Atlas Air 767 down/Texas
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Stabilizer Trim-Possible Failure Mode
About the only way to lose the nut engagement with a ball shaft is to lose the balls out of the nut.
These balls recirculate as the shaft turns, and anything that damages the recirculation pathway has the potential to cause disengagement of the nut from the ball drive shaft.
How could a recirculation pathway be damaged? There is always the potential for FOD in an aircraft. The right foreign object in the correct place and damage can happen despite the best efforts of the design engineers and maintainance engineers.
If the balls are missing from the previously pictured stabilizer actuator nut, the nut would be free to slide, and any position we see it presently would be coincidental.
Should the NTSB held FDR data shows a step jump in stabilizer position, they will be looking closely at this unit to identify the cause. But for now, it needs to sit untouched so as to preserve any potential clues as to what might have happened.
For those who have poor understanding of how a ballscrew system operates, here is an illustration:
These balls recirculate as the shaft turns, and anything that damages the recirculation pathway has the potential to cause disengagement of the nut from the ball drive shaft.
How could a recirculation pathway be damaged? There is always the potential for FOD in an aircraft. The right foreign object in the correct place and damage can happen despite the best efforts of the design engineers and maintainance engineers.
If the balls are missing from the previously pictured stabilizer actuator nut, the nut would be free to slide, and any position we see it presently would be coincidental.
Should the NTSB held FDR data shows a step jump in stabilizer position, they will be looking closely at this unit to identify the cause. But for now, it needs to sit untouched so as to preserve any potential clues as to what might have happened.
For those who have poor understanding of how a ballscrew system operates, here is an illustration:
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A simple nut would probably require frequent maintenance and regular replacement.
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Not on aircraft but I have specified ball screws on robotic production line equipment in a previous job for quite a harsh chemical environment and they were ridiculously reliable. Hundreds of thousands of cycles before noticeable wear without any maintenance at all.
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???
I believe I see ~ 2-3 inches of spindle above the Ball nut. The silver part above the hinge appears to have the same color and diameter as the lower part. With 1,7" per unit and 0,25 units = max ND, I end up with ~2,5 -3 units. Seems like pretty much spot on for that speed and configuration.
That observation does not rally point into the direction of spindle/jackscrew failure nor runaway trim.
On top of that in the footage shown it appears indeed as if there were some kind of recovery attempt going on.
The speed being somewhere around 240kts this becomes more and more strange.
So at the moment it looks like:
- Flight at correct trim (assumption based on position of Jackscrew)
- At 240kts at 6000ft. (assumption based on FR24 data). (Probably Some 60-70kts above stall speed).
- Flight in IMC (Statement early in the process)
- Coming out of the clouds at high speed (>>250kts)
- Flightpath angle in the part visible in the footage initially ~40 - 50°ND, apparently possibly reducing to ~30 - 40°ND.
- Initial altitude in the video difficult to judge but in a wild guess somewhere in the range 2000 - 3000ft.
What plausible scenarios does that leave?
=> Stall as initiating event rather unlikely (too much speed margin, flight attitude pretty stable and straight, albeit hefty ND with rather lowish AoA in the footage)
=> Out of Trim situation somewhat unlikely if the position of the jackscrew represents the state it was in during the terminal phase of the flight.
=> failed jackscrew/ball nut can't be completely ruled out but is not really support by the picture of the jackscrew nor the apparent started recovery => rather unlikely.
At the moment I'm somewhat at a loss what could have reasonably caused that loss of Control.
I believe I see ~ 2-3 inches of spindle above the Ball nut. The silver part above the hinge appears to have the same color and diameter as the lower part. With 1,7" per unit and 0,25 units = max ND, I end up with ~2,5 -3 units. Seems like pretty much spot on for that speed and configuration.
That observation does not rally point into the direction of spindle/jackscrew failure nor runaway trim.
On top of that in the footage shown it appears indeed as if there were some kind of recovery attempt going on.
The speed being somewhere around 240kts this becomes more and more strange.
So at the moment it looks like:
- Flight at correct trim (assumption based on position of Jackscrew)
- At 240kts at 6000ft. (assumption based on FR24 data). (Probably Some 60-70kts above stall speed).
- Flight in IMC (Statement early in the process)
- Coming out of the clouds at high speed (>>250kts)
- Flightpath angle in the part visible in the footage initially ~40 - 50°ND, apparently possibly reducing to ~30 - 40°ND.
- Initial altitude in the video difficult to judge but in a wild guess somewhere in the range 2000 - 3000ft.
What plausible scenarios does that leave?
=> Stall as initiating event rather unlikely (too much speed margin, flight attitude pretty stable and straight, albeit hefty ND with rather lowish AoA in the footage)
=> Out of Trim situation somewhat unlikely if the position of the jackscrew represents the state it was in during the terminal phase of the flight.
=> failed jackscrew/ball nut can't be completely ruled out but is not really support by the picture of the jackscrew nor the apparent started recovery => rather unlikely.
At the moment I'm somewhat at a loss what could have reasonably caused that loss of Control.
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If the stabiliser jackscrew is such a prime suspect for this accident, as evidence by this thread, then why is it just sitting on the hangar floor with all the other wreckage? Surely, the NTSB would have been looking for it, retrieved it and sent it directly to the metallurgists for analysis? I can only assume it doesn't figure as highly in their minds as it does in the minds of PPruners? Or am I missing something?
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Or, the chances that something the PPRuNe Investigative Board have failed to consider increase.
Originally Posted by Raffles S.A
We, the Prunes, want to solve the mystery faster than it takes the NTSB.
Lord Farringdon - I actually asked myself that same question. And then gave it further thought.
And the conclusion I came to is: that the NTSB takes a methodical, organized and scientific approach to investigations. They aren't going to go haring off after any particular piece of evidence until they have collected all the evidence (and it appears that, at the time of the video, half the aircraft more or less was still out in the mud.)
It's my impression (with no direct experience) that the NTSB first and foremost sets out to determine exactly what happened - relying on evidence such as the videos and track data and coms we've also seen and heard, and also things we have not seen and heard here (FDR and CVR recordings if available; actual radar returns; impact pattern(s), witness descriptions, clock times, and so on). And only once they are sure they know what happened in detail, move on to investigating how and why it happened (which is where the physical jackscrew will - or won't - come in). At which point they will go over to the hangar where the parts are stored, and start testing any and all theories that have developed, against the physical evidence of aircraft parts recovered. As well as sifting through maintenance and crew records and other specifics.
Of course, at that point the physical evidence may suggest additional ideas, and the investigators may go back to the tapes, to see if there was an anomaly (sound, crew comment, control movement or disparity) that went unnoticed on the first reading.
In short, at this point, nothing "figures highly in their minds" - or doesn't "figure highly in their minds," one way or the other. The jackscrew isn't going anywhere - it can wait.
cf: "confirmation bias" - and read your Thomas Huxley: "Sit down before fact as a little child, be prepared to give up every conceived notion, follow humbly wherever and whatever abysses nature leads, or you will learn nothing."
And the conclusion I came to is: that the NTSB takes a methodical, organized and scientific approach to investigations. They aren't going to go haring off after any particular piece of evidence until they have collected all the evidence (and it appears that, at the time of the video, half the aircraft more or less was still out in the mud.)
It's my impression (with no direct experience) that the NTSB first and foremost sets out to determine exactly what happened - relying on evidence such as the videos and track data and coms we've also seen and heard, and also things we have not seen and heard here (FDR and CVR recordings if available; actual radar returns; impact pattern(s), witness descriptions, clock times, and so on). And only once they are sure they know what happened in detail, move on to investigating how and why it happened (which is where the physical jackscrew will - or won't - come in). At which point they will go over to the hangar where the parts are stored, and start testing any and all theories that have developed, against the physical evidence of aircraft parts recovered. As well as sifting through maintenance and crew records and other specifics.
Of course, at that point the physical evidence may suggest additional ideas, and the investigators may go back to the tapes, to see if there was an anomaly (sound, crew comment, control movement or disparity) that went unnoticed on the first reading.
In short, at this point, nothing "figures highly in their minds" - or doesn't "figure highly in their minds," one way or the other. The jackscrew isn't going anywhere - it can wait.
cf: "confirmation bias" - and read your Thomas Huxley: "Sit down before fact as a little child, be prepared to give up every conceived notion, follow humbly wherever and whatever abysses nature leads, or you will learn nothing."
The jackscrew is attached to the stabilizer to change the datum trim point of the stabilizer.
In the wreckage photos, the jackscrew is no longer attached.
The question that will need to be answered is whether this failure of attachment happened pre or post-impact.
That is, did impact forces sever the attachment, or did the attachment structurally fail prior to impact and let the stabilizer float aerodynamically free?
In the wreckage photos, the jackscrew is no longer attached.
The question that will need to be answered is whether this failure of attachment happened pre or post-impact.
That is, did impact forces sever the attachment, or did the attachment structurally fail prior to impact and let the stabilizer float aerodynamically free?
In the wreckage photos, the jackscrew is no longer attached.
The question that will need to be answered is whether this failure of attachment happened pre or post-impact.
That is, did impact forces sever the attachment, or did the attachment structurally fail prior to impact and let the stabilizer float aerodynamically free?
The question that will need to be answered is whether this failure of attachment happened pre or post-impact.
That is, did impact forces sever the attachment, or did the attachment structurally fail prior to impact and let the stabilizer float aerodynamically free?
It would be very strange had the jackscrew remained attached to what's left of the stab following an impact like the one that occurred. Occam's Razor applies.
