AW169 crash at King Power Stadium - Preliminary Findings

[Evan]

This is over a year old, but I just came across it.

The cause was not a tail rotor power failure, but a tail rotor pitch runaway. The actuator mechanism became "unscrewed" from the actuator rod that runs throught the power shaft resulting in a full right yaw command, and thus an uncontrollable spin. The report only covers the cause of the mechanical issue in order to issue an AD for inspections. It would seem that the only step the pilot could take at that point would be to cut all power and autorotate, but at low height with no forward speed, that doesn't work so well. Also, unlike a shaft failure where the spin is initiated by main rotor torque alone, this spin was powered by the tail rotor, so it would have been extremely disorienting.

As to the cause, it appears that the castellated nut holding the tail rotor bearing was overtorqued and might have been the initial cause of the failure. That doesn't exactly rule out sabotage though. I'd be interested to know if any others were found in this condition as a result of the mandated fleetwide inspections.

https://assets.publishing.service.go...018_G-VSKP.pdf

[Gabriel]

This is over a year old, but I just came across it.

The cause was not a tail rotor power failure, but a tail rotor pitch runaway. The actuator mechanism became "unscrewed" from the actuator rod that runs throught the power shaft resulting in a full right yaw command, and thus an uncontrollable spin. It would seem that the only step the pilot could take at that point would be to cut all power and autorotate,

I don't see why that would work or why having the rotor spinning thanks to power or thanks to autorotation would make a difference. I don't know this (or any) design in particular, but in general the tail rotor shaft is mechanically and rigidly connected to the main rotor so the tail rotor RPM is always proportional to the main rotor RPM. In that way, as long as the main rotor keeps turning (and there is no failure in the mechanism that transmits the motion form the main rotor to the tail rotor), the tail rotor would keep turning and with a pitch runaway it will keep providing full yaw moment,

[Evan]

I don't see why that would work or why having the rotor spinning thanks to power or thanks to autorotation would make a difference. I don't know this (or any) design in particular, but in general the tail rotor shaft is mechanically and rigidly connected to the main rotor so the tail rotor RPM is always proportional to the main rotor RPM. In that way, as long as the main rotor keeps turning (and there is no failure in the mechanism that transmits the motion form the main rotor to the tail rotor), the tail rotor would keep turning and with a pitch runaway it will keep providing full yaw moment,

I think tail rotor failures are more commonly a destruction of blades due to collision (or enemy fire) or a failure of the powertrain driving them. In that case Newton takes control of yaw and as long as torque is being applied to the main rotor, the helicopter will spin opposite to that force. The pilot can stop this spin by removing power to the main rotor and then, if there is sufficient forward airspeed, rely on the vertical stabiliser to keep a mostly stable heading at least down to the flare.

With this failure, the spin would be in the direction of that force (not a loss of anti-torque but a surge of anti-torque), which, to a pilot trained on 'conventional' tail rotor failure, would be disorienting and might not be immediately recognised as tail rotor failure. That might cost some precious seconds of response. More importantly, in autorotation, when the spinning force created by the main rotor shaft is removed, the yawing force of the still-turning tail rotor at full pitch would continue to cause a right-yawing spin, so there would be no way to remove that spin and establish and control heading during that autorotation at any airspeed.

In this particular crash scenario (an out-of-ground-effect hover at very low height, where autorotation isn't going to save you), it wouldn't change the outcome. But this failure could happen at any time. So this is a failure that could make autorotation impossible at any speed and height.

[Gabriel]

Exactly, so why did you say "It would seem that the only step the pilot could take at that point would be to cut all power and autorotate"? Despite how high and fast they were flying and how quickly the pilot recognized the real nature of the failure (a tail rotor pitch runaway) and how good the pilot, it seems there was no way to "autorotate" our of this situation. Actually, it seems that there is no way out of this situation except if there is a way to disconnect the tail rotor from the main rotor, or perhaps try to make a somehow controlled descent while spinning.

[Evan]

Exactly, so why did you say "It would seem that the only step the pilot could take at that point would be to cut all power and autorotate"? Despite how high and fast they were flying and how quickly the pilot recognized the real nature of the failure (a tail rotor pitch runaway) and how good the pilot, it seems there was no way to "autorotate" our of this situation. Actually, it seems that there is no way out of this situation except if there is a way to disconnect the tail rotor from the main rotor, or perhaps try to make a somehow controlled descent while spinning.

You mean, before I added "but at low height with no forward speed, that doesn't work so well."? The only theorectical way out of this one I can imagine is to keep power on and reduce collective to bring it slowly back down, but you would have to have a superhuman immunity to centrifugal effects. As I said, my concern is that this failure makes autorotation impossible even where it would normally be effective.

[Gabriel]

You mean, before I added "but at low height with no forward speed, that doesn't work so well."?

Yes. I don't see how autorotation would work well in this situation (runaway tail rotor pitch) regardless of whatever height and speed.

Here is your full quote, which I still don't understand (and based in your posterior comments, it seems that you don't either)

The cause was not a tail rotor power failure, but a tail rotor pitch runaway. The actuator mechanism became "unscrewed" from the actuator rod that runs throught the power shaft resulting in a full right yaw command, and thus an uncontrollable spin. The report only covers the cause of the mechanical issue in order to issue an AD for inspections. It would seem that the only step the pilot could take at that point would be to cut all power and autorotate, but at low height with no forward speed, that doesn't work so well.

Even if you want to try to descend while accepting the spin (not that you have many options, unless if it possible to disconnect the tail rotor), why would you totally cut power and autorotate? You can descend with power or at least add power at the end to try to flare and cushion the crash.

Another point: Even if you were super-human to resist all the bad physiological things related to the spinning, the fact that the helicopter is spinning dramatically affects the way it behaves and responds to pitch and roll inputs. Stability, gyroscopic effects, static and dynamic un-balance, all affect stability and control.

It feels to me that we are trying to see what a pilot could have done after the plane lost a wing. I don't know if there is anything that could have been done here (regardless of speed, altitude, knowledge, startle, reaction time, physiological effects) that would have had an impact in the outcome.

[Evan]

Yes. I don't see how autorotation would work well in this situation (runaway tail rotor pitch) regardless of whatever height and speed.

Here is your full quote, which I still don't understand (and based in your posterior comments, it seems that you don't either)



Even if you want to try to descend while accepting the spin (not that you have many options, unless if it possible to disconnect the tail rotor), why would you totally cut power and autorotate? You can descend with power or at least add power at the end to try to flare and cushion the crash.

Another point: Even if you were super-human to resist all the bad physiological things related to the spinning, the fact that the helicopter is spinning dramatically affects the way it behaves and responds to pitch and roll inputs. Stability, gyroscopic effects, static and dynamic un-balance, all affect stability and control.

It feels to me that we are trying to see what a pilot could have done after the plane lost a wing. I don't know if there is anything that could have been done here (regardless of speed, altitude, knowledge, startle, reaction time, physiological effects) that would have had an impact in the outcome.

Does this help?:

It would seem that the only step the pilot could take at that point would be to cut all power and autorotate, but at low height with no forward speed, that doesn't work so well.

i.e, it doesn't work.

The only thing a pilot could be expected to do is to "cut all power and [proceed to apply the procedure to] autorotate". It won't help, but that's what the blue font part is about. That appears to be what this pilot did. I'm not suggesting that he could have done anything else for a better outcome. I'm saying that was all he could do. This was not survivable.

More importantly, I'm saying this is an undefended failure condition with a very low chance of recovery or survivability at any height or airspeed. Perhaps it doesn't have to be.

Perhaps the FAR's could require that a control force is needed to maintain tail rotor pitch (perhaps by spring-loading them to nuetral). This way, if the actuator rod ever became detached from the servo, it would result in a neutral tail rotor force and allow for normal, tail rotor out autorotation using aerodynamic yaw stability whenever autorotation is possible.

[Evan]

AAIB has released its final report (in two volumes) on this crash.

See if you can spot the trend:

The investigation found the following contributory factors for this accident:

1. The load survey flight test results were not shared by the helicopter manufacturer with the bearing manufacturer in order to validate the original analysis of the theoretical load spectrum and assess the continued suitability of the bearing for this application, nor were they required to be by the regulatory requirements and guidance.
2. There were no design or test requirements in Certification Specification 29 which explicitly addressed rolling contact fatigue in bearings identified as critical parts; while the certification testing of the duplex bearing met the airworthiness authority’s acceptable means of compliance, it was not sufficiently representative of operational demands to identify the failure mode.
3. The manufacturer of the helicopter did not implement a routine inspection requirement for critical part bearings removed from service to review their condition against original design and certification assumptions, nor were they required to by the regulatory requirements and guidance.
4. Although the failure of the duplex bearing was classified as catastrophic in the certification failure analysis, the various failure sequences and possible risk reduction and mitigation measures within the wider tail rotor control system were not fully considered in the certification process; the regulatory guidance stated that this was not required.

Eight Safety Recommendations have been made in this report. These have been made to EASA to address weaknesses or omissions identified in the regulations for the certification of large helicopters - Certification Specification 29. The recommendations address the main findings of the investigation and include: validation of design data by suppliers post‑test; premature rolling contact fatigue in bearings; life limits, load spectrum safety margin and inspection programmes for critical parts; and assessment and mitigation of catastrophic failure modes in systems.

Yes, 3WE, we do need more regulation. Especially regarding the robustness of oh-so-very-critical flight controls.

https://www.gov.uk/aaib-reports/aircraft-accident-report-aar-1-slash-2023-leonardo-aw169-g-vskp

[3WE]

Yes, 3WE, we do need more regulation. Especially regarding the robustness of oh-so-very-critical flight controls.

[url]https://www.gov.uk/aaib-reports/aircraft-accident-rep3;

Darn, I was thinking you were going for an automated recovery system, since you saw options for landing the helicopter that the stupid pilots didn't exercise. Maybe another day.

Indeed, I hate it when controls break. Just the other day, the control surface that directs the left half of an airplane away from hard, underlying, surfaces broke.

[Evan]

Darn, I was thinking you were going for an automated recovery system, since you saw options for landing the helicopter that the stupid pilots didn't exercise. Maybe another day.

Nothing a pilot can do to recover from this. Hence the need for very robust design. This cannot happen.

[3WE]

This cannot happen.

This DID happen.

[Evan]

This DID happen.

Correct. Therefore, something is very wrong and regulations need to improve, because otherwise, it can and did and will happen. And it cannot happen.

[Gabriel]

Correct. Therefore, something is very wrong and regulations need to improve, because otherwise, it can and did and will happen. And it cannot happen.

I think the verb you are looking for is "must".

[3WE]

I think the verb you are looking for is "must".

…and I’m sure the authorities are addressing it with the utmost urgency.