Originally posted by 3WE
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Lion Air 737-Max missing, presumed down in the sea near CGK (Jakarta)
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Boeing would like to call attention to an AOA failure condition that can occur during manual flight only....
Additionally, pilots are reminded that an erroneous AOA can cause some or all of the following indications and effects:
- Automatic disengagement of autopilot.
So it can put itself in this failure mode!
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Originally posted by vaztr View PostBoeing would like to call attention to an AOA failure condition that can occur during manual flight only....
Additionally, pilots are reminded that an erroneous AOA can cause some or all of the following indications and effects:
- Automatic disengagement of autopilot.
So it can put itself in this failure mode!
The 737MAX has two AoA vanes.
The loss of autoflight is because the autoflight must always have multiple sensors in agreement on air data. It will not rely on a single source that might be erroneous, so, if one vane doesn't match the other, no autoflight. Also, the NG uses AoA vane data to calibrate other air data values, which also wouldn't match.
The uncommanded stabilizer trim (see the AvHerald repost above) that we are discussing here occurs, of course, in manual flight only, and apparently it does not require agreement on air data, although I pray that's not true...
The A320 has three AoA vanes, so a single failure can be fail-passive. The A350 and—I think— the A380 have four.
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From the most recent statements, it seems like what we are looking at is a ground repair to correct pitot probe issues that resulted in a damaged, defective or otherwise malfunctioning AoA vane. The pitot issues on the previous four flights might not have played a role in the crash.
The maintenance log mentions flushing the pitots. Hopefully this was done by the book, in a way to avoid water ingress into the AoA vanes. Otherwise, they could have iced up, although maybe not at 5000ft in Indonesia...
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Originally posted by Evan View PostFrom the most recent statements, it seems like what we are looking at is a ground repair to correct pitot probe issues that resulted in a damaged, defective or otherwise malfunctioning AoA vane. The pitot issues on the previous four flights might not have played a role in the crash.
The maintenance log mentions flushing the pitots.
Apart from the remark of unreliable airspeed and altitude, which prompted the flushing of the captain's static ports, an entry for elevator feel computer light illuminated was written down by the flight crew of [the previous flight], maintenance opened and cleaned a cannon plug connector for the elevator feel computer
Additionally, pilots are reminded that an erroneous AOA can cause some or all of the following indications and effects:
- ...
- FEEL DIFF PRESS light.
I would say that a sudden change from about +2500 fpm to about -3500 fpm in about 20 seconds (that's -6000 fpm per second or -100 fps or an acceleration -3.1g which corresponds to a load factor of -2.1) would be a sudden, very likely uncompounded, fall. And since it happens 3.5 minutes after lift off with the plane barely above 5000 ft, I would say that this qualifies as shortly after take-off too.
There were testimony from different passengers saying that the plane suddenly started to fall out of the sky and that they thought that was the end and all these typical comments that normally don't mean much in the sense of reliable data for an investigation, but some times it does if it is consistent with other hard data.
One could say that perhaps this was the result not of a real fall but of a false altitude and vertical speed indication caused by unreliable air data, but the way the GROUNDspeed varies during the incident makes me think this was not the case.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
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Originally posted by Gabriel View PostThere was more than that in the previous flights...
Combine with this:
And with this (re: ADS-B data from previous flight):
I would like to know what, if any, ground maintenance was done in response to the previous three instances.
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Some more info on the bulletin and the system involved
https://leehamnews.com/2018/11/07/bo...-air-accident/
In the comments the author mentions a hypothetical scenario where the plane could be spinning in a severe stall, and then one AoA vane would be blocked and would get no airflow, while the other would be at a high alpha, making it difficult to set up a logic for AoA data discrepancies. To me it seems two vanes is not enough.
But that auto trim stall prevention system is a bit of a shock to me, if it is what happened. What if the pilots also had UAS? And the autopilot was off. In such situations, to my knowledge, most systems are designed to give pilots full control authority.
Am I the only one who thinks there would have been a spectacular reaction on the internet over this if it had happened on an Airbus.
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Originally posted by Black Ram View PostAm I the only one who thinks there would have been a spectacular reaction on the internet over this if it had happened on an Airbus.
What if the pilots also had UAS?
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Originally posted by Black Ram View PostAm I the only one who thinks there would have been a spectacular reaction on the internet over this if it had happened on an Airbus.
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Originally posted by Evan View PostYes, it certainly looks like an incidence of erroneous aoa data there as well.
I would like to know what, if any, ground maintenance was done in response to the previous three instances.
Apart from the remark of unreliable airspeed and altitude, which prompted the flushing of the captain's static ports, an entry for elevator feel computer light illuminated was written down by the flight crew of [the previous flight], maintenance opened and cleaned a cannon plug connector for the elevator feel computer
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
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Originally posted by Black Ram View Posta hypothetical scenario where the plane could be spinning in a severe stall, and then one AoA vane would be blocked and would get no airflow, while the other would be at a high alpha, making it difficult to set up a logic for AoA data discrepancies.
But that auto trim stall prevention system is a bit of a shock to me, if it is what happened. What if the pilots also had UAS? And the autopilot was off. In such situations, to my knowledge, most systems are designed to give pilots full control authority.
This is an issue in Airbus too. AF had the trim automatically and stealthy move almost full nose-up. In certain conditions it might have reached the full-nose up position and deactivate there and the only clue for the pilots would be a "manual trim" message in the EICAS.
Trim runaway has been an issue and potential risk since WW2.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
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Originally posted by Evan View PostYes, it certainly looks like an incidence of erroneous aoa data there as well.
I would like to know what, if any, ground maintenance was done in response to the previous three instances.
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I was right to hesitate to join the thread. The response from some long standing contributors was to sneer and having sneered not to respond when I offered some clarification of my background.Notably, they failed to respond to my question as to whether pilots have the ability to go back to basic principles when the systems fail and to establish and maintain the aircraft in a safe and stable attitude.Instead, the response to this crash seems to concentrate on a retrospective forensic examination of how the " systems" failed. When you are sitting at the front and the system has landed you up shit creek it might be more pertinent to remember before anything else what keeps the aircraft flying. Having spent all those hours in the cockpit, don't you have a picture of how the view outside should look or what the artificial horizon might tell you?
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Originally posted by pegasus View PostI was right to hesitate to join the thread. The response from some long standing contributors was to sneer and having sneered not to respond when I offered some clarification of my background.Notably, they failed to respond to my question as to whether pilots have the ability to go back to basic principles when the systems fail and to establish and maintain the aircraft in a safe and stable attitude.Instead, the response to this crash seems to concentrate on a retrospective forensic examination of how the " systems" failed. When you are sitting at the front and the system has landed you up shit creek it might be more pertinent to remember before anything else what keeps the aircraft flying. Having spent all those hours in the cockpit, don't you have a picture of how the view outside should look or what the artificial horizon might tell you?
Yes.
Except for the one's who aren't very well trained in systems. You seem to be suggesting that, when systems fail, all you have to know is basic airmanship, the skills every glider pilot should excel in. That's a popular belief on this forum. But it isn't true.
Sometimes systems are doing things that might confuse you or trip you up or scramble your situational awareness. If you know the systems as well as the airmanship, you know how to get these system failures under control and out of the way. That is part of stabilizing a modern airliner.
In the hypothetical case we are building here, that a system anomaly led to an uncommanded and unwarranted stabilizer movement, the pilots who know how to stabilize the aircraft would respond, in part, by cutting off the automatic stab trim. Every pilot who is properly trained on the 737NG/MAX has practiced the runaway pitch procedure very well.
And for the ones who haven't, the answer is:
No.
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