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.
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 aircrafts 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.
Damaged on the ground
| Status: | Accident investigation report completed and information captured |
| Date: | Monday 26 February 2007 |
| Time: | 10:00 |
| Type: |  Boeing 777-222ER |
| Operator: | United Airlines |
| Registration: | N786UA |
| 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 |
| Aircraft damage: | Minor |
| Aircraft 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 |
| ? |
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 aircrafts 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:
 |
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Classification:
Electrical system problem Damaged on the ground
Sources:
» SKYbrary Photos
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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