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.