What happened
On 3 July 2019, a Boeing 777-236, registration G-YMMU, was preparing for a scheduled passenger flight from London Heathrow to Bengaluru. Prior to departure, the aircraft was operating with the APU bleed air valve and the right air conditioning pack isolated due to previous occurrences of similar issues.
While the aircraft was still on the ground, the third pilot noted an unusual odour, though the commander and co-pilot did not initially detect anything. However, shortly after takeoff, all three pilots experienced a strong, organic, and oily smell accompanied by a metallic taste. As the aircraft climbed through 2,000 ft, the intensity of the odour increased. In response, the co-pilot donned an oxygen mask, and the commander subsequently passed control of the aircraft to him. After confirming that the cabin crew had not detected the smell in the passenger area, the remaining pilots also donned oxygen masks and initiated the 'smoke, fire or fumes' checklist.
Because the aircraft was significantly above its maximum landing weight, the crew decided to jettison fuel to reach a safe landing mass rather than attempting an immediate overweight descent. During this process, the crew opted not to switch off the left air conditioning pack to avoid depressurising the cabin while wearing masks. Following the fuel reduction, the aircraft returned to Heathrow and landed safely. Notably, cabin crew later reported detecting the fumes near a forward door and along the left side of the cabin during the final approach.
The investigation
The investigation focused on the recurring nature of these events, as G-YMMU had experienced several similar incidents in the preceding weeks. Previous events included an oily smell during an approach from Buenos Aires and the detection of diesel fumes while holding at Heathrow.
Engineers conducted extensive testing, including ground engine runs and a dedicated flight test. During the flight test, fumes were specifically associated with the left air conditioning pack. Following these findings, the aircraft's configuration was altered, and various components within the air conditioning system were replaced. Despite these efforts and multiple subsequent inspections, the precise origin of the odour remained elusive.