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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 7th Nov 2018, 20:05
  #701 (permalink)  
 
Join Date: Nov 2018
Location: Shropshire
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First time poster, long time lurker here!

Hopefully I can answer some of the questions posted about the TRA (Tail Rotor Actuator).

The input lever will drive a dual main control valve to control both hydraulic systems. The position feedback is via the trunnion block (90). I would imagine, and I don’t know this specific actuator, that the control valve will be spring biased to return the actuator to a neutral position if the pilot input is lost. If the position feedback is lost the actuator will run away to a hardover position.

If the washer (140) was incorrectly installed position control would still be present, but the null and full stroke positions of the actuator would be affected . This will be installed and checked (visually and by test) at the manufacturer, as would the nut (60) and cotter pin (50). These parts would never be removed from the actuator unless it was undergoing some repair work (removed from the airframe).

As far as a nut going missing, I think that the thread is handed such that the rotation of the Tail Rotor always tightens the rod into nut. If the nut is missing then I would guess that it was a mechanical failure somewhere.
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Old 7th Nov 2018, 20:56
  #702 (permalink)  
 
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#697 https://www.pprune.org/10304706-post697.html

full link of EASA E- AD https://ad.easa.europa.eu/ad/2018-0241-E
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Old 7th Nov 2018, 21:01
  #703 (permalink)  
 
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Originally Posted by bumpthump View Post
GRAYHORIZONSHELI Do you actually mean “they found a missing nut” or “they found that a nut is missing”?

I would assume (and surely at this stage of the game, it’s still an assumption) it would be the latter.

I stress again, I am not a pilot, but I am a mech engineer (albeit from a different discipline). As a slight aside, I have used split pins and wire locks on many a track motorcycle in my time, so as well as those two safety measures, there is (as you stated) the correct torque setting for the nut. On top of that, I don’t know if the aviation world use it or not, but on track bikes, it was also common practice to apply a further safeguard in the form of an appropriate thread locking fluid (yes, usually from the Loctite company).
As stated in my post earlier in this thread, I am not a pilot. I am however, a groundsman, and have worked in stadium environments on and off for many years. What I can tell you is that if standard post match practices were carried out (they may well not have been, given the circumstances), there is a very good chance that any missing components would have been found, if they were on the pitch itself.

Immediately following matches at this level, pitches are effectively 'hoovered' with mowers to remove debris - this debris includes organic debris i'e., tufts of grass etc, and also objects thrown onto the pitch - coins is the common issue in this respect. If a nut, or other component was on the pitch, it would have been found during the post match 'clean up, or failing that, during the next cut as the mower blades would 'find' it.

I suspect however that all pitch maintenance activities would have been suspended immediately given the nature and location of the incident. That said, if there were any components on the pitch, they would most definitely have been found by now.
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Old 7th Nov 2018, 23:03
  #704 (permalink)  
 
Join Date: Nov 2008
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Originally Posted by GeordieMike View Post


As far as a nut going missing, I think that the thread is handed such that the rotation of the Tail Rotor always tightens the rod into nut. If the nut is missing then I would guess that it was a mechanical failure somewhere.
Remember, this is a static component (control servo and feedback link) that transfers the input to the pitch control, there is no loading at this location.
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Old 7th Nov 2018, 23:06
  #705 (permalink)  
 
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Originally Posted by Mike78 View Post
As stated in my post earlier in this thread, I am not a pilot. I am however, a groundsman, and have worked in stadium environments on and off for many years. What I can tell you is that if standard post match practices were carried out (they may well not have been, given the circumstances), there is a very good chance that any missing components would have been found, if they were on the pitch itself.

Immediately following matches at this level, pitches are effectively 'hoovered' with mowers to remove debris - this debris includes organic debris i'e., tufts of grass etc, and also objects thrown onto the pitch - coins is the common issue in this respect. If a nut, or other component was on the pitch, it would have been found during the post match 'clean up, or failing that, during the next cut as the mower blades would 'find' it.

I suspect however that all pitch maintenance activities would have been suspended immediately given the nature and location of the incident. That said, if there were any components on the pitch, they would most definitely have been found by now.
Yes, for sure. If anything was found on the pitch by the groundsmen, it would most certainly be with the AAIB for investigation now.
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Old 7th Nov 2018, 23:46
  #706 (permalink)  
 
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Those familiar with this particular aircraft have already stated that this particular component requires the removal of a cover for inspection. That being the case, should any component part company from its design location, there is every likelihood that it would be retained by the cover and remain with the aircraft rather than be found on the stadium playing surface. Would that be (yet another) fair assumption?
Again, in terms of scale, I don’t know how big or small this component actually is.
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Old 7th Nov 2018, 23:56
  #707 (permalink)  
 
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Originally Posted by GeordieMike View Post
First time poster, long time lurker here!
...
...
If the washer (140) was incorrectly installed position control would still be present, but the null and full stroke positions of the actuator would be affected . This will be installed and checked (visually and by test) at the manufacturer, as would the nut (60) and cotter pin (50). These parts would never be removed from the actuator unless it was undergoing some repair work (removed from the airframe).
...
...
Thanks for the input.
What is the approximate full stroke of the actuator relative to the washer thickness? Would the missing washer offset on null and full stroke positions be subtle/adjustable at some other point or clearly way out? In any case it is hard to fathom leaving the factory that way or with missing lock means.
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Old 8th Nov 2018, 01:00
  #708 (permalink)  
 
Join Date: Sep 2017
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Originally Posted by Mike78 View Post
As stated in my post earlier in this thread, I am not a pilot. I am however, a groundsman, and have worked in stadium environments on and off for many years. What I can tell you is that if standard post match practices were carried out (they may well not have been, given the circumstances), there is a very good chance that any missing components would have been found, if they were on the pitch itself.

Immediately following matches at this level, pitches are effectively 'hoovered' with mowers to remove debris - this debris includes organic debris i'e., tufts of grass etc, and also objects thrown onto the pitch - coins is the common issue in this respect. If a nut, or other component was on the pitch, it would have been found during the post match 'clean up, or failing that, during the next cut as the mower blades would 'find' it.

I suspect however that all pitch maintenance activities would have been suspended immediately given the nature and location of the incident. That said, if there were any components on the pitch, they would most definitely have been found by now.

dont make the assumption, or imply that i did, that it fell out into the grass. more than likely it stayed within the cowlings, at least until the crash anyways
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Old 8th Nov 2018, 04:14
  #709 (permalink)  
 
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Here is a question.

Can someone explain the process / thinking around the AD? In so much that the commentary for the reason of the AD says:-

Reason: An accident occurred on an AW169 helicopter, the root cause of which has not been identified and the technical investigation is still ongoing. While the helicopter was on a take-off phase at low forward speed, a loss of yaw control has been observed.

As a precautionary measure, Leonardo issued ASB 169-120 for AW169 helicopters to provide inspection instructions to check correct installation of the tail rotor (TR) servo-actuator
What I don't understand is the aircraft crashes almost 2 weeks ago. If the AD is truly a precautionary element what prompts the thinking of this 2 weeks later? Surely either its a precaution thinking out loud type AD, in which case why not flag earlier? Or they are concerned about something they have seen more recently, in which case why not just say?? It just seems bizarre.
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Old 8th Nov 2018, 05:31
  #710 (permalink)  
 
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Originally Posted by Pittsextra View Post
Here is a question.
Originally Posted by Pittsextra View Post

Can someone explain the process / thinking around the AD? In so much that the commentary for the reason of the AD says:-



What I don't understand is the aircraft crashes almost 2 weeks ago. If the AD is truly a precautionary element what prompts the thinking of this 2 weeks later? Surely either its a precaution thinking out loud type AD, in which case why not flag earlier? Or they are concerned about something they have seen more recently, in which case why not just say?? It just seems bizarre.


Accident happened late Saturday 27th October.
Leonardo release their SB Monday 5th November.
EASA release their EAD Wed 7th November.

Just over a week for Leonardo to react. I guess just evidence gathering between.




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Old 8th Nov 2018, 05:33
  #711 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
Thanks for the input.
What is the approximate full stroke of the actuator relative to the washer thickness? Would the missing washer offset on null and full stroke positions be subtle/adjustable at some other point or clearly way out? In any case it is hard to fathom leaving the factory that way or with missing lock means.
There’s no specific mention of checking the washer in the SB. Only the nut, the pin and the lock wire.
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Old 8th Nov 2018, 08:48
  #712 (permalink)  
 
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Originally Posted by Mitchaa View Post
[RIGHT]

Accident happened late Saturday 27th October.
Leonardo release their SB Monday 5th November.
EASA release their EAD Wed 7th November.

Just over a week for Leonardo to react. I guess just evidence gathering between.
Yeah that would be fair but what evidence? The video footage was available almost instantly and so now evidence surely means physical evidence from the accident aircraft or of prior events that have been caught prior to an accident with other in service aircraft. It can't be the former if the narrative around the AD is faithful and if its the latter does that take a week to get out with the commitment of intelligent minds?

It would be incredible if this isn't off the back of something seen or suspected and so why it can't be said more plainly - or indeed co-ordinated via the AAIB goodness knows... Hey Ho.
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Old 8th Nov 2018, 12:13
  #713 (permalink)  
 
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Originally Posted by Mitchaa View Post


There’s no specific mention of checking the washer in the SB. Only the nut, the pin and the lock wire.
True enough, the reason I was wondering about the washer is that it is the only thing that might not be obvious if misassembled since a washer is often seen under a nut. Of course that does suggest that if it was suspect it would have been mentioned in the AD.

Most likely is that they suspect a fault in the feedback link but may not know exactly what at this stage.
Hard to fathom how pin and lockwire could be missed on assembly so might be a failure.
Here is one scenario: If the lockwire was incorrectly installed could it eventually wear into the locking pin, possibly on the head side?
.
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Old 8th Nov 2018, 12:43
  #714 (permalink)  
 
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So much conjecture.....
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Old 8th Nov 2018, 12:48
  #715 (permalink)  
 
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It is a slow news week....
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Old 8th Nov 2018, 16:17
  #716 (permalink)  
 
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I think TR blades tend towards zero pitch when uncontrolled. Gazelle definitely did in manual. Lynx as well, albeit with a powerful spring bias unit? Sea King? Is it generic or type specific?
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Old 8th Nov 2018, 16:33
  #717 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
Thanks for the input.
What is the approximate full stroke of the actuator relative to the washer thickness? Would the missing washer offset on null and full stroke positions be subtle/adjustable at some other point or clearly way out? In any case it is hard to fathom leaving the factory that way or with missing lock means.
I don’t know this specific product but I would expect the stroke to be around 50 to 100 mm. the washer would be a small fraction of this.

I don’t think you would be able to adjust the input mechanism to account for the missing component and the result would be a setting should be far enough out to be identified and rectified.
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Old 8th Nov 2018, 16:38
  #718 (permalink)  
 
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Originally Posted by Mitchaa View Post


Remember, this is a static component (control servo and feedback link) that transfers the input to the pitch control, there is no loading at this location.
Yes, the servo actuator is static but I think there will be some torque transmitted down the control rod as the attachment (bearing assembly?) from the control rod to rotor pitch mechanism will not have zero friction.
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Old 8th Nov 2018, 19:48
  #719 (permalink)  
 
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CAA PAPER 2003/1
Helicopter Tail Rotor Failures
is well worth a read. Here are some extracts.

There are two major types of TRF:
a) A TR drive failure (TRDF) is a failure within the TR drive system with consequent
(usually total) loss of TR thrust. Example causes are internal fatigue or external
impact resulting in a broken drive shaft.
b) A TR control failure (TRCF) is a failure within the TR control system such that
normal pilot control of TR thrust has been partially or totally lost. Example causes
are internal wear or external impact resulting in a severed control cable. The
resultant TR applied pitch, or power, could be free to fluctuate, or may be fixed
anywhere between high pitch (HP) or low pitch (LP) setting, including that of the
current trim pitch (TP).
Both of these TRFs are time critical emergencies. The pilot has to identify and
diagnose the TRF type and react with the correct control strategy within a few
seconds (or less), to prevent the aircraft departing into an uncontrollable flight state.
Even if the pilot recovers from the initial transients, yaw (pedal) control will have been
lost and the ability to manoeuvre safely and carry out a safe landing will have been
significantly degraded. The TR and its drive and control systems are clearly flight
critical components and should be designed so that their probability of failure is
‘extremely remote’. The airworthiness design requirements for UK military and civil
aircraft define ‘extremely remote’ as being less than 10-6 [1] and between 10-7 and
10-9 [2,3] per flight hour respectively.

Recovery from the failure transient
For TRDFs, and TRCFs where the post-failure pitch angle of the TR blades is different
from the pre-failure trim position, the immediate effect is a yaw response. That is, (for
anticlockwise main rotors), nose to starboard following a TRDF or LP TRCF, and nose
to port following a HP TRCF. The level of initial yaw acceleration will depend on the
nature of the failure, and the level of yaw rate and attitude build-up will depend on the
forward speed. In hover, an unchecked TRDF will result in the yawing moment from
the main rotor torque reaction spinning the fuselage at rates in excess of 100° sec-1,
perhaps even as high as 150-200° sec-1. Typically, the higher the forward speed, the
lower the yaw rate and attitude excursion as any natural directional stability of the
aircraft will tend to reduce the severity of the motion. However, this is only true up to
some value of sideslip, beyond which it is possible that directional stability can
reverse, resulting in increased yaw rate and attitude excursions. Evidence from the
Lynx TRF AFS trial [5] suggests that the ability of the pilot to successfully manage a
forward flight failure is strongly related to the extent of the initial yaw/sideslip
transient. If this exceeds 90°, then the pilot is unlikely to be able to recover, as the
flight control problem is exacerbated by disorientation; if the yaw rate reduces to zero
below about 30° yaw angle, then the pilot has a much greater chance of recovering
from the failure. Accompanying the yaw excursions will be pitch and roll motion,
which can further increase the risk of disorientation. An additional effect of any roll
attitude transient is an increase in the main rotor disc angle of incidence, leading to
an increased risk of the rotor over-speeding as the pilot reduces main rotor collective
to contain the effects of the failure. The extent of the attitude excursions depends on
the aerodynamic design characteristics of the fuselage and vertical stabiliser, the
resulting directional stability, the type of attitude stabilisation present in the flight
control system and the pilot’s control actions
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Old 8th Nov 2018, 21:33
  #720 (permalink)  
 
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Chronus, #3, loss of tail rotor blade ,followed milliseconds later by loss of other blade(s),and probably the gearbox.. The C of G of the a/c will move to somewhere beyond the nose of the a/c ....
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