What aircraft was involved and where did it happen?

The accident on 2006-11-08 involved a BAe 146-300, registration G-OINV, at Descent into Inverness, GB.

What was the probable cause of the accident?

The loss of electrical power to flight displays and cockpit lighting was likely caused by Generator 1 dropping offline due to the electrical load of the APU start, which may have caused the bus fail relay on the GCU to lock out.

Electrical Power Loss During Descent into Inverness

A BAe 146-300 experienced a sudden loss of cockpit displays and lighting during the APU start sequence while approaching Inverness.

What happened

On 8 November 2006, a BAe 146-300, registration G-OINV, was performing a passenger flight from London Gatwick to Inverness. While descending through FL70, the flight crew initiated the approach procedure, which included starting the Auxiliary Power Unit (APU). Approximately five seconds after the APU start sequence began, the aircraft experienced a sudden loss of electrical power to all Primary Flying Displays, Navigation Displays, and cockpit lighting. No warning indications were provided to the crew during this event.

The commander immediately issued a ‘mayday’ call to Inverness Air Traffic Control. The aircraft remained above the cloud layer in Visual Meteorological Conditions. Within approximately 15 seconds, the commander successfully restored electrical power by manually switching the APU off and resetting Generator 1 and Generator 4. While cabin lighting remained stable, the crew noted a temporary loss of power to the galley. Following the successful landing at Inverness, the ‘mayday’ was cancelled.

The investigation

The investigation, which included analysis by the aircraft manufacturer, focused on the electrical system and the sequence of the APU start. Investigators examined the Generator Control Unit (GCU) and the contactor, both of which were stripped for inspection. While the GCU showed no faults, the contactor revealed worn surfaces on the three sets of contacts and an out-of-limit voltage drop. However, there was no evidence of melting or fusing of the components.

Further testing under both hot and cold conditions failed to replicate the intermittent fault. The investigation also reviewed a previous incident involving the same aircraft in August 2006, where a generator failure had led to the replacement of the No 1 GCU. Following the recent event, the operator replaced the No 1 bus tie contactor and the No 1 GCU, after which the aircraft returned to service without further issues.

Findings

The electrical failure occurred during the high-current demand of the APU start.
The most likely cause was Generator 1 dropping offline due to the high current required for the APU start, potentially triggering a bus fail relay lock-out on the GCU.
Prior to the crew's manual intervention, the Battery, Emergency DC, and AC2 busbars remained powered, but the AC1 busbars and essential DC busbars were unpowered.
No circuit breakers were found to have tripped during the event.