Comment by WalterBright
1 day ago
> The computer overrode the inputs.
This is incorrect. The manual stabilizer trim thumb switches override MCAS.
1 day ago
> The computer overrode the inputs.
This is incorrect. The manual stabilizer trim thumb switches override MCAS.
Are we not in agreement? MCAS overrode the inputs and the thumb switches could override MCAS?
The pilot's inputs are the thumb switches, and they override MCAS.
Additionally, the stab trim cutoff switch overrode both MCAS and the thumb switches.
Using both easily and successfully averts MCAS crashes, as proven in the first incident (there were three, but only two are reported on).
There were also a lot of MCAS near misses.
3 replies →
MCAS autonomously adjusts trim downward. The trim switches override MCAS, but when released, MCAS can resume trimming down again. The trim adjustments don't "override" the pilot's elevator inputs (MCAS has no direct control over the elevators), but they can make the controls so heavy that it's impossible to pull up.
If MCAS is running the trim, the thumb switches override it.
MCAS affects the stabilizer, the thumb switches affect the stabilizer, the cutoff switch affects the stabilizer.
The elevators are controlled by the control column and the autopilot.
> The trim switches override MCAS, but when released, MCAS can resume trimming down again.
That is correct. That is why the procedure is to return the trim to normal with the thumb switches, then turn off the trim system. That's it. That's all there is to it.
> but they can make the controls so heavy that it's impossible to pull up.
Almost right - the trim has more authority than the elevators. The trim's ability to travel far is to provide great ability to get out of trouble. I don't really know what factors the aerodynamics guys used to calculate the max travel required. I do know there is a travel limiter on it (as I worked on that, too!) which reduces the max travel at higher speeds, because otherwise it can rip the tail section off, which is a big no-no.
There are sooo many constraints on the design of an airplane I sometimes wonder how anyone manages to make one that works at all. The Wright Bros calculated that their machine would fly, and it did, barely. Their contemporaries did seat of the pants design, which is why they failed.
3 replies →