Originally Posted by
flyingchanges
Just because it is not running continuously doesn't mean it is not a runaway. How many times does it need to do something that you have not asked for to qualify?
If the lane keeping function on your car tried to pull you into the ditch 3 or 4 times would you just let it keep on doing that or would you see a pattern and turn it off?
That using the cutout switches is the correct remedy both for erroneous MCAS activation and for stabiliser trim runaway does not make the two things the same. They are in fact very distinct and have very different symptoms:
erroneous MCAS activation:
- is intermittent
- runs for max 10s at a time
- can be stopped temporarily by pilot trim input
- is then inhibited for 5s
None of the above is true for a trim runaway. It
- runs constantly
- does not ever stop (unless it reaches the mechanical stop, is turned off by the cutout switches or is mechanically arrested by grasping and holding the trim wheel.)
The fact that at least one (and possibly two) crew did not find the remedy obvious, makes it an obvious design problem, and blaming the crew is the most useless thing to do if you want to find out how to prevent a recurrence. It is only ever the right thing to do if you want to feel smugly superior.
To reiterate: frequent automatic trim is normal on a 737 (STS, Mach Trim, ...), and is in no way comparable to a lane assist function going haywire. You comparison would be more appropriate if we were talking about the autopilot initiating a turn when none was called for.
Bernd