Ethiopian Prelim Report
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Mansfield
Just a minor technical point. If a heavily loaded jackscrew cannot move in the nose up direction, that does not mean it cannot move in the nose down direction. Consider the case of a scissors jack under a heavy vehicle, and you get the picture.
The two ANU trim inputs shortly before the end indicate that a) the master trim switches were back on, and b) the trim motor was capable of moving the stabilizer. I really don’t know whether the manual trim can be frozen while the trim motor still works, but I suspect it would be the other way around.
Mansfield
Just a minor technical point. If a heavily loaded jackscrew cannot move in the nose up direction, that does not mean it cannot move in the nose down direction. Consider the case of a scissors jack under a heavy vehicle, and you get the picture.
Just a minor technical point. If a heavily loaded jackscrew cannot move in the nose up direction, that does not mean it cannot move in the nose down direction. Consider the case of a scissors jack under a heavy vehicle, and you get the picture.
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Mansfield
Just a minor technical point. If a heavily loaded jackscrew cannot move in the nose up direction, that does not mean it cannot move in the nose down direction. Consider the case of a scissors jack under a heavy vehicle, and you get the picture.
Just a minor technical point. If a heavily loaded jackscrew cannot move in the nose up direction, that does not mean it cannot move in the nose down direction. Consider the case of a scissors jack under a heavy vehicle, and you get the picture.
Mansfield, #42,
Your points about the AoA vane are well made. However for the complete picture it would be necessary to compare vane failure ‘rates’ between NG and MAX. Short of that, 3 failures so early in a new aircraft’s life is suspicious, but not beyond probability.
Whilst the weight of evidence points towards the vane, we should not forget wiring.
As for crew procedures, the use of the trim runaway drill was a hasty response to an accident. With hindsight it appears very inadequate, e.g. use of autopilot, which is very important for trim runaway, but less so for MCAS.
We should not blame crews for not following any checklist, for as much as the industry overly depends on crews being able to manage ‘all’ abnormal situations, so too that they will follow procedures - a grossly misused assumption.
For the human factors (cognitive) aspects, because we cannot know what the crew perceived and deduced, all that should be concluded is that a crew acted as they saw the situation at that time, with all of the confusing inputs, alerts and distractions, which now ‘we’ cannot comprehend - thats our limitation not the crew’s.
There is growing weight of argument that with trim already offset, then neither electric or wheel will be effective - speed dependant, note the ‘equivalent safety case’.
A simplified overview of the situation / dilemma (with hindsight) - from alf above.
With incorrect MACAS operation using yoke trim before selecting trim cutout would loose the battle with MCAS 10 to 5 against (misleading procedure - ‘can be used’).
Using manual trim wheel after trim cutout, then the (one) pilot might be unable to restore trim to give an acceptable stick force for the other pilot, as speed and trim ineffectiveness increase.
Thus there is a finely balanced point where cutout is used and aircraft trim can be recovered, or after cutout it is unlikely that the aircraft can be controlled. The first Lion flight was the ‘right’ side of the line - three crew, the second Lion flight understandably not so.
Others have argued a range of scenarios for switching and trim operation re fdr; we don’t know, we might only deduce. The important aspect for aircraft control is what the tail did, not that a switch was operated.
Re crews’ recording technical issues; this could be influenced by the widely used electronic log and self test - are there manufacturer’s recommendations re this (or sales pitch); compare with other operators.
I would not oppose your final suspicions, but suggest that they might better be phrased as a worldwide industry issue, and less interpretable as applying to particular operators.
Your points about the AoA vane are well made. However for the complete picture it would be necessary to compare vane failure ‘rates’ between NG and MAX. Short of that, 3 failures so early in a new aircraft’s life is suspicious, but not beyond probability.
Whilst the weight of evidence points towards the vane, we should not forget wiring.
As for crew procedures, the use of the trim runaway drill was a hasty response to an accident. With hindsight it appears very inadequate, e.g. use of autopilot, which is very important for trim runaway, but less so for MCAS.
We should not blame crews for not following any checklist, for as much as the industry overly depends on crews being able to manage ‘all’ abnormal situations, so too that they will follow procedures - a grossly misused assumption.
For the human factors (cognitive) aspects, because we cannot know what the crew perceived and deduced, all that should be concluded is that a crew acted as they saw the situation at that time, with all of the confusing inputs, alerts and distractions, which now ‘we’ cannot comprehend - thats our limitation not the crew’s.
There is growing weight of argument that with trim already offset, then neither electric or wheel will be effective - speed dependant, note the ‘equivalent safety case’.
A simplified overview of the situation / dilemma (with hindsight) - from alf above.
With incorrect MACAS operation using yoke trim before selecting trim cutout would loose the battle with MCAS 10 to 5 against (misleading procedure - ‘can be used’).
Using manual trim wheel after trim cutout, then the (one) pilot might be unable to restore trim to give an acceptable stick force for the other pilot, as speed and trim ineffectiveness increase.
Thus there is a finely balanced point where cutout is used and aircraft trim can be recovered, or after cutout it is unlikely that the aircraft can be controlled. The first Lion flight was the ‘right’ side of the line - three crew, the second Lion flight understandably not so.
Others have argued a range of scenarios for switching and trim operation re fdr; we don’t know, we might only deduce. The important aspect for aircraft control is what the tail did, not that a switch was operated.
Re crews’ recording technical issues; this could be influenced by the widely used electronic log and self test - are there manufacturer’s recommendations re this (or sales pitch); compare with other operators.
I would not oppose your final suspicions, but suggest that they might better be phrased as a worldwide industry issue, and less interpretable as applying to particular operators.
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The stabiliser responded normally to the captains manual (electric) trim ANU just a gnats cock below VMO (just before the stab trim was selected to cutout). So the motor would seem to be equal to the task.
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I think your assertion contains several logical inconsistencies:
1. The momentary trim inputs may be an artifact of the FDR data sampling algorithm, or the crew stopped trying when the trim wheels didn't turn after a few seconds. We don't really know for sure, and IMO you can't build an argument on such evidence.
2. The recorded increase in the stabiliser ANU position may also be a data artifact, or backlash in the cables and pulleys, or the trim motor locking mechanism. There are a number of suggestions from other sources (including EASA), that what you describe is physically impossible at high airspeeds and aerodynamic loads.
3. Re-enabling the trim switches after runaway trim, is not documented in any procedure, so cannot count towards the certification process. They could re-write the procedures, but currently it seems to fail certification under those extreme conditions.
1. The momentary trim inputs may be an artifact of the FDR data sampling algorithm, or the crew stopped trying when the trim wheels didn't turn after a few seconds. We don't really know for sure, and IMO you can't build an argument on such evidence.
2. The recorded increase in the stabiliser ANU position may also be a data artifact, or backlash in the cables and pulleys, or the trim motor locking mechanism. There are a number of suggestions from other sources (including EASA), that what you describe is physically impossible at high airspeeds and aerodynamic loads.
3. Re-enabling the trim switches after runaway trim, is not documented in any procedure, so cannot count towards the certification process. They could re-write the procedures, but currently it seems to fail certification under those extreme conditions.
2. The move between 2.1 and 2.3 when the trim was disabled certainly may be any of those. I doubt it would have had a significant effect at normal airspeeds, but was exacerbated near VMO.
3. Again, I disagree with you. They re-enabled the trim, but did NOT attempt to use the yoke trim switches after doing so! In similar dire straits I may have re-enabled the trim, but NOT until after I had the yoke trim switches held in the ANU position. If the trim did not IMMEDIATELY begin to go toward ANU, I would have cut out the trim again.
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As I mentioned earlier, I have to wonder whether the Ethiopian captain knew the trim had been reinstated. No way to really know unless the CVR transcript has more evidence. Regarding reinstating the trim cutoff switches, the captain of LNI043 did just what you said; he turned them back on, noted a trim moving toward AND, and shut them off again...permanently.
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I have a very strong feeling that the designers of the MCAS programming had no idea of the effect of speed upon the forces required to move the stabilizer trim, indicated by the fact that they slightly changed the algorithm to account for speed sometime during the certification process.
The original designers may have done calculations for high speed and less trim authority and found it was ok.
The "extreme high speed" regime is probably also the reason why MCAS apparently has no altitude dependency, since as-designed it would only engage at high altitude. Erroneous activation at low alt is clearly more hazardous, but possibility of AOA and IAS failing high, to give erroneous activation at low alt, might be suitably remote (despite the slight dependency in the ADIRU).
Later on in flight testing, a problem was found at low mach as well, so someone decided it should activate there (and therefore possibly at low alt), and with much more authority. That change quite possibly didn't require code changes, just configuration data. The sort of change you can get an "intern" to do... in fact the more junior the team member you can find the better - junior staff tend to bend awfully (and understandably) easily when leaned on from on high.
Did they redo the entire safety case for the new activation regime and authority levels and then just "forget" to update the certification documents, or did they "forget" to update a lot more than the cert docs? Did they just decide that the late discovery that the dynamite packing cases had a few screws missing wasn't a problem because they had a fully certified hammer, they'd just have to hit them a bit harder?
