Synopsis
On 8 November 2006, at 2100 UTC, a BAe 146-300 aircraft, registration G-OINV, experienced a loss of electrical power during descent into Inverness. The aircraft was on a public transport passenger flight from London Gatwick, carrying 5 crew and 71 passengers. No injuries were reported and no damage occurred.
History of the Flight
The aircraft was inbound to Inverness at Flight Level 70 and was above cloud. The Auxiliary Power Unit (APU) was started as part of approach checks. Approximately five seconds into the APU start procedure, there was a loss of electrical power to all primary flying displays, navigation displays, and cockpit lighting, with no warnings shown. The commander made a 'mayday' call to Inverness to report the loss of electrics, and the reply from ATC was heard by both pilots. The aircraft was maintained above cloud in Visual Meteorological Conditions.
The commander managed to regain electrical power about 15 seconds later. He then switched the APU off and reset Generator 1 (GEN 1) and Generator 4 (GEN 4), after which electrical power to all flight deck displays returned to normal. The cabin crew confirmed that cabin lighting remained illuminated but galley power was temporarily lost. The approach to Inverness continued; once established on the Instrument Landing System (ILS) and the airfield was in sight, the 'mayday' was cancelled. After landing, a Ground Power Unit (GPU) was requested and no attempt was made to start the APU. At no stage were any circuit breakers found to be tripped.
Aircraft Information
The BAe 146 has three electrical generators: GEN 1 and GEN 4 mounted on engines No 1 and No 4 respectively, supplying the AC1 and AC2 busbars. The APU generator (APU GEN) can also supply power. The two AC busbars can be linked by bus tie contactors. The captain's displays are supplied from busbars fed from GEN 1, and the co-pilot's displays from busbars fed from GEN 2. Each generator has a Generator Control Unit (GCU) that locks out if a fault is detected. A daily check is carried out by the flight crew to ensure a dormant lock-out has not occurred.
Subsequent Maintenance Activity
On the ground, the only identifiable fault was a possible problem on the ground service bus. The operator replaced the No 1 bus tie contactor, but this did not rectify the fault. Attention then focused on the No 1 GCU, which was replaced. This cleared the fault, and the aircraft returned to service without recurrence. Both the contactor and the GCU were sent for strip inspection. No fault was found with the GCU. The contacts in the contactor were worn to varying levels with an out-of-limit voltage drop across all three sets of contacts, but no evidence of melting or fusing. Tests under hot and cold conditions found no intermittent fault.
Safety Actions
The investigation was inconclusive. The manufacturer recommended that the operator carry out more detailed tests on the system, including the requirements of Service Information Leaflet (SIL) 24/47, which notes typical causes such as an open phase in the contactor or wiring chafes in the generator circuit. These tests were scheduled for the aircraft's next deep maintenance check.