What happened
On 5 October 2015, a Boeing 747-400, registration G-BNLW, arrived at Cape Town International Airport for passenger transport operations. During the ground handling process, ground crew attempted to connect a single ground power unit (GPU) by plugging two sockets into the aircraft's electrical receptacles. The crew reported difficulty inserting the No. 1 lead, noting that the socket required wiggling to achieve connection. Shortly after the connection was established, smoke was observed emanating from the GPU. Upon disconnecting the plugs, the ground crew noted that the connectors were warm. While an initial inspection by an avionics engineer found no immediate issues, a subsequent attempt to use a different GPU failed as the aircraft would not accept the power. The aircraft was later cleared for service and flew to London Heathrow Airport without further incident.
The investigation
The investigation, conducted by the aircraft operator, included a detailed examination of the aircraft upon its arrival at Heathrow. Inspectors focused on the electrical and equipment (E/E) bay, located beneath the fuselage skin near the nose gear. This inspection revealed significant sooting and heat damage to the power cables. The investigation established that the insulation within the underfloor compartment had briefly ignited, though the limited air volume around the wiring likely prevented the fire from spreading further. While the aircraft's external receptacles showed evidence of electrical arcing and severe overheating on the neutral phases, structural testing confirmed that the aircraft's structural integrity remained intact.
Findings
- The primary cause of the damage was the misalignment of the GPU socket within the aircraft's receptacle during the connection process.
- This misalignment likely caused 115V AC power to be delivered to a pin with minimal load, triggering a momentary power surge and electrical arcing.
- The heat damage was concentrated on the power cables and the ground power receptacle pins.
Safety action
- The operator has instructed ground personnel to monitor aircraft receptacles and GPU sockets for any signs of heat damage.
- Ground staff have been directed to ensure all connectors are properly aligned before completing the connection of ground power.