What happened
On 26 February 2007, a Boeing 777-222, registration N786UA, was undergoing engine start-up procedures at London (Heathrow) Airport. Following pushback with the auxiliary power unit running, the crew started both engines in quick succession. Approximately 40 seconds after the engines reached a stable idle, the flight crew observed flickering instrument displays and heard a low-pitched, intermittent growle originating from the aft right side of the flight deck.
Shortly thereafter, the Engine Indication and Crew Alerting System (EICAS) issued a caution indicating a failure of the right Main AC Bus. The crew attempted to reset the right generator control switch, but the electrical failure persisted. Within minutes, the crew detected a faint electrical burning smell and an equipment cooling override message. The commander subsequently ordered the shutdown of the right engine. Ground personnel observed smoke emerging from the aircraft's vent, and the fire service was summoned. After the aircraft was parked, the crew disembarked via steps as the batteries were disconnected.
The investigation
An investigation by the AAIB established that while the aircraft's smoke detectors did not initially trigger a master warning, smoke was visible in the flight deck and emanating from the aircraft's vents. Upon inspection of the Main Equipment Centre (MEC), fire service personnel found the area filled with smoke. After the smoke cleared, investigators discovered significant fire and heat damage to the P200 power distribution panel.
Detailed examination of the removed P200 panel revealed that the right generator circuit breaker (RGCB) and right bus tie breaker (RBTB) contactors had suffered extreme heating and electrical arcing, which destroyed their main moveable contacts. The intensity of the fire was sufficient to melt and vaporize parts of these components, causing molten metal to drip onto the insulation blankets below. This resulted in extensive damage to the floor panel, cooling system ducting, wire bundles, and several structural frames and stringers.
Findings
- The primary cause of the fire was a failure within either the RBTB or RGCB contactors.
- The RGCB and RBTB had not been replaced since the aircraft's manufacture in 1997, having completed 6,622 flight cycles and 43,519 flight hours.
- There was no specific maintenance requirement to replace these contactors after a set period of time or number of cycles.
- The aircraft was equipped with an ELMS I power panel, whereas previous reported overheating incidents involved the modified ELMS II version.
- The smoke detection system's effectiveness was limited because the cooling system relies on a pressure differential to vent smoke, which is ineffective while the aircraft is stationary on the ground.