An electrical fire beneath the flightdeck floor of a United Airlines Boeing 777-200 reached a temperature of more than 1,000°C (1.832°F) without generating any fire or smoke warnings for the pilots, says a UK Air Accidents Investigation Branch report.

The event occurred at London Heathrow airport on 26 February 2007 just after the crew had started the engines, and the fire was identified when a tug driver told the crew that smoke was coming from the main equipment centre vent.

The fire began when intense arcing and short-circuiting took place in and around the contacts of the right generator circuit breaker and right bus tie breaker in the P200 power panel just after the power from the integrated drive generator came on line following engine start. Molten metal from the P200 panel started dripping on to the insulation blankets below it, which ignited despite their flame-retardant characteristics.

United 777-200
 © United Airlines

The crew received several electrical system warnings on their engine indicating and crew alerting system (EICAS) displays, including a separate indication when the right integrated drive generator power supply was interrupted, and another to show that the bus tie connection had tripped. The flight data recorder shows that 40s after the failure the main equipment centre smoke detector activated, tripping the centre's cooling fan system, which generated a warning message on the EICAS. The crew shut down the right engine and taxied the aircraft (N786UA) to a nearby stand, where all on board were safely evacuated via steps.

The AAIB say the initial cause of the fire was the failure of contacts in the right generator circuit breaker and right bus tie breaker, but the root cause of that failure is not known because of the damage caused by the intense heat.

One recommendation is that a tray must be sited below the power panels to catch drops of molten metal should a contact fail again, because this will prevent the drips igniting the insulation blanket below them.

Among other recommendations, however, the AAIB says that the crew should be warned directly of smoke in the main equipment centre, and the power panels should be designed to isolate automatically contacts that are short-circuiting. The US Federal Aviation Administration and Boeing are acting on these.

The AAIB notes that, because power panel contacts had previously failed, the manufacturer was already seeking remedies before the Heathrow event.

Source: Flight International