by Chris » Thu Sep 30, 2010 3:02 pm
Well, in the case of altitude hold, the PID loop should automatically command an elevator deflection to pitch the airplane to the correct angle of attack. The AoA vane would just give you a measure of what that angle is since the elevator deflection doesn't necessarily correspond to AoA.
So yea, it makes sense that using pitch hold, you would descend or ascend, unless you chose the exat right pitch to create the lift = weight, and thus hold altitude.