Primary finding
Probable cause
The fatigue fracture of a main gear wheel hub.
Investigator assessment
Analysis narrative
During the landing/taxi sequence, the left main gear wheel hub experienced a fatigue fracture. The cracks which led to the fracture originated in the hub grease dam groove. The fracturing of the hub damaged other brake compenents, which allowed hydraulic fluid to leak onto a hot brake pad, where it was ignited.
Source record
Factual narrative
On April 17, 1994, approximately 1855 Pacific daylight time (PDT), an Alaska Airlines McDonnell Douglas MD-83, N931AS, experienced a fire in the area of the left main gear while taxiing to the passenger loading gate at Seattle-Tacoma International Airport, Seattle, Washington. After stopping at the gate and requesting fire equipment, the captain ordered an emergency evacuation of the aircraft. There were no reported injuries to the 132 passengers, six crew members, or the single jump seat occupant. The aircraft, which departed San Francisco International Airport on an IFR flight plan at 1655 PDT, was operating in visual meteorological conditions at the time of the incident. This mishap did not involve the activation of an ELT. According to tower personnel, smoke was seen coming from the area of the left main gear as the aircraft taxied to the gate. The tower notified the aircrew of the situation and, as the aircraft stopped at the gate, flames began coming from the same area, prompting the captain to initiate an emergency evacuation. Approximately half of the occupants evacuated through the normal jetway exit, while the other half exited using the tail cone slide. The investigation has revealed that the left main gear wheel hub experienced a fatigue fracture which originated in the hub grease dam groove. Subsequent to the hub failure, hydraulic fluid, which was leaking from a damaged brake component, was ignited by a hot brake pad. -