On final approach to land at Auckland, the crew of the Boeing 767 observed the right alternating current (AC) electrical bus fail indication. The crew started the auxiliary power unit (APU) to supply additional electrical power and the approach and landing were completed without further incident. The non-normal checklist was actioned on the ground and the right AC bus power was restored. The customer service manager later advised the pilot in command that the emergency lighting illuminated briefly during the failure but then went out. This left the cabin in total darkness throughout the incident until right AC power was restored.
Maintenance investigation could not fault the electrical systems during ground tests. It is likely that there may have been an electrical earth occurring in the right generator feeder wires or terminal. The right generator control unit (GCU) would then have isolated the right generator, leaving the right AC bus without power. A fault would then be automatically detected in either the current to or from the right AC bus and the right bus tie breaker (BTB) would remain open. This would ensure the right AC bus was isolated and could not be powered by the left engine generator. The APU generator came on-line just before touchdown and automatically powered the right AC bus, however not all the engine indicating and crew alerting system (EICAS) messages were cleared with the APU powering the right AC bus. The messages cleared when the non-normal checklist was completed.
On this production series B767 aircraft, if power to the right AC bus fails then all cabin lighting is extinguished. On later production series B767, the left AC bus supplies cabin side wall lighting and the right AC bus supplies cabin overhead lighting, therefore there is some cabin lighting if either AC bus fails.
When the right AC bus lost power so too did the right direct current (DC) bus. However, as no faults were sensed in the DC electrical system the DC tie relay automatically closed to power the right DC bus from left DC bus. This sequence normally takes 11.5 seconds. The emergency lighting circuit, sensing initially there was no DC electrical power, momentarily activated while electrical system switching took place (the 11.5-second changeover). Once the DC tie relay closed the right DC bus became powered by the left DC bus and the emergency lighting then automatically extinguished. The emergency lights are designed not to light up continuously unless all DC electrical power is lost.
As no definite fault could be isolated for the right AC bus failure, the performance of the aircraft electrical system was being monitored by Engineering Maintenance Watch and the company Flight Safety Department
|Date:||10 May 2000||Investigation status:||Completed|
|Time:||1113 hours UTC|
|Release date:||01 August 2000||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Sydney, NSW|
|Destination||Auckland, NEW ZEALAND|