What happened
On 28 November 1998, a Boeing 747-438, registration VH-OJD, was performing a public transport flight departing from London Heathrow Airport. Approximately 45 minutes into the climb, the crew observed an amber 'equipment cooling' message on the Engine Indication and Crew Alerting System (EICAS), accompanied by a flashing attention light.
Following a consultation with the airline's maintenance organization, the flight crew elected to return to London Heathrow. The aircraft landed without further incident, and no injuries were reported to the crew or passengers.
The investigation
Subsequent analysis of the aircraft's Central Maintenance Computer identified a fault involving the ground exhaust valve of the Electronic Equipment (E/E) bay cooling system. Investigators also discovered that a related circuit breaker had tripped. This valve, which manages airflow for cooling and smoke clearance, is situated in an open bay beneath the forward cargo compartment floor.
Initial maintenance attempts to rectify the issue—including manually closing the valve, installing a shorting link, and replacing the valve, relay, and ECS card—failed to clear the warning. An overnight inspection at Heathrow allowed for more intensive troubleshooting. During wiring continuity checks, investigators located damaged wires near the exhaust valve. These wires had been improperly installed beneath a bilge thermal insulation blanket, hidden against the fuselage skin.
Evidence showed that a localized fire had occurred between the outer film of the insulation blanket and the airframe. The fire consumed a section of the blanket's outer film, measuring roughly 18 by 6 inches, but the underlying insulating foam remained largely intact. The presence of condensation in the aircraft bilge may have helped limit the fire's spread. The investigation noted that the damaged 24-gauge wires had been sooted by combustion products, though they were repairable via in-line splices.
Findings
- The primary cause of the electrical fault was damaged wiring located beneath the thermal insulation blanket.
- The wires were likely damaged by being stepped on during previous maintenance activities, as the area is difficult to see due to the blanket's placement.
- The fire was fueled by the combustion of the polyester film used in the lightweight insulation blanket, which had been recently replaced with a custom-made version by the operator.
- While the insulating foam was fire-retardant, the outer polyester film of the blanket was susceptible to burning under the conditions present during the incident.