Does anyone know if this aircraft was equipped with the Garmin or Stec autopilot? I would suspect the Garmin... but nevertheless, I think the below is applicable to each type.
The
flight manual says:
Pitch trim is provided by adjusting the neutral position of the compression spring cartridge in the elevator control system by means of an electric motor. It is possible to easily override full trim or autopilot inputs by using normal control inputs
An electric motor changes the neutral position of the spring cartridge attached to the elevator control horn.
Elevator (pitch) trim operates on 28 VDC supplied through the 2-amp PITCH TRIM circuit breaker on ESS BUS 2.
The autopilot interfaces the pitch trim system via a pitch trim adapter (also powered via the PITCH TRIM CB), allowing the AP to drive pitch trim. A failure of this module could cause trim runaway as described, which is not solved by hitting the AP disconnect. By selecting approach flap, the autopilot could have been attempting to maintain VS and subsequently feeding in pitch servo, meanwhile commanding forward pitch trim. However if the forward pitch trim function had failed, this would result in a pitch up when the AP was disconnected.
I also see that the autopilot minimum use height on approach is 400' AGL, so it would seem sensible that this pilot was practicing its use in VMC, prior to finding himself in that situation while in the soup....