What happened
On 15 September 2006, an Airbus A319-111, registration G-EZAC, was operating a commercial passenger flight from Alicante, Spain, to Bristol, UK. The aircraft had been dispatched under the provisions of the operator's Minimum Equipment List (MEL) because the No 1 Integrated Drive Generator (IDG1) had failed on a previous flight. To compensate for the missing generator, the crew was utilizing the APU generator to power the AC1 busbar.
While cruising at Flight Level 320, the aircraft experienced a sudden electrical disturbance. A 'clunk' was heard, followed by the immediate loss of several critical services, including the Captain's Primary Flight Display, the autopilot, and the autothrust system. Crucially, the loss of the AC1 busbar also resulted in the loss of all radio telephony (RTF) communications, as the aircraft's communication equipment relied on the DC ESS busbar, which was de-energised during the event.
In response to the failure, the crew attempted to re-establish power by selecting the AC ESS FEED push button to alternate; however, this action was unsuccessful. The commander subsequently selected the emergency transponder code 7700 to alert air traffic control of the situation. Despite the loss of primary flight displays and communications, the crew managed the situation and landed the aircraft safely at Bristol.
The investigation
The AAIB investigation examined the aircraft's electrical distribution system and flight data. Data from the flight recorders confirmed that the AC1, AC ESS, and DC ESS busbars de-energised simultaneously. The investigation focused on why the APU generator, which was intended to provide backup power, failed to maintain the necessary busbar stability.
Laboratory testing of the Generator Control Unit (GCU1) revealed an intermittent fault where the unit incorrectly detected a fault current. This caused the monitoring system to misinterpret a fault within the GCU itself as a fault in the generator line contactor, leading the system to lock the APU generator off the AC1 busbar. Additionally, a fault was identified in the Ground Power/APU Generator Control Unit (GAPCU).
Findings
- The primary cause of the electrical failure was the incorrect interpretation of a GCU fault as an external system fault by the monitoring logic.
- The failure of the GCU1 to correctly monitor the system led to the unintended disconnection of the APU generator.
- The loss of the AC1 busbar caused a cascading loss of essential services, including all radio communications.
- The aircraft was operating in a configuration where the remaining power source was vulnerable to a single-point failure in the control units.
Safety action
- Safety Recommendation 2006-142: It is recommended that Airbus revise the fault monitoring logic of the Generator Control Unit to prevent the system from misinterpreting internal GCU faults as external faults.
- Safety Recommendation 2006-143: It is recommended that Airbus introduce a modification to automatically transfer the electrical feed to the AC ESS busbar if the No 1 Main AC busbar is lost.
- Safety Recommendation 2006-144: It is recommended that Airbus advise operators that RTF communications may be lost due to reliance on a single busbar.
- Safety Recommendation 2006-145: It is recommended that Airbus implement modifications to improve the level of power supply segregation for radio communications.