Originally Posted by
wilco77
Thanks that makes sense.
Why do we also disconnect PFC if the airplane cannot be trimmed?
In normal mode, pitch trim is achieved by changing the trim reference speed. The PFCs move the elevators to trim the aircraft and the stabiliser then moves to streamline the elevator. The PFCs need airspeed data to perform that function. If you can trim the aircraft, well and good. If not, then you must disconnect the PFCs and revert to direct mode to restore pitch trim. In direct mode, the pitch trim switches move the stabiliser directly.