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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. |
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 |
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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Originally Posted by Asturias56
(Post 11497967)
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 |
Originally Posted by DOUBLE BOGEY
(Post 11498734)
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 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. |
Originally Posted by noooby
(Post 11498808)
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. Unless I am wrong (happened once, not nice) all Leonard’s have this latent failure mode. DB |
Originally Posted by noooby
(Post 11498808)
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. 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. |
Originally Posted by noooby
(Post 11498808)
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. |
Originally Posted by DOUBLE BOGEY
(Post 11498734)
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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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 |
Originally Posted by DOUBLE BOGEY
(Post 11498734)
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 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. |
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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Originally Posted by 212man
(Post 11498819)
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...
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.
Originally Posted by DOUBLE BOGEY
(Post 11498734)
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. |
1 Attachment(s)
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. |
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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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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