Originally posted by Evan
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Lion Air 737-Max missing, presumed down in the sea near CGK (Jakarta)
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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.
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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
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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 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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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 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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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.
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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 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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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 3WE View PostI don't like seeing math on short-period radar data (like less than 5 min)...I've seen too many crazy "readouts" on ordinary flights from Flightaware/Flightradar/etc.
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Originally posted by pegasus View Post...if the training equips the pilots to revert to basic principles.
Then, in the real world I see pilots who overwhelmingly twist knobs as they fly precise vectors and standard procedures and use navigation systems.
I don't think they lack training or lack KNOWLEDGE, but I sometimes wonder if they just totally get out of the habit of flying.
I know that's a wrong statement as so many folks commit to "hand flying below 10,000 feet" (or many variants of that rule)…
Even then- you get people who totally lock in on the flight director- and their brain disconnects...and we even see folks chase bad flight director indications when 'traditional instruments' are clearly and correctly screaming foul...
You are SOOOOOOOOO SOOOOOO so so so so so so used to vanilla flying and knob twisting that you just don't pick up on weird stuff fast enough...
(And acknowledging that folks get good simulator and CRM training)…
Not sure what I'm saying other than maybe it's NOT the training, but that you can get comfortable with your YEARS of routines and then excrement transpires.
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Originally posted by Gabriel View Post***I would say that a sudden change from about +2500 fpm to about -3500 fpm in about 20 seconds Gabbieaeroengineeringmath***
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Originally posted by Aviation HeraldOn Nov 7th 2018 Boeing issued an Operations Manual Bulletin (OMB) to all Boeing 737 MAX Operators stating that the investigation into the crash of PK-LQP found one of the Angle of Attack Sensors had provided incorrect readings, which could cause the aircraft's trim system to uncommandedly trim nose down in order to avoid a stall during manual flight. The OMB directs "operators to existing flight crew procedures to address circumstances where there is erroneous input from an AOA sensor." The OMB reiterates the Stabilizer Runaway non-normal checklist.
The flight Crew Operations Manual Bulletin TBC-19 reads:
The Indonesian National Transportation Safety Committee has indicated that Lion Air flight 610 experienced erroneous AOA data. Boeing would like to call attention to an AOA failure condition that can occur during manual flight only.
This bulletin directs flight crews to existing procedures to address this condition. In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds. The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 seconds after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down stabilizer continue to occur unless the stabilizer trim system is deactivated through use of both STAB TRIM CUTOUT switches in accordance with the existing procedures in the Runaway Stabilizer NNC. It is possible for the stabilizer to reach the nose down limit unless the system inputs are counteracted completely by pilot trim inputs and both STAB TRIM CUTOUT switches are moved to CUTOUT.
Additionally, pilots are reminded that an erroneous AOA can cause some or all of the following indications and effects:
- Continuous or intermittent stick shaker on the affected side only.
- Minimum speed bar (red and black) on the affected side only.
- Increasing nose down control forces.
- Inability to engage autopilot.
- Automatic disengagement of autopilot.
- IAS DISAGREE alert.
- ALT DISAGREE alert.
- AOA DISAGREE alert (if the AOA indicator option is installed)
- FEEL DIFF PRESS light.
In the event an uncommanded nose down stabilizer trim is experienced on the 737-8 /-9, in conjunction with one or more of the above indications or effects, do the Runaway Stabilizer NNC ensuring that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight.
In manual flight, the NG (and I assume the MAX) will command a forward column trim force 4X the normal one and will continue to trim the THS (incrementally?) to a full nose down position. You don't want a single sensor failure to cause this to happen.
Onside stickshaker activation, fine, but not this.
If the pilots have good situational awareness, they can stop this by using the trim cutoff switches. Normally, electrical trim can also be stopped by pulling or pushing against the trim direction, but maybe not in the excessive AoA regime. They can also just grab the trim wheel and stop it.
SItuational awareness should come from the stickshaker, the sudden column force and the trim wheel activation. Hopefully the AoA indicator is now standard on the MAX. It probably isn't...
But also... it results in an airspeed and altitude disagree warning. Just to help scramble your situational awareness I guess. And no AoA indicator: no AoA disagree warning. Brilliant.
Hopefully the report will tell us that a single failure cannot cause an uncommanded full-nose-down stabilizer trim.
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