What happened
While operating at FL310 during the cruise phase of flight, an electrical fire occurred on board the aircraft. To combat the flames, the flight crew performed an emergency procedure by turning off both generators and the battery switch. Following the fire, the flight diverted to Des Moines, Iowa, where the aircraft landed without further incident. All occupants were not injured.
The investigation
Post-flight inspections of the aircraft revealed that the cross tie relay, along with four other relays and their associated wiring, had been charred. This damage was located in the left main equipment rack near the left forward cabin door.
Investigators determined that the fire originated from a phase-to-phase short within the cross tie relay arc box. The investigation found that metal dust, which are products of relay contact wear, had migrated through the arc box, causing the short circuit.
Findings
Technical discrepancies regarding maintenance intervals were identified during the investigation. The manufacturer of the relay recommended a 7,000-hour time between overhaul (TBO) specifically when the component is utilized as a cross tie relay. Additionally, the aircraft manufacturer recommended a 7,000-hour TBO for the relay regardless of its application.
However, the operator was utilizing a 14,000-hour TBO. This extended interval was part of an FAA-approved TBO reliability program designed for relays installed in either a vertical or horizontal orientation. At the time of the failure, the relay had accumulated 7,775.26 hours since its last overhaul.