Inside the Effort to Fix the Troubled Boeing 737 MAX

Bill Watson

Bill Watson

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The first thread on this was too long for me to follow.
Is this about correct?
The original install used one AOA sensor input to the computer - which would give erroneous annunciations and control inputs when that sensor failed; this was all software v. 11.
The fix proposed uses two sensors and if there is a miscompare, the system stands down; this is all using software v. 12.1

And, in each occurrence leading up to this point, had the pilot disabled the system (switch? CB?) as described in a manual(?) when he noticed it behaving erratically, normal control would have been achieved.

please correct as needed.
 
Let'sgoflying! said:
And, in each occurrence leading up to this point, had the pilot disabled the system (switch? CB?) as described in a manual(?) when he noticed it behaving erratically, normal control would have been achieved.
Click to expand...
Stabilizer Trim Cutout switches.
 
Let'sgoflying! said:
And, in each occurrence leading up to this point, had the pilot disabled the system (switch? CB?) as described in a manual(?) when he noticed it behaving erratically, normal control would have been achieved..
Click to expand...

Well none of the pilots knew that not doing it fast enough would result in out-of-trim forces too powerful to overcome with the manual trim wheel or elevator.
 
dmspilot said:
Well none of the pilots knew that not doing it fast enough would result in out-of-trim forces too powerful to overcome with the manual trim wheel or elevator.
Click to expand...
They apparently also didn't know that if the nose starts becoming "heavy" they can use the trim thumb-switches on the yoke to stop the nose-down trim inputs and keep the aircraft in a trimmed-state indefinitely, even during unschedule MCAS activations.
 
Larry in TN said:
They apparently also didn't know that if the nose starts becoming "heavy" they can use the trim thumb-switches on the yoke to stop the nose-down trim inputs and keep the aircraft in a trimmed-state indefinitely, even during unschedule MCAS activations.
Click to expand...
It seems like the captain of the Lion Air accident flight may have known, but failed to communicate it to his first officer.
 
Palmpilot said:
It seems like the captain of the Lion Air accident flight may have known, but failed to communicate it to his first officer.
Click to expand...

This is the biggest tragedy - the captain had it under control. I speculate about how the transition led to the crash, and rather suspect that the captain was instinctively using airmanship (recognizing the need for down trim and applying it as needed), but weak ADM (never converting the symptom - repeated need for nose-up trim - into action - disabling the automatic pitch trim). Of course, this whole time, he’s flying with the stick shaker shaking away.

Actually, even worse is the failure of the crew of the prior flight to either (1) abort their flight and immediately land for correction, or (2) properly and fully report the symptoms, thus allowing the MX guys to properly diagnose and repair.
 
Larry in TN said:
They apparently also didn't know that if the nose starts becoming "heavy" they can use the trim thumb-switches on the yoke to stop the nose-down trim inputs and keep the aircraft in a trimmed-state indefinitely, even during unschedule MCAS activations.
Click to expand...
How much authority do the thumb-switch trims have? To me this is the big question.
 
I haven't paid a lot of attention to the subject at hand so I may have missed this, but did this problem occur during certification/test flights.??
 
Zeldman said:
I haven't paid a lot of attention to the subject at hand so I may have missed this, but did this problem occur during certification/test flights.??
Click to expand...

Don't think they would take off with a broken AoA sensor or had a way to simulate one so I would guess no.
 
Zeldman said:
did this problem occur during certification/test flights.??
Click to expand...
No. But during the flight tests of the MCAS the test pilots determined the MCAS actuation rate was too low, so they increased the rate. However, in order to get the MCAS to actuate the aircraft must be put into an extreme flight condition at the edge of its certified flight envelop. The Lion and Ethiopia accidents both had a faulty AOA sensor that caused the MCAS to actuate outside of its designed intent.
 
dmspilot said:
The thumb switches override MCAS.
Click to expand...
Then it is truly unfortunate that the pilots did not hang on to that switch and land at the nearest safe place. How did they not know to do that? I am mystified.
 
So...with the modified MCAS, is it still meeting the TC? Or shhh, don't talk about that?
 
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