Re:Boeing 737 MAX crash and the rejection of ridiculous data
by chupa-chups on Wednesday April 10, 2019 @03:47PM
(#19628134)
This is not the problem. MCAS in itself sadly is the problem.
They had to solve the issue that the pitch-up moment shouldn't be accelerating on its own just depending on AoA, which was (aerodynamically) inevitable without automated adjustments due to the placement of the engines.
They had to resort to the worst possible cludge, since they weren't even allowed to add new electrical systems (which would have caused a recertification and/or a retraining), so they resorted to an already existing system (assisted trimming by autopilot).
The effect of MCAS is only "fixed" by manually adjusting the trim, which involves moving a jackscrew which holds the last section of the elevator at a fixed angle. Unfortunately the required force of moving this jackscrew increases with the airspeed. There is no easy way out. A bit more background can be found here: https://www.satcom.guru/2018/11/stabilizer-trim.html
Btw, the extra price item was an AoA disagree warning. This is related to MCAS in a way like an odometer failure to traction control. It wouldn't have helped, if pilots stuck to their memory items and checklist (which they have to: they recognize runaway trim and have to react accordingly).
To get a slight feeling what it means to have runaway trim (without assistance which they had to disable in concert with MCAS) please have a look at this video:
https://vimeo.com/329558134
They had to solve the issue that the pitch-up moment shouldn't be accelerating on its own just depending on AoA, which was (aerodynamically) inevitable without automated adjustments due to the placement of the engines.
They had to resort to the worst possible cludge, since they weren't even allowed to add new electrical systems (which would have caused a recertification and/or a retraining), so they resorted to an already existing system (assisted trimming by autopilot).
The effect of MCAS is only "fixed" by manually adjusting the trim, which involves moving a jackscrew which holds the last section of the elevator at a fixed angle. Unfortunately the required force of moving this jackscrew increases with the airspeed. There is no easy way out. A bit more background can be found here: https://www.satcom.guru/2018/11/stabilizer-trim.html
Btw, the extra price item was an AoA disagree warning. This is related to MCAS in a way like an odometer failure to traction control. It wouldn't have helped, if pilots stuck to their memory items and checklist (which they have to: they recognize runaway trim and have to react accordingly).
To get a slight feeling what it means to have runaway trim (without assistance which they had to disable in concert with MCAS) please have a look at this video: https://vimeo.com/329558134