What happened
On 15 September 2006, an Airbus A319-111, registration G-EZAC, was operating a commercial passenger flight near Nantes, France. The aircraft had been dispatched under the Minimum Equipment List (MEL) because the No 1 Integrated Drive Generator (IDG) had failed on a previous flight, leaving the APU generator to supply the AC1 busbar.
While cruising at Flight Level 320, the crew heard a loud 'clunk' followed by a series of system failures. A malfunction caused the loss of the Captain’s Primary Flight Display, the navigation display, the autopilot, and the autothrust system. Crucially, all means of radio telephony (RTF) communication were lost. The crew attempted to reconfigure the electrical system using the AC ESS FEED push button, but the action was unsuccessful. Due to the lack of communications, the crew elected to use the emergency transponder code and continue to the destination.
During the approach to Bristol, the landing gear failed to extend, necessitating the use of the emergency extension system. The aircraft landed safely without any injuries to the 6 crew members or 138 passengers.
The investigation
The AAIB investigation examined flight recorder data and conducted laboratory testing on electrical components. The investigation established that the aircraft's electrical distribution system suffered a significant disruption when the AC1, AC ESS, and DC ESS busbars de-energised.
Testing of the Generator Control Unit (GCU1) and the Ground Power/APU Generator Control Unit (GAPCU) revealed a manufacturing defect in a Static Read-Only-Memory (SRAM) component. This hardware fault caused the monitoring system to incorrectly interpret a fault in the GCU itself as a fault in the external generator line contactor. Consequently, the system opened the bus transfer contactor, disconnecting the APU generator from the AC1 busbar.
Findings
- The primary cause of the electrical failure was a faulty monitoring logic within the GCU1, which misidentified an internal unit fault as an external system fault.
- A manufacturing defect in the SRAM component of the GCU and GAPCU caused the system to incorrectly trigger the isolation of the APU generator.
- The loss of the AC1 busbar resulted in the loss of essential flight instruments and all radio communications.
- The aircraft was operating with one IDG inoperative under MEL provisions at the time of the incident.
Safety action
Following the investigation, the AAIB issued three safety recommendations:
- Safety Recommendation 2006-142: To Airbus, requesting a revision of the fault monitoring logic in the Generator Control Unit to prevent the misinterpretation of internal faults as external system faults.
- Safety Recommendation 2006-143: To Airbus, recommending an automatic transfer of the electrical feed to the AC Essential busbar during a No 1 Main AC busbar failure.
- Safety Recommendation 2006-144: To Airbus, advising operators to ensure that radio communications systems have improved power supply segregation.