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Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

reuters.com

11–20 of 272 posts

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#11
post #6

Earlier quoted context omitted.

Was the trim wheel not visibly spinning out of their control? Simply because the symptoms are different should mean that stopping it via taking manual control is a recurring option.

The spin was not visible to these pilots presumably because it was happening in small increments with a delay between them.

Exactly. It was not "runaway" in the sense of "continuously doing the same." Something was happening, from the point of view of the pilot and co-pilot "at random." (Not to mention that on the previous models exactly such pattern was effectively impossible to happen). The faulty non-redundant sensor was producing false reading and that was the input to the to the pilots completely unknown MCAS which then moved the nose down based on the difference between the input and MCAS' expected target, turning "the correction" on only from time to time.

The presence of the third, off-duty pilot on the flight before additionally explains even better how in that case the crash was avoided. While the pilot and co-pilot were at the controls, the third one, looking from behind, didn't have the direct duties to do, so he had enough time to both just observe everything that was going on and the less immediate stress to allow him to come to the solution that worked.

Like when one programmer spends half an hour on something "not working" and complains to the colleague who just takes a look and immediately sees what the first one hasn't for half an hour. Additionally the movement of the trim wheel is actually easier to follow from the place in the back (see https://news.ycombinator.com/item?id=19440045 ).

And there's also a possibility that the false sensor reading and MCAS together produced a slightly different differential signal which resulted in more obvious manifestation of the problem. Possibly both elements contributed to the lucky outcome in that previous flight.

In the old times of civil aviation there was the third person on duty in the cockpit, the "flight-engineer." As the planes got more computerized controls, they started to be certified to fly with two-pilot crews. The computerized systems, when working, do reduce the chance of the crash and overall number of accidents did decrease simply because the humans were on the average less often in control, and therefore there was less chance to do anything wrong.

The problem is when the unreported computerized controls secretly depend on the single sensor that can be faulty. And when they actively confuse the pilot and ruin the flight instead of helping him.

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#12
post #2

This appears to answer the question of why, given that there is a "memory item" for dealing with runaway trim that would have helped, it wasn't followed by the crew: > “They didn’t seem to know the trim was moving down,” the third source said. “They thought only about airspeed and altitude. That was the only thing they talked about.” If this is true, not only did the pilots not realize the trim was the cause of the d…

Was the trim wheel not visibly spinning out of their control? Simply because the symptoms are different should mean that stopping it via taking manual control is a recurring option.

Part of the problem is that the MCAS system has a 20-second delay after the last time the pilot touched controls for the flaps, and it adjusts itself to a new position and then stops. There are no other indicators that it's happening, and since it's noncontinuous and has a delay, it would be easy to miss the only visual sign of the trim wheel readjusting itself once, especially since the wheel itself is by the pilot's thigh outside of forward line of sight.

In a normal runaway trim situation, by comparison, the trim wheel continues obviously spinning as long as the problem is going on, so it only takes a single glance at any time to notice it.

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#13
The thing I don't understand about that MCAS system is why it would continue to drive the nose of the plane down even when the pilot is reefing on the stick to pull up. The cruise control in my car automatically disengages when I hit the brakes, it doesn't continue to accelerate to keep the car going 60 while I am jamming on the brakes, in an emergency, I don't have time to find the off switch. Similarly, I would think that in an emergency, it should automatically disengage if the pilot pulls hard on the stick, in an emergency, the pilot doesn't have time to read the manual to find the off switch for the thing. It seems crazy to me that they wouldn't have a safety override like that.

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#14
post #9

Earlier quoted context omitted.

Was the trim wheel not visibly spinning out of their control? Simply because the symptoms are different should mean that stopping it via taking manual control is a recurring option.

I doubt it would have been "out of control", it would have just been slow changes over time as trim increased. The pilots might not have been fighting it much either, if they thought that airspeed was the problem, and not pitch.

The graph on page 14 of this report[0] is pretty telling about how slow the changes were... Note Trim Manual, Trim Automatic, and Pitch Trim Position.

[0] https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-...

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#15
What I don't understand whenever I read anything related to the MCAS system is why does it not check if the plane is in a dangerous nose dive using other sensors the plane might have? Does the average plane not have a gyroscope? My sub 500 dollar phone has both a gyroscope and an inertial measurement unit (accelerometer).

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#16

One of the factors that has been quickly glossed over is the low level of experience of some of the pilots in both the Indonesian and Ethopian accidents. In the USA, due to much higher levels of experience pilots have been correcting for the MCAS actions effectively and complaining about it to NASA (in order to avoid being branded as whistle-blowers by their airlines).

[deleted]

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#17

What I don't understand whenever I read anything related to the MCAS system is why does it not check if the plane is in a dangerous nose dive using other sensors the plane might have? Does the average plane not have a gyroscope? My sub 500 dollar phone has both a gyroscope and an inertial measurement unit (accelerometer).

Because MCAS never assumes the input sensors could be faulty.

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#18

One of the factors that has been quickly glossed over is the low level of experience of some of the pilots in both the Indonesian and Ethopian accidents. In the USA, due to much higher levels of experience pilots have been correcting for the MCAS actions effectively and complaining about it to NASA (in order to avoid being branded as whistle-blowers by their airlines).

I’d be highly concerned of the competency of any pilot who complains to NASA.

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#19
post #7
post #4

In the other news, on the previous day on the same plane there was a retired pilot flying in cockpit who correctly diagnosed problem & told how to disable malfunctioning system: https://www.bloomberg.com/news/articles/2019-03-19/how-an-ex...

That's the most amazing revelation to me so far. The same plane had an identical uncontrolled-dive problem the day before, which required an emergency power cut to the improperly-engaged motor, and somehow that incident didn't ground the plane or even result in any effective advisory to the next crew. Further, the Bloomberg story reports: "The Indonesia safety committee report said the plane had had multiple failures…

[deleted]

Re: Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

#20

The thing I don't understand about that MCAS system is why it would continue to drive the nose of the plane down even when the pilot is reefing on the stick to pull up. The cruise control in my car automatically disengages when I hit the brakes, it doesn't continue to accelerate to keep the car going 60 while I am jamming on the brakes, in an emergency, I don't have time to find the off switch. Similarly, I would thi…

Flight augmentation systems work a different way. Those take into consideration all inputs and average it out. Same when two pilots give different instructions.
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