What happened
On 4 January 1998, a BAC One Eleven 501EX, registration G-AWYS, was operating a scheduled public transport flight from Belfast to Birmingham. While cruising, the flight crew heard several popping sounds and noticed smoke emerging from the 'hat-ray' stowage compartment located behind the commander's seat.
The senior cabin attendant discovered an orange flame on the lower shelf of the compartment, specifically near the area where pilot smoke hoods are stored. After an initial attempt to extinguish the flame with BCF extinguisher failed, two additional bursts were required to suppress the fire. During the process, the attendant inhaled smoke and extinguisher fumes. The passengers remained calm throughout the incident.
Following the fire, the flight crew donned oxygen masks and declared a 'Mayday', diverting the aircraft to Liverpool. The aircraft landed safely and without further incident.
The investigation
Investigators examined the stowage area and discovered a small, padded plastic oxygen mask pouch that had fallen through a one-inch gap between the top shelf and the fuselage trim. The pouch had lodged behind an electrical relay panel.
Upon inspection, the pouch showed burn marks consistent with contact with two electrical cables. The insulation on these cables was charred, and soot was present on adjacent wiring. The investigation focused on the relay responsible for cabin sidewall lighting, which had been part of a previous modification to the aircraft.
Findings
- The investigation established that the oxygen mask pouch had fallen into a gap behind the relay mounting panel.
- The relay in question was approximately nine years old and was likely at the end of its operational life, exhibiting high voltage drops (290 mV compared to the expected 150-175 mV).
- The pouch acted as a thermal insulation blanket, preventing the natural dissipation of heat from the cables.
- This heat buildup led to the combustion of the pouch material and the cable insulation.
- The circuit breakers did not trip because there was no significant change in the electrical current, only an increase in localized temperature.
- The 'popping' sounds heard by the crew were attributed to the combustion of the pouch material.