On 26 February 2007, a Boeing 777-222 operated by United Airlines, after pushback from the stand at London Heathrow Airport, experienced internal failure of an electrical component which subsequently led to under-floor fire. The aircraft returned to a stand where was attended by the Airfield Fire Service and the passengers were evacuated.
This is an extract from the aircraft accident Report (2/2009) published by the Aircraft Accident Investigation Branch (AAIB), UK:
The accident occurred during engine start after pushback from the stand. After the right generator came online an electrical failure occurred in the right main bus. The failure resulted in severe internal arcing and short circuits inside the two main power cont actors of the right main bus. The heat generated during the failure resulted in the contactor casings becoming compromised, causing molten metal droplets to fall down onto the insulation blankets below. The insulation blankets ignited and a fire spread underneath a floor panel to the opposite electrical panel (P205) (see Figure 1), causing heat and fire damage to structure, cooling ducts and wiring.
Figure 1 - Location of P200 power panel and nearby panels in the Main Equipment Center (MEC). Source: AAIB 2/2009.
The flight crew responded to the bus failure and a burning smell by shutting down the right engine and taxiing to a nearby stand. The Airfield Fire Service attended the aircraft when it arrived on stand and entered the Main Equipment Centre where they discovered significant smoke but no fire. The passengers were evacuated uneventfully via steps.
The Report identifies the following causal factors:
- An internal failure of the Right Generator Circuit Breaker or Right Bus Tie Breaker contactor on the P200 power panel inside the Main Equipment Centre resulted in severe internal arcing and short-circuits which melted the contactor casings. The root cause of contactor failure could not be determined.
- The open base of the P200 power panel allowed molten metal droplets from the failed contactors to drop down onto the insulation blankets and ignite them.
- The aircraft’s electrical protection system was not designed to detect and rapidly remove power from a contactor suffering from severe internal arcing and short-circuits.
- The contactors had internal design features that probably contributed to the uncontained failures.
The accident Report provides several recommendations (See Further Reading, AAIB 2/2009, page 100) addressing design of electrical systems by manufacturer and replacement/use of specific hardware and software on Boeing 777 aircraft.