Bonanza 36
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Commercial
Flight Crew Last 90 Days 12; Flight Crew Total 2200; Flight Crew Type 221
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action
Aircraft
I was in level flight at 5000 feet MSL on an IFR flight plan talking to Departure Control. The S-TEC autopilot was in altitude hold mode and had been in such for at least a few minutes. Suddenly; the autopilot pitched up the plane. I caught the altitude increase at approximately 5350 feet MSL. There was not a conflict or even any ATC communication. This is the second time; however; this has happened with this autopilot; but it was at least 20 hours ago. I had flown it and others had during this time with no problems. Approximately 20 hours ago; I was in the plane and the same thing happened in cruise. It was [at] a slower altitude increase and I caught it at about 125 feet altitude increase. I am reporting this to see if there have been other problems with S-TEC AP's in altitude hold mode.
Beechcraft pilot reported an altitude excursion due to an autopilot malfunction.