What happened
On 22 October 2005, an Airbus A319-131, registration G-EUOB, was climbing from London Heathrow Airport when the flight crew experienced a significant electrical failure. The incident began with a loss of AC Bus 1, which triggered a cascade of system failures across the cockpit.
As the aircraft climbed, the crew observed the simultaneous blanking of the primary flight and navigation displays for both the commander and the co-pilot. Additionally, the upper Engine/Warning Display (ECAM) became unavailable. This left the crew with limited visual information on the main electronic instrument system, forcing them to rely on standby instruments and manual procedures to manage the aircraft' and monitor engine parameters.
The investigation
The AAIB investigation focused on the electrical power generation and distribution systems, specifically examining the Generator Control Unit (GCU) and the Electronic Instrument System (EIS). Investigators analyzed flight data from the Digital Flight Data Recorder (DFDR) and the Cockpit Voice Recorder (CVR) to reconstruct the sequence of the electrical failure.
Technical examinations were conducted on the aircraft's electrical network, including the AC Essential bus and the behavior of the Display Management Computers (DMCs). The investigation also looked into the certification standards for the A320-series and the adequacy of the standby instrument power supplies. Furthermore, the investigators reviewed the procedures provided in the Flight Crew Operating Manual (FCOM) regarding the recovery of the AC Essential bus.
Findings
- The primary cause of the incident was a failure in the electrical power distribution system involving the AC Bus 1.
- The failure of the AC Bus 1 resulted in the simultaneous loss of the primary flight displays (PFDs) and navigation displays for both pilots.
- The loss of the upper ECAM display prevented the crew from receiving automated system monitoring and failure management instructions.
- The investigation identified that the Generator Control Unit (GCU) was susceptible to false differential protection trips.
- There were concerns regarding the power supply configuration for the standby artificial horizon, which could lead to its loss of function if the DC Essential bus were lost.
Safety action
Following the investigation, several safety recommendations were issued:
- Airbus was advised to review the Master Minimum Equipment List (MMEL) regarding the dispatch of aircraft with an inoperative lower ECAM display to ensure crews have clear recovery instructions.
- Recommendations were made to modify the GCUs to reduce susceptibility to false differential protection trips and to improve fault recording.
- The investigation prompted a modification (Service Bulletin SB A320-24-1120) to provide automatic reconfiguration of the power supply to the AC ESS bus in the event of an AC Bus 1 failure.