What happened
On 6 March 2015, a Boeing 777-236, registration G-RAES, was operating a commercial passenger flight from London Heathrow Airport. During the taxi, takeoff, and initial climb phases, the flight crew noticed that the flight deck temperature was rising and airflow from the cockpit vents was extremely low. Despite attempts to cycle the air conditioning packs, trim air, and recirculation fan switches, the temperature control remained ineffective.
As the aircraft reached its initial cruise altitude, the three members of the flight deck crew began experiencing physical symptoms, including headaches, nausea, light-endedness, and difficulty concentrating. To mitigate these effects, the crew temporarily opened the cockpit door to allow for better ventilation and to check for potential electronic cooling issues. While the temperature dropped, the crew's physical condition did not significantly improve. To prevent pilot incapacitation, the pilot flying began using an oxygen mask.
Unable to resolve the issue through technical support from maintenance control, the crew decided to return to Heathrow. They declared a PAN PAN, jettisoned fuel over the Irish Sea, and performed a safe landing at the airport. No injuries were reported among the 221 passengers or the 14 crew members, and there was no damage to the aircraft.
The investigation
The AAIB investigation focused on the air conditioning system, specifically the delivery of air from the left pack to the cockpit via the forward freight bay. Engineers discovered that the distribution ducting forward of panel P310 in the Main Equipment Centre was completely devoid of airflow. Upon inspecting the ducting, investigators found a significant accumulation of debris, including wire, bubble wrap, and insulation material, which was obstructing the duct approximately 12 inches upstream from the panel. The brittle, blackened state of the material suggested the blockage had been present for a considerable period.
Technical records revealed that this specific aircraft had experienced several recent reports of inadequate airflow and high temperatures throughout February 2015. Previous maintenance actions had included cleaning sensors and adjusting louvres, but these did not address the underlying obstruction.
Findings
- The flight deck ventilation failure was caused by debris accumulation in the underfloor ducting.
- The source and timing of the debris entering the system could not be determined.
- The aircraft had a recent history of similar airflow complaints that had been addressed with temporary fixes rather than identifying the physical blockage.