see 16 min onwards
Also see ‘jackscrew’ at approx 5 min
Like the little dog !!
Airborne_Again wrote:
BUT just before control was lost, there were two short manual trim-up commands that did cause a slight change of pitch trim in the up direction, shortly followed by another MCAS trim-down command which did cause a downward pitch trim change
So from reading between the lines and looking at the FDR readout it appears that the stabiliser trim was re-enabled at some point – either by pilot action or some other mechanism.
Exactly.
I think this is what happened:
Qu
At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working.
WHICH manual trim? On the FDR there is a try to input manual ELECTRIC trim which could not work, as the cut offs were switched off.
Manual trim should be using the trim wheel. The trim stayed in the 2.1° position for almost 2 minutes with the captain pulling on the yoke to keep the nose up. Why did nobody trim nose up manually with the wheel in this time? Or was there a small input from 2.1 to 2.3 degree and as it was so slow, someone thought it did not work?
So what exactly did the FO try after this? And what happens, if you switch the cut offs back on?
If that is what happens, that is the smoking gun and we have a MCAS induced incident which turned into an accident by crew error.
If not, then Boeing has yet another question to answer.
Mooney_Driver wrote:
the First-Officer replied that it is not working.
Given they seem to be able to distinguish when the trim switch on the control column is activated from the FDR/CVR it might suggest the FO could not overcome the airloads using the manual trim wheel.
Is it possible that the trim wheel cannot be manually rotated, due to aerodynamic load on the THS?
Certainly if the THS is trimmed too far out then a pilot cannot control the aircraft in pitch (if the airspeed is also high enough) but I am talking about the THS, not the elevator.
On the wider topic, I am wondering whether MCAS ever gets activated in normal operations, or whether it is supposed to protect only against excessive pitch-up which would not happen if the plane is flown properly. One crashed at TO+6 mins and the other at TO+12 mins – both high AoA phases. Obviously if Boeing say this 737 is like the others “and if you do what you could do in an old 737 then MCAS will activate and save the situation” then they are going to have to make some big changes. But the Q is still worth asking. Of course it is non-PC because both crashes were 3rd World airlines, but there isn’t anything one can do about that.
Ted wrote:
Given they seem to be able to distinguish when the trim switch on the control column is activated from the FDR/CVR it might suggest the FO could not overcome the airloads using the manual trim wheel.
Yes but that does not explain why the autotrim came back. I am starting to get a really funny feeling here. I can not imagine they put the cut off switches back and if so, they would have trimmed up as much as they could (which inhibits MCAS until you release the electric trim) yet we have two events when the stab trim was moved first by a little and then down in rapid trim after the cut outs were done. This does not compute. Not in the least.
Peter wrote:
both high AoA phases.
The actual AOA had nothing to do with it, it was the “fake” AOA coming from corrupt sensors. That is perfectly clear now. What caused the sensors to fail will be another investigation, in the Lion Air case they had ample evidence before and let the plane fly, in this case there was nothing until after lift off. What if they had a bird strike on the sensor? We don’t know and likely will never know.
Peter wrote:
Obviously if Boeing say this 737 is like the others “and if you do what you could do in an old 737 then MCAS will activate and save the situation” then they are going to have to make some big changes.
Well that is wrong and we know that. The trim system is totally redesigned. There is no trim brake. MCAS has a much higher trim speed than before. And I have a feeling that is not all by a long run.
The big question right now is, how is it possible that after deactivation of the trim via the trim cut out switches did they have 2 minutes later again trim input by autotrim?
And of course how on earth did Boeing get away with creating a system which will fly the airplane into the ground when it thinks it stalls? In this day and age, this is simply crazy. What happened to pitch and power to recheck the situaiton before letting a single AOA sensor kill a whole plane?
Mcas was supposed to act by 0.6° steps from what I have read somewhere, and it has been documented this way, but has been changed to 2.1 step after flight tests, actually to compensate lift created by lower part of the engine intake at high AoA.
Now I hear that the manual handwheel is not possible to use over 250 kt due to air loads on the elevator and the stabilizer….
not only for this model but for all of them.
If true, it leaves me speechless.
If a pilot cannot turn the trim wheel then neither can the servo.
This article https://theaircurrent.com/aviation-safety/vestigal-design-issue-clouds-737-max-crash-investigations/ explains possible control lock-up at high speed and mis-trimmed elevator. It also refers to a vers old 737 manual from the eighties where this situation is described and where there is a so colled “roler-coaster” procedure to solve locked up elevator. High speed was definitely a factor here. I am curious whether the “roller-coaster” procesure is in actual 737 manual but I doubt it. It is funny as the airframe remains the same. It was probably politically incorrect so they removed it…
Peter wrote:
If a pilot cannot turn the trim wheel then neither can the servo.
Why – would think the jackscrew motor (stabiliser) has more torque than pilots turning trim wheel ‘cranks’.
I assume the ‘trimwheel’ is connected to the stabiiser and the thumb trim (yoke) to the elevator.
As I questioned before I’m not sure what input from pilot / autopilot / MCAS etc is controlling what at the back of the aircraft ????
