What happened
On 15 September 2006, an Airbus A319-111, registration G-EZAC, operating a scheduled flight from Alicante, Spain, to Bristol, UK, experienced a significant electrical system failure. While cruising at Flight Level 320 near Nantes, France, the aircraft's No 1 engine-driven generator tripped offline. This event triggered a cascade of system degradations, including the loss of the aircraft's transponder and the de-energisation of the AC Essential busbar.
The failure had a profound impact on the flight deck. All radio communications became inoperative, and the Captain’s electronic flight instrument displays went blank. The loss of the transponder signal meant the aircraft was not visible to certain radar controllers for approximately 10 minutes, creating a period of reduced separation from another aircraft. Furthermore, the crew found it impossible to manually reconfigure the electrical system to restore power, as the push-button selectors provided no clear indication of their position and failed to execute the necessary power transfer.
Despite the increased workload and the loss of critical automated systems, the crew managed to land the aircraft safely at Bristol.
The investigation
An investigation was conducted by the AAIB with assistance from the aircraft manufacturer, Airbus, and the French BEA. Investigators examined the electrical architecture and the history of the aircraft's maintenance. It was discovered that the aircraft had been dispatched from Alicante with the No 1 generator selected off, using the APU generator as a substitute, which was permitted under the operator's Minimum Equipment List (MEL).
The investigation focused on the No 1 Generator Control Unit (GCU). Findings revealed that an intermittent fault within this component had caused the generator to trip. Crucially, the investigation found that the GCU had been repeatedly removed from service due to similar intermittent faults but had been returned to service without the underlying issue being resolved. Additionally, the investigators looked into why the crew could not restore power, noting that the electrical system design required a manual reconfiguration that proved ineffective under the circumstances.
Findings
- An intermittent fault in the No 1 Generator Control Unit caused the loss of the left electrical network.
- The aircraft's electrical system design required a manual reconfiguration of the electrical feed to the AC Essential busbar, which, when triggered by the loss of the No 1 AC bus, caused multiple systems to fail.
- The flight crew was unable to reconfigure the system, for reasons that could not be established.
- The aircraft was dispatched with a main generator inoperative under provisions that did not require an investigation into the history of previous electrical faults.
- There were inadequate measures in place to identify Generator Control Units that were repeatedly rejected from service due to recurring faults.
Safety action
Several safety recommendations were issued to address the systemic issues identified, including:
- A recommendation for Airbus to revise the fault monitoring logic of the Generator Control Unit to prevent the system from misinterpreting internal faults as external busbar failures.
- A recommendation for a modification to allow for the automatic transfer of the electrical feed to the AC Essential busbar if the No 1 Main AC bus fails.
- A recommendation to modify maintenance and overhaul procedures to ensure that components with high service rejection rates are not repeatedly returned to service without finding the root cause.