Accident
Last updated: 24 April 2014
Statuts:Enquête Officielle
Date:lundi 26 février 2007
Heure:10:00 UTC
Type/Sous-type:Boeing 777-222ER
Compagnie:United Air Lines
Immatriculation: N786UA
Numéro de série: 26938/52
Année de Fabrication: 1997-03-23 (9 years 11 months)
Heures de vol:43519
Cycles:6622
Moteurs: 2 Pratt & Whitney PW4090
Equipage:victimes: 0 / à bord: 20
Passagers:victimes: 0 / à bord: 185
Total:victimes: 0 / à bord: 205
Dégats de l'appareil: Mineurs
Conséquences: Repaired
Lieu de l'accident:London-Heathrow Airport (LHR) (   Royaume Uni)
Phase de vol: En attente (STD)
Nature:Transport de Passagers Intern.
Aéroport de départ:London-Heathrow Airport (LHR/EGLL), Royaume Uni
Aéroport de destination:?
Détails:
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.

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.

Sources:

Official accident investigation report
investigating agency: Air Accidents Investigation Branch (AAIB) - U.K.
report status: Final
report number: AAIB AAR 2/2009
report released:16-FEB-2009
duration of investigation:721 days (1 year 11.9 months)
download report: Report on the accident to Boeing 777-222, registration N786UA at London Heathrow Airport on 26 February 2007 (AAIB AAR 2/2009)
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