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