Incident Overview
On 30 December 2005, an Airbus A340-642 (registration G-VGOA) operating a scheduled passenger flight from London Heathrow to Los Angeles experienced an anomalous condition shortly after takeoff. During rotation, a section of the passenger cabin filled with a light white mist, accompanied by an 'oily' smell reported by cabin crew. The Flight Services Manager informed the commander of 'smoke in the cabin', though the flight deck only detected the smell. The commander levelled off at a safe altitude and sent the relief first officer to assess the cabin, where the mist had dissipated but the odour lingered.
Crew Actions and Return
The commander decided to return to Heathrow and declared a PAN urgency call to Air Traffic Control. The crew executed the 'Smoke/Fumes Removal' checklist and informed the FSM and passengers. The aircraft was above maximum landing weight, so ATC vectored it to a fuel jettison area. Approximately 83 tonnes of fuel were jettisoned over 60 minutes, a rate slower than the aircraft manual's published figure. No system faults appeared on the ECAM. The subsequent approach and landing were uneventful. A fire service vehicle inspected the aircraft on the runway, finding nothing unusual, and the aircraft taxied to a remote stand for passenger disembarkation.
Aircraft Examination and Findings
Operator maintenance engineers conducted high-power engine runs with bleed air selected from each engine in turn, but no leaks or fumes were detected. The APU, which was standardly operating during takeoff until 1,500 ft, was likewise test-run with no issues. Galley equipment and in-flight entertainment were also ruled out. The APU had not been recently serviced, eliminating over-servicing. To isolate the cause, the aircraft completed seven subsequent flights with the APU inoperative, during which no mist or smells were reported. During an ‘A5’ check, a slight oily smell reappeared when APU bleed air was selected, but inspection of the APU bay showed no oil leaks. The aircraft returned to service with the APU still off.
On a later flight to Johannesburg, additional downtime allowed a detailed inspection of the APU and associated ducts, which found no faults. However, a trace of unknown fluid was detected on the fuselage underside, aft of the APU inlet. After cleaning and a ground run, no mist or smell occurred. A flight test with APU operating also produced no issues.
Approximately one week later, during routine APU inspection, an engineer noticed a small drain hole (about 1/8 inch in diameter) located aft of the curved APU diverter. The hole was difficult to spot. When the engineer attempted to check if the hole was clear, almost a litre of an oil-water mixture drained out. The panel containing the drain hole was removed, revealing a wet and contaminated interior. The operator suspected that a buildup of dirt and dried oil had blocked the hole.
Discussion
The crew handled the situation effectively and received 'excellent support' from ATC. The blocked drain hole likely prevented oil and other contaminants from draining overboard. During takeoff vibration, the blockage may have suddenly released, allowing the accumulated fluid to be ingested by the running APU. This contaminant could then pass into the bleed air duct and cabin air conditioning system, producing the oily smell and mist.
Follow-up Action
In response, the operator introduced a new maintenance task requiring inspection of the drain hole at every A check. The aircraft manufacturer was informed. The reason for the slower-than-expected fuel jettison rate remained undetermined at the time of the AAIB report.