Sky Arrow N445SA incident near Marietta, Georgia

Probable cause

An electrical fire due to the lack of a positive locking mechanism for the fuse retainer in the main fuse block, which resulted in voltage fluctuations that caused thermal damage to the avionics cooling fan. Contributing to the incident was the student pilot's inadvertent shutdown of the engine at low altitude.

NTSB analysis

Shortly after takeoff, when the special light sport airplane was climbing through an altitude of approximately 1,000 feet above ground level (agl), the certificated flight instructor (CFI) and the student smelled and saw electrical smoke in the cockpit. The CFI began a turn back to the departure airport, and declared an emergency to the air traffic control tower. The CFI, in the rear seat, and without access to the electrical switches or circuit breakers, asked the student in the front seat to "turn everything off except the radio." After they received clearance to return to the airport, the CFI asked the student to turn off the radio; the student turned off the radio and the master switch. Shortly thereafter, the CFI instructed the student to turn on the master switch and radio, but "they would not work," and when the student cycled the master switch, the engine ceased operation. The CFI stated that due to the low altitude at the time of the power loss, he focused on landing safely, and did not attempt to troubleshoot the situation or verify the student pilot's actions. The airplane landed uneventfully on a road. Post-incident examination and testing revealed that the main fuse block for the airplane's electrical system exhibited intermittent contact, which, due to the architecture of the electrical system, caused the airplane system voltage to peak at values 5 to 8 volts above the normal value of 12 volts. The over-voltage condition resulted in overheating and thermal damage to an avionics cooling fan. The fuse block was not intended for aviation applications, and was installed without a positive locking mechanism for the fuse retainer. After the incident, the airplane manufacturer issued a service bulletin that recommended safety-wiring the main fuse block fuse retainer in the closed position, and also incorporated that change into the production line. No electrical or mechanical mechanism that related the fuse block malfunction or the over-voltage condition to engine operation or failure was identified. After the fuse block and damaged cooling fan were replaced with new units, the incident airplane and engine functioned normally. It was likely that the student inadvertently and unknowingly shut down the engine when he attempted to comply with the CFI's instructions.

Text from the NTSB. Open the NTSB docket for ERA09IA326 for the full record.