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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 6th Sep 2023, 16:21
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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
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Old 6th Sep 2023, 16:40
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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.
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Old 7th Sep 2023, 13:57
  #1243 (permalink)  
 
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Originally Posted by Asturias56
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.
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Old 7th Sep 2023, 16:39
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Originally Posted by DOUBLE BOGEY
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.
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.
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Old 7th Sep 2023, 16:49
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Originally Posted by noooby
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, thatís 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 Leonardís have this latent failure mode.

DB
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Old 7th Sep 2023, 16:51
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Originally Posted by noooby
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 helicopterís 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?
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Old 7th Sep 2023, 16:52
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Originally Posted by noooby
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.
Nooby yes the elastomeric are stiff. However they were overcome by the servo, last in-it being made was power pedal, disconnects from the control rod and the valve remains open and motoring to full deflection.
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Old 7th Sep 2023, 18:00
  #1248 (permalink)  

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Originally Posted by DOUBLE BOGEY
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.
It went to full negative pitch. This meant that even with the engines off, it would still have been totally out of control in yaw. As I previously wrote, broadly equivalent to a full hard-over of a powerful rudder on a fixed wing aircraft.
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Old 7th Sep 2023, 18:43
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See section 1.12.2 on Page 126, itís covered there. Essentially the rate of descent exceeded the stroke rate of the landing gear so itís ability to absorb impact forces was limits.
K
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Old 7th Sep 2023, 19:17
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Originally Posted by DOUBLE BOGEY
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 10į 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 Ďrun oní landing without
any additional damage occurring to the helicopter.
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Old 7th Sep 2023, 19:26
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Eric

I knew Eric of old and as soon as I heard about this crash I knew it was mechanical not Pilot error. He was a sound steady Pilot. RIP.
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Old 7th Sep 2023, 20:34
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Originally Posted by 212man
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...
They do, Page 126 from the report (my bold for emphasis):
Impact assessment
The helicopter manufacturer carried out an assessment of the deceleration loads experienced during the impact sequence using recorded and calculated data provided by the AAIB. The assessment considered the loads on the helicopter’s structure in the region of the fuel tanks and rear row of passenger seats. It also considered the possible differences had the helicopter’s landing gear been extended.

The assessment identified that with the landing gear extended, there was no significant decrease in the forces transmitted through the helicopter’s structure. The manufacturer stated that this was because the calculated rate of deceleration and the forces involved exceeded the landing gear’s ability to react, deform and dissipate the impact energy.
And to address this:
Originally Posted by DOUBLE BOGEY
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.

The report states (page 27) that the forces experienced by the tail rotor control bearing in the axial direction (pitch changing direction) are all stabilising (centrifugal and aerodynamic loads seek 0 pitch, and the elastomeric loads seek 0 deformation on the elastomeric bearing). However, the hydraulic force on the tail rotor control rod was still present to overcome these forces and the tail rotor pitch continued beyond full 'non power pedal' deflection as the tail rotor control stops were no longer interacting with the tail rotor pitch control rod. Page 160 of the report (my bold for emphasis):
The actuator lever mechanism is designed to act as mechanical feedback for the hydraulic actuator, closing off hydraulic pressure once the movement of the control shaft matches the pilot’s pedal input. As the lever was now completely disconnected from the control shaft, the shaft continued to move under hydraulic pressure without restriction.

Last edited by ApolloHeli; 7th Sep 2023 at 20:43. Reason: Formatting
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Old 10th Sep 2023, 18:04
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This is an extraordinary failure by the certification authorities and the manufacturer. Attached is a paper from the CAA from 2003 where precisely this gap in certification requirements in was identified, and a rulemaking task initiated in what was JAR at the time. Clearly nothing came of that, or if it did, it was lost (again !) in the transition to EASA. Then it seems that Leonardo's own design department 'forgot' its own experience on the AW139 where it installed the mitigation of the LH thread nut.

But there is an operational issue that seems to have been overlooked, and that is that the helicopter was 75ft (I think) above its planned TDP and commencing a turn before Vmini or Vy. Every pilot who flies a CatA departure profile knows without being told that having everything hanging out at the top of the backup is a very vulnerable position to be in, and is therefore in a hurry to get through that and away to the safety of forward flight. I guess we'll never know what happened here, but I would have been interested to see if the helicopter had been regularly operated this way.

Finally its pure speculation, but the fact that VSKP had the worst degradation of its duplex bearing for the fewest hours of any machines in the fleet might not be unconnected with the nature of this stadium operation with its long drawn out backup at high power.
Attached Files
File Type: pdf
CAPAP2003_01.pdf (1.98 MB, 29 views)
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Old 10th Sep 2023, 21:51
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The H175 doesn't have any information regarding what a crew can expect if there is a loss of TR control. There is no drill, and there is no simulation. Yet there is a single cable run, and a clearly identifiable hydraulic failure sequence that will both lead to no TR control should things go wrong. As Wide Mouth Frog indicates, is there a gaping "critical single point failure mode" not covered in CS-29 regulation?
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Old 15th Sep 2023, 01:44
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Come on you TREs and you FO(T)Is out there. What do you think ? Nobody is risking anything, the CAA already laid it out in lavender in 2003. How happy are you that these wide gaps exist in certification of new helicopters, grandfathered designs in older helicopters, and training ? And how happy are you that the regulatory bodies and apparently at least two manufacturers (Airbus and Leonardo) have utterly let down the pilot community ?
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