What happened
On 30 December 2005, an Airbus A340-642, registration G-VGOA, was performing a scheduled passenger flight from London Heathrow to Los Angeles. During the takeoff rotation, a light white mist appeared in a section of the cabin, accompanied by an oily odour reported by the cabin crew. While the flight crew noted the smell on the flight deck, no visible smoke or mist was present there.
After reaching a safe altitude, the commander requested a relief officer to inspect the cabin. The officer reported that the mist had dissipated, though the oily smell remained. Because the aircraft was above its maximum landing weight, the crew was vectored by Air Traffic Control to jettison fuel. Following the disposal of 83 tonnes of fuel, the aircraft returned to Heathrow. The landing was unevent and no injuries were sustained by the 18 crew members or 308 passengers. Fire services attended the aircraft upon arrival, but no immediate issues were found during visual inspections.
The investigation
Maintenance engineers conducted high-power engine runs and tested the auxiliary power unit (APU) and associated air conditioning ducts, but no leaks or fumes were detected during these tests. However, a trace of an unknown fluid was found on the underside of the fuselage, aft of the APU inlet.
One week later, during a routine inspection, an engineer discovered a small, 1/8-inch drain hole located behind the APU diverter. When the engineer attempted to clear the hole, approximately one litre of an oil-water mixture drained out. The area inside the panel was found to be wet and contaminated. It was determined that a buildup of dirt and dried oil had blocked the hole, causing fluids to accumulate.
Findings
- A blockage in the drain hole, caused by a buildup of dirt and oil, prevented contaminants from draining overboard.
- The ingestion of an oil-water mixture into the APU inlet allowed the fluid to enter the bleed air duct and the cabin air conditioning system.
- The sudden release of this accumulated fluid, potentially triggered by aircraft vibrations during takeoff, caused the observed mist and odour.