Amateur-built Lancair IV-P N811HB accident near Murrieta/Temecu, California

Probable cause

A total loss of engine power while on approach due to the electrical overload of a fuse caused by the builder's inadequate design and installation of the electrical ignition system.

NTSB analysis

The pilot reported that the engine lost power while on approach for landing in the amateur-built experimental airplane. Unable to reach the airport, the pilot initiated a forced landing in hilly desert terrain resulting in structural damage to the airframe. Inspection of the modified engine revealed that it was equipped with an electronic ignition system consisting of two capacitive discharge modules with direct crank sensors. Two five-amp fuses in the electrical circuit for the ignition system were found blown. Inspection of the electrical system found no failures of the physical airplane wiring. The pilot, who built the airplane, stated that he designed the ignition system's electrical circuit with the intention of providing electrical redundancy to the ignition system. Examination and testing of the design as installed demonstrated that a minor voltage differential between the main and redundant power source created a condition that routed the current flow for both ignition sources through a single fuse, resulting in a single point of failure. According to the ignition manufacturer, the recommended wiring configuration that would allow ignition power redundancy did not include the addition of a common bus bar as was found on the accident airplane. The positioning of the automotive fuses on the firewall prevented the pilot from accurately identifying the electrical failure.

Text from the NTSB. Open the NTSB docket for LAX08LA066 for the full record.