Incident Overview
The aircraft was on base leg when the auxiliary power unit (APU) was started. The first officer called for landing gear and flaps to be extended to 15 degrees. As the gear and flaps deployed, the aircraft experienced a complete loss of electrical power. The APU did not start, and the battery voltage indicated between 17 and 18 volts. The flight crew performed checklist procedures but could not restore electrical power. A go-around was executed, and the subsequent landing was completed without further incident.
Electrical System Examination
Post-incident examination of the electrical system revealed that the DC voltmeter in the cockpit was reading 2 volts higher than actual. The battery was found to be discharged due to a loss of electrolyte. Additionally, there were latent shorted failures of the No. 1 and No. 2 generator control unit (GCU) blocking diodes. The specific failure mode of these diodes was not determined.
Sequence of Failures
The failure of the diodes resulted in an excessive current draw from both GCUs during the flight crew's attempt to start the APU. This caused an immediate overload of the AC power supply input fuses internal to each GCU. Consequently, the No. 1 and No. 2 generators were lost. The battery became completely discharged, leading to the total loss of electrical power.