Works been a bugger, so I can't say this is still true with latest patch, but...
In previous patch, if you had combat trim set to manual mode in the flight box (same box with auto-take off and tracers), if you turn combat trim on, it will stay on UNTIL you engage auto-pilot. When you disengage auto-pilot combat trim turns back off.
Test it yourself. Turn combat trim on, hit x, then x again. Betcha combat trim light goes off.