AAIB Report: Boeing 747-433, G-ONEE

Boeing 747-433 (G-ONEE), water ingress leading to loss of two AC buses, and ice leading to unreliable airspeed, after departure from London Heathrow Airport, 8 February 2024

The flight involved a freighter variant of the Boeing 747-400 that was due to fly from London Heathrow Airport to Hong Kong International Airport. The aircraft suffered numerous electrical issues which began during the takeoff roll and worsened during the climb. These were caused by the loss of AC electrical buses 1 and 4, which resulted in multiple failure messages for the flight crew to manage as well as the loss of all pitot tube heating. The flight crew carried out numerous, concurrent checklists to deal with the system failures, declared a PAN and started to divert to Amsterdam Schiphol Airport. When a pressurisation issue caused the crew to initiate a descent, the aircraft entered icing conditions and the pitot tubes froze, resulting in all three airspeed indicators indicating an erroneously low airspeed. The autopilot remained engaged and pitched the aircraft nose-down to regain indicated airspeed. The pitch reached 7.6° nose-down with an associated 6,240 ft/min descent rate, before the aircraft exited icing conditions, correct airspeed was regained and the autopilot pitched up. The descent had resulted in an 18 kt overspeed. The flight crew declared a MAYDAY, dealt successfully with a flap issue and landed safely at Schiphol Airport.