of tail rotor
control occurred while the helicopter was in forward flight. The reduced engine
torque demand, the vertical tail surface aerodynamically contributing to the yaw
control and the force generated by the default blade position, were sufficient to
allow the pilot to maintain forward flight and perform a ‘run on’ landing without
any additional damage occurring to the helicopter.