Incident Summary
A pilot preparing for an intended flight found the aircraft's batteries dead and requested an external power cart from the fixed base operator (FBO). The pilot completed a waiver but did not recall discussing the aircraft's voltage requirement with FBO personnel. The FBO personnel stated that their external power cart had a minimum voltage of 28 volts and asked the pilot if his aircraft's electrical system was 28 volts, to which the pilot replied yes. The aircraft actually had a 12-volt system.
Sequence of Events
A passenger remained outside the aircraft while the pilot was inside and a ground power unit (GPU) with a minimum of 28 volts was connected to the external power receptacle. The pilot told the line service person to turn on the power and immediately noticed the alternator circuit breaker had tripped. An individual from the FBO heard a bang and saw smoke coming from the aircraft. The passenger later told the pilot that the line service person was frozen, but the passenger was able to disconnect the GPU from the aircraft and noted arcing during the disconnection.
The pilot reported seeing smoke at the rear of the aircraft. The passenger, who was outside, stated that when he came off the wing an explosion occurred, and fire and smoke were coming from the top of the aircraft. The pilot exited the aircraft and used a fire extinguisher from it to extinguish the fire. Fire department personnel responded but the fire had already been extinguished before their arrival.
Postaccident Findings
A postaccident inspection of the airframe revealed structural damage above a battery. Following the accident, the FBO added a line to their liability release indicating "Aircraft voltage system is _____________ volts." The exact voltage requirement was left blank to be filled in by the pilot.