What happened
On 26 February 2007, a Boeing 777-222, registration N786UA, operated by United Airlines, was undergoing engine start procedures at London Heathrow Airport following pushback from its stand. Shortly after the right engine reached the required speed for the generator to come online, a significant electrical failure occurred.
The flight crew observed multiple EICAS messages and electrical system abnormalities, including the momentary blanking of primary flight displays. As the electrical fault progressed, the crew noted an unusual noise and a distinct smell of electrical burning. Smoke was detected within the Main Equipment Centre (MEC), triggering the equipment cooling override. The tug driver, observing heavy smoke emanating from the aircraft's vents, alerted the crew via the interphone.
In response to the developing situation, the crew shut down the right engine and subsequently the left engine. The aircraft was eventually taxied to a stop, and passengers were instructed to disembark. There were no fatalities or injuries during the incident.
The investigation
An investigation by the AAIB examined the electrical power distribution and control system of the aircraft, specifically focusing on the contactors and power panels. The investigators analyzed flight recorder data, including the CVR, FDR, and NVM, to establish a timeline of the electrical failures.
Technical examinations were conducted on the aircraft's wiring, insulation blankets, and the specific electrical components involved, such as the RGCB and RBTB contactors. The investigation also included laboratory tests on insulation blanket materials and endurance testing of contactors to understand the mechanisms of failure and the potential for arcing.