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Accident description
Last updated: 11 December 2017
Status:Final
Date:Monday 26 February 2007
Time:10:00 UTC
Type:Silhouette image of generic B772 model; specific model in this crash may look slightly different
Boeing 777-222ER
Operator:United Air Lines
Registration: N786UA
C/n / msn: 26938/52
First flight: 1997-03-23 (9 years 11 months)
Total airframe hrs:43519
Cycles:6622
Engines: 2 Pratt & Whitney PW4090
Crew:Fatalities: 0 / Occupants: 20
Passengers:Fatalities: 0 / Occupants: 185
Total:Fatalities: 0 / Occupants: 205
Airplane damage: Minor
Airplane fate: Repaired
Location:London-Heathrow Airport (LHR) (   United Kingdom)
Phase: Standing (STD)
Nature:International Scheduled Passenger
Departure airport:London-Heathrow Airport (LHR/EGLL), United Kingdom
Destination airport:?
Narrative:
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 contactors 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), causing heat and fire damage to structure, cooling ducts and wiring. 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.

Probable Cause:

CAUSAL FACTORS:
1. 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.
2. 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.
3. 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.
4. The contactors had internal design features that probably contributed to the uncontained failures.

Accident investigation:
cover
Investigating agency: AAIB
Status: Investigation completed
Duration: 1 year and 12 months
Accident number: AAIB AAR 2/2009
Download report: Final report

Sources:
» SKYbrary 


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This information is not presented as the Flight Safety Foundation or the Aviation Safety Network’s opinion as to the cause of the accident. It is preliminary and is based on the facts as they are known at this time.
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