Short circuit leads to power loss

This June 2007 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.

Aircraft: Piper Comanche.
Location: Ramona, Calif.
Injuries: None.
Aircraft damage: Substantial.

What reportedly happened: The pilot was performing a test flight to confirm the operation of newly installed avionics. According to the pilot, during the return flight to the airport, the engine began to “backfire” and then lost all power. The pilot went through the emergency procedures using the checklist, but was unable to restart the engine. He made a forced landing in a dirt field with the landing gear retracted.

Both fuel tanks were found to be approximately half full. The post-accident examination revealed a short circuit of both magneto grounding leads in the electrical wiring between the magneto switch and the magneto. The wiring harnesses were original to the airplane, which was manufactured in 1964. Investigators were able to successfully start the engine and perform a run-up under two separate conditions. The first engine start occurred after investigators disconnected both p-leads at their respective magneto terminals. The second occurred after new electrical wire was connected from the magneto switch to both magnetos.

A review of the engine logbook revealed that the engine had been returned to service following a 100-hour inspection about a month prior to the accident. The engine logbook entry indicated that maintenance personnel checked wires, cables, and hoses for condition and security.

Probable cause: A total loss of engine power due to a short circuit between both magneto ground leads. A factor in the accident was the inadequate annual/100-hour maintenance inspection regarding the wiring.

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