737MAX Stab Trim architecture

[infrequentflyer789]

Originally Posted by Loose rivets

Both on R&N but this one has some historic discussion with gums et al.

https://www.pprune.org/archive/index.php/t-576817.html

Thanks for the pointer. This bit jumped out for me:

Ok, so for aircraft with trimmable horizontal stabilisers (THS) you must never hold the trim switch engaged for extended periods, but must release the switch frequently, so as to allow the column to return to neutral - and allow the forces to diminish.

So, that process would be:

1. "Blip" the thumb-switch for a second or two, then release;
2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;

Then repeat. Blip, relax, assess. Blip, relax, assess.

But this is NOT how pilots are taught to trim an aircraft. When pilots first learn to fly, one of the first things they learn is to hold the attitude, then trim until the control forces go to zero.

But it seems that trimming a large, THS aircraft that way can be fatal.

"Blip, relax, assess. Blip, relax, assess."

One wonders what the result of that trim technique would be with MCAS in play...