Originally Posted by
CBreezy
Because the heading bug controls actual flight characteristics whereas, in most situations, the ALTO knob does not. Again, why not set your own flaps or lower your own gear?
Because unlike the flaps, the altitude pre-selector does control the flight path of the aircraft. I just had a loss of separation because as the altitude captured, I looked at the approach plate and the PM got an airspeed assignment confused with a new altitude and caused the aircraft to dip below the assigned altitude. At the same time, a new controller climbed an aircraft too soon so we got an RA because we ended up going 100 feet below our altitude.