Originally Posted by
GordonR_Cape
Alchad
I am quoting from memory, but the explanation somewhere on this forum is:
1. On the NG there are only two inputs for electrical trim, the autopilot and the yokes. Each has their own switch, wired in parallel.
2. On the MAX there are three inputs, if you add MCAS. Since adding a third switch is complicated, they wired the switches in series. This provides an extra level of inhibition of runaway trim, since one of the two could theoretically get stuck in the on position (due to short circuits).
I hope this is correct and helpful. The implications of these changes are beyond my scope.
Edit: I posted simultaneously with
infrequentflyer789 whose comment overlaps with and supports my recollection.
Think this was it post 2439 on the Ethiopian thread
@
RUTUS, Since there are no other electrical connections in that diagram, the logical conclusion is that indeed on the NG any kind of automatic trim changes can be disabled by the AP cutout switch, and also by the column cutout switch connected in series with it.
I can't find a similar diagram for the MAX, but I remember reading that the cutout system has been redesigned. For example the two cutout switches have been renamed, from MAIN ELECT and AUTO PILOT, to PRI and B/U (primary and backup).
And, if I remember correctly, those two switches don't longer have independent functionality on the MAX, because they are connected together in series. If one of them gets stuck or fails shorted, the other can act a backup for it, so on the MAX both manual cutout switches would disable any kind of electric trim, manual or automatic.
I don't have further details about that, and I wouldn't want to speculate about exactly how it works on the MAX in combination with the column cutout switches, but this has been discussed previously in the Lion Air thread, you may try to look there for more details.