Baron 58/58TC
Flight Crew Multiengine; Flight Crew Commercial; Flight Crew Instrument
Flight Crew Last 90 Days 60; Flight Crew Total 1900; Flight Crew Type 450
Time Pressure
Aircraft Equipment Problem Less Severe; Airspace Violation All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
VFR departure normal. Right turn at shoreline. I departed with flight following but was informed ATC would be too busy along my route and was told to squawk VFR. Limited to the Class Bravo shelf; I set my altitude select to 4500 ft. and engaged the autopilot in the climb. Monitoring the climb; I saw the plane continue the climb past 4500 ft. and where I expected it to settle back to 4500 ft.; instead go through 4600 ft. upon which I disengaged the auto pilot and pushed on the yoke. However; the trim up elevator wouldn't relax; I couldn't turn the trim wheel and all I could do was push hard against the yoke as I busted Bravo and the plane climbed at least another 400 ft. with the trim stuck. I pulled the circuit breaker but it still wouldn't release and I manually pushed the yoke hard to get the plane below the bravo. I cycled the breaker and the switch and finally got the trim under control. I hand flew at 4500 ft. until passing the shelf and continued on my flight climbing to 8500 ft. with no difficulties. Later; I determined that I have a faulty autopilot altitude switch. Getting it fixed tomorrow. In retrospect; I feel I handled the problem calmly but expeditiously.
BE58 pilot reported overshooting altitude during climb due to autopilot runaway.