PA34 pilot reports runaway electric trim at 6000 ft. Control of aircraft is lost until trim motor circuit breaker trips.

Narrative

The aircraft departed IFR; in VMC; on a ferry flight. The autopilot was unserviceable; but electric elevator trim worked. The aircraft was maintaining 6000 ft; as cleared; then recleared to 11000 ft. The electric trim was used to pitch up; but the aircraft started descending; so continuous nose-up trim was selected; to try to correct this. Very quickly; the aircraft was descending at more than 1000 FPM; and required all the pilot's strength to pull the control column back; to avoid pitching down. ATC was advised that it was not possible to climb; due to control problems. The elevator trim wheel appeared jammed in the forward position; and would not move. Greater force was used; the trim wheel was freed; and was set to a position that allowed the aircraft to continue under normal control. The electric trim circuit breaker had popped. ATC was told of the control problem; and that the aircraft was returning to the departure airport. After landing and closing down; the control problem was investigated; and found to be a defective switch on the left-hand control column. If moved forward; the trim wheel motored forward. If moved back; the trim wheel still motored forward. The flight continued the next day; with the electric trim CB pulled. Comments: The pilot was an experienced CFI; MEII; with over 1500 hours of multi time; of which about half was in Senecas. It was extremely fortunate that the incident occurred at 6000 ft. If it had happened on takeoff; in IMC; it would have probably caused a crash. This was a very serious (and probably very rare) failure mode of the trim switch. It could have been responsible for unexplained accidents in the past; and I believe it is essential to notify the NTSB of this incident. I suggest a controlled experiment is conducted to replicate the handling characteristics of full forward trim; when using climb power.