Incident Summary
During an engine start procedure, the pilot noticed the starter was operating but did not engage the engine. He turned off the magnetos and battery, then exited the airplane to investigate. The pilot did not chock or otherwise secure the airplane. After turning the propeller slightly, he began to walk away. At that point, the engine started unexpectedly, and the airplane moved across the ramp before colliding with a parked Beech 77.
Investigation Findings
An inspection of the airplane revealed that the left magneto would not ground out with the ignition switch, resulting in a "hot mag" condition. This condition allowed the engine to fire despite the magnetos being switched off.
Probable Cause
The investigation determined the probable cause to be a hot magneto condition caused by the left magneto's failure to ground out with the ignition switch.