The only way trim tabs will stay where you put them is if you turn auto pilot off.
Auto pilot sets your trim. If you change the trim manually with auto pilot on then the trim will go back to where the auto pilot sets it. To manually trim the aircraft you need CT and auto pilot to both be off.
When you change positions to gunners or F6 in a bomber, the auto-level trim kicks in and moves the trim tabs.
Bozon: I tried turning off the CT (while it was on by default) trimming it out, going to F3 mode, then F1. The CT light was still off.
F3 mode does not engage the auto-level. Moving to any gunner position, or entering the bomb sight (F6) will.
Therefore, if you manually trim your plane for a missing engine, you cannot move to any of the above mentioned positions. If you do, you'll be out of trim while there (auto-trim does not compensate for the feathered engine) and you'll have to manually re-trim when you move back to the pilot seat.
Easy to test - manually move one trim tab to its full extent, go to a gunner/F6 position, move back to pilot and looks at the trim tabs.
