What happened
During a test flight intended to verify the operation of newly installed avionics, the pilot was returning to the airport when the engine began backfiring and subsequently lost all power. The pilot attempted to follow the emergency checklist but was unable to restart the engine. Consequently, the pilot performed a forced landing in a dirt field with the landing gear retracted. There were no injuries reported.
The investigation
Upon inspection, both fuel tanks were found to be approximately one-half full. Examination of the airframe revealed a short circuit affecting both magneto grounding leads within the electrical wiring located between the magneto switch and the magnetos. Investigators determined that the wiring harnesses were original to the 1964 aircraft.
During testing, investigators successfully started the engine and performed a run-up under two different conditions. The first successful start occurred after disconnecting both p-leads at their respective magneto terminals. The second successful start was achieved after installing new electrical wiring from the magneto switch to both magnetos. Investigators noted that the general condition of the p-lead electrical wiring was poor. A review of the engine logbook showed that the engine had been returned to service following a 100-hour inspection approximately one month before the accident, with entries indicating that maintenance personnel had checked wires, cables, and hoses for condition and security.