What happened
On 30 August 1999, a Boeing 747-236B, registration G-BDXL, was performing a passenger flight departing from London Heathrow. Shortly after the climb-out phase, cabin crew alerted the flight deck to a burning smell originating from the right-hand side of the central cabin area. This report was accompanied by observations of a hot sidewall panel and heated floor sections, which the flight engineer subsequently confirmed.
In response to the situation, the pilot declared a PAN-PAN call and initiated an emergency return to London Heathrow. To manage the aircraft's weight for the descent, approximately 12,000 kg of fuel was jettisoned. During the approach, the crew shut down the number 1 and number 3 air conditioning packs. Following this action, the cabin crew noted that the smell of burning had decreased. The aircraft landed at Heathrow without further incident, and the airport fire service attended the aircraft while it was parked at the stand.
The investigation
Investigators examined the cabin area where the heat had been detected. The inspection revealed that the source of the heat was a leak of high-temperature air. The investigation established that the defect was the result of the partial disintegration of composite material within a trim air duct. This duct was transporting air at temperatures of approximately 200°C.
Findings
- The loss of structural rigidity in the composite trim air duct allowed hot air to escape from behind a clamp.
- The leaking air entered an area of the cabin that contained no sensitive equipment.
- The acoustic insulation in the affected area remained unburnt.
- The aircraft sustained only minor trim damage.