What happened
On 8 August 1999, a Boeing 777-236, registration G-VIIE, was performing a scheduled passenger flight from London Heathrow. While the aircraft was climbing through flight level 260, the crew and passengers noticed a blue/grey haze accompanied by acrid-smelling fumes within both the cockpit and the cabin.
In response to the situation, the pilot declared a 'PAN' emergency and initiated a return to London Heathrow. To mitigate the effects of the fumes, the flight crew immediately utilized onboard oxygen. The crew followed established procedures for smoke removal and fuel jettisoning. The aircraft subsequently performed a standard autoland at Heathrow. Following the landing, emergency services inspected the aircraft and found no visible signs of smoke. The passengers and crew, totaling 276 people, disembarked normally without injury.
The investigation
Following the incident, ground inspections were conducted involving the operation of the engines, the air-conditioning packs, and the Auxiliary Power Unit (APU). During these tests, the reported defect could not be reproduced. Investigators found no evidence of oil residue or contamination within the pneumatic ducting for the engines or the APU.
However, a detailed inspection of the APU buffer air circuit identified that the vent tube was obstructed. Due to the history of oil contamination issues related to the APU buffer air system—which provides pressure to the rear carbon seal of the APU load compressor—maintenance crews decided to replace the APU as a preventative measure. Additionally, the dual heat exchanger for the left air-conditioning pack was replaced due to suspected oil contamination. Subsequent ground runs following these replacements showed no further odors.
Findings
- The primary cause of the smoke and fumes was a blocked vent tube within the APU buffer air circuit.
- While the defect was not reproducible during ground testing, the obstruction in the buffer air system was identified as the source of the issue.