Lion Air 737 Max Accident
11-13-2018, 12:45 PM
#51
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Originally Posted by JetEagle
Am I the only one who finds it odd that the 737 control yolk is missing a pitch trim disconnect button. Can someone who flies Boeing enlighten me as to the human factors philosophy in excluding a button that nearly every other airplane has on their controls.
"Every other airplane"?Don't remember such a control on the Airbus stick and you're correct, they're not on the Boeing yoke, nor are they called "trim disconnect". I think you might be referring to the stabilizer disconnect controls. They're not on the yoke....because they're not. I guess Boeing decided they had a better place to put them. Usually on the aft part of the throttle quadrant somewhere.
11-13-2018, 01:09 PM
#52
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Originally Posted by JetEagle
Am I the only one who finds it odd that the 737 control yolk is missing a pitch trim disconnect button. Can someone who flies Boeing enlighten me as to the human factors philosophy in excluding a button that nearly every other airplane has on their controls.
It’s a 200 but the 300/400 have it in the same place.
11-13-2018, 02:53 PM
#54
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So after doing some more reading on this, it appears this new system is a variation of the existing elevator feel system and the speed trim system. Essentially, it moves the stabilizer via the trim system and the trim wheels will rotate when this is occurring. The pilot should be able to stop the automatic movement of the stabilizer by using the thumb switches, and if that doesn't work, the cutout switches.
The system is slightly different, but the procedure for shutting it off is the same as it was on the NG.
It can drive the stabilizer in 10 second increments, which would be a decent amount of trim and stick force to overcome.
The pilots on the previous flight used the cutout switches to regain control and were successful.
Then you would have to pop out the handle and manually trim the airplane.
The system is slightly different, but the procedure for shutting it off is the same as it was on the NG.
It can drive the stabilizer in 10 second increments, which would be a decent amount of trim and stick force to overcome.
The pilots on the previous flight used the cutout switches to regain control and were successful.
Then you would have to pop out the handle and manually trim the airplane.
11-13-2018, 04:15 PM
#55
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From the Emergency AD note (2018-23-51, Nov 7, 2018):
"This emergency AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer."
But the AOA sensor was changed prior to the accident flight. (?)
"This emergency AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer."
But the AOA sensor was changed prior to the accident flight. (?)
11-13-2018, 04:18 PM
#56
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Originally Posted by 1wife2airlines
When did Boeing decide not to put a mechanical stab trim brake in their 737 series.
11-13-2018, 04:27 PM
#57
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Originally Posted by Adlerdriver
So, you're saying at one time, 737 aircraft had a mechanical stab trim brake?
11-13-2018, 04:54 PM
#58
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Originally Posted by 1wife2airlines
I was under that assumption, not having flown it, that the ability to stop the stabilizer from trimming by moving the elevator opposite it was the same mechanical system as is on the B727. Is it? If not was it an electrical system that Boeing then removed.
I’m pretty sure that’s how I remember it working. Moving the control column in the opposite direction of undesired trim motion, automatically cuts out those trim inputs. Does that constitute a “mechanical brake” in your view?
757/767, 777 all have the same STAB cutout switches that remove hydraulic power from the stabilizer (never flown 747 or 787 but they look to be the same as well). Those are memory items switches if we receive warning of uncommanded/unscheduled stab movement. Once they're off, hydraulic power is removed from the stabilizer trim control module. I don't believe it's possible for the stab to move due to air loads (because of the actuator design).
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