Passengers had boarded the BAe 146 prior to departure. When the
pilot in command selected the start master switch to No. 1 engine
in preparation for engine start, the aircraft's AC power supply
immediately failed. The indications included the "APU GEN OFF LINE"
annunciator and cockpit/cabin emergency lighting illuminating. The
auxiliary power unit (APU) generator was reselected on to restore
AC power but immediately after the switch selection was made, the
AC power failed again. The crew also noticed a small amount of
smoke drifting past the cockpit overhead emergency lighting. They
immediately turned off the start power and began disembarking the
passengers.
While the passengers were disembarking, the co-pilot checked the
electrical equipment bay located on the outside of the aircraft. He
found a small fire in an electrical rack, which he extinguished
with the cockpit's portable fire extinguisher. The co-pilot also
disconnected the aircraft batteries. The off-airport rescue and
fire fighting service (RFFS) was called and remained in attendance
until the arrival of engineering staff.
Inspection by maintenance personnel revealed that the remote
control circuit breaker (RCCB) which controls the AC-powered
hydraulic pump had failed.
The RCCB was forwarded to the ATSB and dismantled. It was found
to have been subjected to extreme heat, which destroyed two of the
three main AC contacts within the RCCB. The level of internal
damage precluded determination of why the RCCB had failed. However,
it was found that as a consequence of the RCCB design, the three
main contactor chambers were open to air, dirt and moisture during
normal operations. The investigation could not determine if this
design feature was a factor in the electrical malfunction.
The AC-powered hydraulic pump internal thermal switch wire was
found to be pinched between the impeller housing and the stator,
effectively creating a short circuit to ground. The effect of this
short circuit would only be noticed when the pump had exceeded an
operating temperature of 204 degrees Celsius. Although the pump did
not display any outward signs of excessive heat, it did exhibit a
general state of deterioration commensurate with the extended time
in service for this unit. Clearly the RCCB was subjected to
excessive current load. This caused a catastrophic internal failure
and the subsequent heat generated by the failure led to molten
metal escaping from the RCCB main contactor compartment. The molten
metal then flowed across two energised power cables, which resulted
in the short-circuiting of two AC power phases.
The RCCB was located in an equipment bay that was not monitored
by fire or smoke detection devices. The technical crew was alerted
to the fire by smoke in the cockpit, system failures and a fire
that the co-pilot noticed when he gained access to the RCCB through
an external bay door. It was possible to access the equipment bay,
which held the RCCB, from the cockpit. If a similar problem were to
occur, opening the access door during flight would introduce more
oxygen to the fire and vent smoke and noxious fumes into cabin,
threatening crew and passengers.
The airframe manufacturer's failure-trend data for the RCCB was
examined and it was determined that the equipment exhibited very
high reliability in service. Consequently, the probability of
recurrence of this type of failure was considered to be low.
