Originally Posted by
fgrieu
The control column trim switches are not the "cutout" switches. I'm inclined to believe that central cutout switches have authority over MCAS, but control column trim switches do not, contrary to
But again, do we have an authoritative answer, based on some circuit diagram, manual, first-hand knowledge?
Yes, we do. We are talking 3 sets of switches in series here: 1) thumb (normally open), 2) control column "extreme deflection" (normally closed) and 3)pedestal cutouts (normally closed).
What I meant is that if you electrically trim nose up with your thumb, MCAS stops trimming nose down right then, so you win. Plus, there is a 5 second delay between you releasing your thumb and MCAS trying to nosedive again. That's your timeframe to kill it with pedestal cutout switches.
I don't know where I learnt it first, but that fact is "authoritatively" shown on FDR from both lionair flights, to begin with. Also, reductio ad adsurdum: Otherwise no way the scenario is survivable for any amount of time with cutout switches to "normal". 60 seconds after first MCAS activation you are diving vertical, regardless of elevator position.
In fact, if you could
hear it trim, (you cant because stick shaker) it would be quite trivial to discover that, as soon as you trim with your thumb switch, it stops. I'd bet the accidents would have not happened.
The only ongoing debate now* is (and as far as I know, there is no consensus yet but there will be soon) if you were slow with your thumb and got to a point in which you need
a lot of pull to remain level, your control column "extreme deflection" trim cutout switches may inhibit your trim inputs and then (possibly) MCAS wins by lack of opposition. (because at that moment you will not recall yet another system working silently against you and release control column to regain the ability to trim).
Which, if comfirmed, is a frightening situation, because it only takes one panicking pilot pulling too hard to prevent both from trimming up and wining the fight against MCAS. Thats a good theory to explain what happened in the last seconds of the first crash.
*Edit: I'm quite convinced after examining again the schematics, that MCAS vs thumb trim switch battle is not affected by control column deflection switches.