Fire damage in United Airlines B777-222 (N786UA) flight UA955
United Airlines flight UA955 : 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.
The RGCB and Right Bus Tie Breaker (RBTB) suffered from severe internal arcing and short circuits which generated temperatures in excess of 1,000°C, and resulted in uncontained failures.
The investigation identified 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.
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This incident provides a clue to the disappearance of Malaysian Flight MH370 since it is now established by Australia’s ATSB through analysis of Satellite signals traffic to & sometimes from MH370 that MH370 experienced electrical failures. Particularly the SAT phone call at 18:39 UTC which MH370 first acknowledged, then disconnected due to unstable voltage fluctuations. If in that aircraft just like United Airlines Flight UA955, there had been smoke in the MEC causing MH370 to automatically vent cabin air to clear smoke from the MEC there may well have been a cause for hypoxic flight.