History of the Flight
The aircraft was pushed back from the stand with the auxiliary power unit (APU) running, the towbar was disconnected, and both engines were started in quick succession. The flight crew, comprising a commander, operating co-pilot, and relief co-pilot, reported that the engine starts appeared normal. At about the time when the engine integrated drive generators (IDGs) would normally come online, the flight crew saw the instrument displays flicker and heard a low-pitched, intermittent growling noise from the aft right side of the flight deck. A few seconds later, they received an Engine Indication and Crew Alerting System (EICAS) caution for 'ELEC AC BUS R', indicating that the right Main AC Bus had failed. The right 'GEN CTRL OFF' light also illuminated on the overhead panel, and subsequently the 'ISLN' caption on the 'R BUS TIE' switch illuminated.
The Flight Data Recorder (FDR) revealed that 40 seconds after both engines had stabilised at ground idle, the smoke detector inside the Main Equipment Centre (MEC) detected smoke. The Cockpit Voice Recorder (CVR) recorded sounds of equipment powering down and crew comments regarding the failure of the right main bus. The flight crew completed the EICAS checklist for the right AC bus failure, selecting the right generator control switch to off and then on again. About two and a half minutes after the electrical failure, they became aware of a faint electrical burning smell and shortly afterwards noticed the 'EQUIP COOLING OVRD' message on the EICAS. The commander then ordered the co-pilot to shut down the right engine.
The ground handling crew observed smoke emanating from the MEC vent at the front of the aircraft and alerted the flight crew. Two minutes later, Air Traffic Control (ATC) advised that smoke had been seen coming from the aircraft and that the fire service had been requested. The aircraft was taxied onto a nearby stand using the left engine. Once on stand, the flight crew shut down the left engine and the APU, by which time light smoke was present in the flight deck. ATC further advised that smoke had been seen coming from the forward outflow valve. Approximately twelve and a half minutes after the electrical failure, the batteries were switched off, and the passengers and crew disembarked via steps placed at door 2L.
Airfield Fire Service personnel checked the aircraft's MEC, which was filled with smoke, but did not detect any fire. They manually opened the forward cargo compartment and removed two cargo pallets to check for additional signs of fire, but none were found. The smoke slowly cleared in the MEC, revealing signs of fire damage.
Damage to the Aircraft
An inspection inside the MEC after the accident revealed extensive heat and fire damage to the P200 power panel. The worst affected components of the power panel were the right generator circuit breaker (RGCB) and right bus tie breaker (RBTB) contactors, parts of which had melted and vaporised. There was evidence that molten metal had dripped down onto the insulation blankets beneath this panel. Extensive fire damage to the fire-retardant insulation blankets located behind and beneath the power panel under the floor had occurred. Nearby components including a floor panel, equipment cooling system ducting, other wire bundles, and some structural frames and stringers in the vicinity were later determined to have suffered sufficient heat damage to require replacement.
Detailed Examination
The P200 power panel was removed for a more detailed examination. The examination revealed that both the RBTB and RGCB had suffered from extreme heating and electrical arcing. The main moveable contacts within both contactors were destroyed. There was some insulation damage to the bus bars within the panel in the vicinity of the RBTB. The damage to the surrounding components of the power panel appeared to be a consequence of a failure within either the RBTB or RGCB.
History of the Contactors
The serial numbers on the RBTB and RGCB contactors were unreadable as a result of the fire damage. An initial inspection of the aircraft's maintenance records revealed that neither component had been replaced since the aircraft was manufactured in 1997. At the time of the accident, the aircraft had completed 6,622 flight cycles and flown for 43,519 hours. The RBTB and RGCB share the same contactor part number, and there is no maintenance requirement to replace either contactor after a fixed time or flight cycle period.
Previous Incidents and Preventative Action
Prior to this accident, the aircraft manufacturer was involved in investigating 11 in-service reports of power panel overheat events, three of which involved major damage to the panels. The affected panels were the P200 and P300, and the affected contactors were the RBTB, Auxiliary Power Breaker (APB), and Primary External Power Contactor (PEPC). These previous events all involved Boeing 777 aircraft fitted with ELMS II power panel, a modified version of the ELMS I power panel fitted to N786UA, although many components including the contactors are the same. As a result of these incidents, the aircraft manufacturer published details of preventative action that operators could take in its 777 Fleet Team Digest No 777-FTD-24-06005.
Further Investigation
The AAIB is working with the US National Transportation Safety Board (NTSB), the aircraft manufacturer, the aircraft operator, the power panel manufacturer, and the contactor manufacturer to determine the cause of the failures within the electrical power system. Further investigation is also being carried out into understanding how the fire spread and how to improve fire protection within the MEC.