Originally Posted by
SLFinAZ
I've got a fairly basic question, my understanding is that a fundamental concept in aircraft design is that a control surface should never be able to be neutralized let alone over ridden by it's trim tab(s). Yet from some of what I've read on this thread it seems that that it's actually possible with the 737 for the force of the trim tab to exceed the capability of the main control surface?? That makes no sense to me at all...
ELI5YO:
Someone else already gave you the wiki page. On most "newer' big aircraft there is a moving horizontal stabilizer. If you trim, or the auto-trim works, it sets the movable horizontal stabilizer so that you don't have to push or pull on the elevator. there is no trim tab. The stabilizer is a lot bigger than the elevator, so if something goes wrong with the setting of the stabilizer, like a runaway trim due to an air data/aoa failure it could overpower the capabilty of the elevator/pilot input. Normally the emergency checklist would direct you to do something to switch the auto-trim off and regain manual control (on the B737 elevator cutout switches).