What happened
On 26 February 2007, a Boeing 777-222, registration N786UA, was undergoing engine start procedures at London (Heathrow) Airport. While the aircraft was being pushed back with the auxiliary power unit running, the crew initiated the start of both engines in quick succession. During this process, the flight crew noted flickering instrument displays and heard an intermittent growling noise originating from the aft right side of the flight deck.
Shortly after, an Engine Indication and Crew Alerting System (EICAS) caution appeared, indicating a failure of the right Main AC Bus. The crew also observed a 'GEN CTRL OFF' light, signifying a loss of power from the right engine-driven generator. As the aircraft taxied, the crew noticed a faint electrical burning smell and an 'EQUIP COOLING OVRD' message. The commander subsequently ordered the shutdown of the right engine.
Ground handling personnel observed smoke emerging from the aircraft's forward vent. Following the incident, the aircraft was moved to a nearby stand, and the passengers and crew disembarked safely. There were no fatalities and no injuries among the 185 passengers or 20 crew members.
The investigation
The AAIB investigation focused on the electrical power distribution system and the damage sustained within the Main Equipment Centre (MEC). Investigators found that while the aircraft was stationary, the smoke detection system in the MEC triggered, though the initial inspection by the Fire Service did not immediately detect an active fire. Upon manual inspection of the forward cargo compartment, evidence of fire damage was discovered.
Detailed examination of the P200 electrical power panel revealed that the right bus tie breaker and the right generator circuit breaker had suffered extreme heating and electrical arcing. The damage was severe enough that the main moveable contacts within these contactors were destroyed, and molten metal had dripped onto the underlying insulation blankets. The heat was sufficient to damage nearby structural frames, stringers, and equipment cooling ducts, necessitating their replacement.
Findings
- The primary cause of the electrical failure was extreme heating and electrical arcing within the right bus tie breaker and right generator circuit breaker.
- The fire damage to the P200 power panel and surrounding aircraft structure was a direct consequence of the failure within these contactors.
- Both affected contactors had been in service since the aircraft's manufacture in 1997 and had no specific maintenance requirement for replacement based on time or flight cycles.
- The aircraft was equipped with an ELMS I power panel, whereas previous reported overheat incidents in the fleet involved the modified ELMS II version.