What happened
On 6 February 2007, an Airbus A319-111, registration G-EZIU, was operating a commercial flight from Barcelona to Liverpool. While crossing the south coast of England, the flight crew began receiving a continuous stream of ECAM caution messages accompanied by aural tones. The warnings included engine N2 over-limit and EGT over-limit messages, as well as fuel flow and engine N1 discrepancies.
As the crew attempted to manage the situation, the captain's Primary Flight Display and Navigation Display transitioned to red indications, similar to those seen during inertial reference system alignment. The crew also noted intermittent landing gear and trim warnings. Due to the high frequency of the alerts—with approximately 460 Master Caution chimes recorded by the CVR—the crew was unable to effectively process the checklists.
Fearing the engine issues were genuine, the commander considered shutting down the number 2 engine but ultimately decided to divert to London Stansted due to more favorable weather. The aircraft landed safely without further incident, and no injuries were reported among the 78 passengers or 6 crew members.
The investigation
The AAIB investigation focused on the electronic instrument system and the cause of the conflicting engine data. Analysis of the Flight Data Recorder (FDR) confirmed that engine parameters were actually within normal limits and that no actual engine limits had been exceeded.
Technical examination of the aircraft's Display Management Computers (DMC) revealed that the issue was not mechanical. When the crew switched the captain's instruments from DMC 1 to the standby DMC 3, the erroneous messages ceased. Subsequent investigation of the removed DMC 1 showed that it had previously declared input data as invalid during a similar incident in 2006.
Findings
- The primary cause of the warnings was corruption in the SRAM of the PUMA 0 circuit board within DMC 1, which led to the transmission of invalid input data and corrupted output data.
- This corruption caused the Flight Warning Computers to detect false discrepancies between engine parameters and the thresholds set by the DMC.
- The flight crew was unable to identify the display system as the source of the error because the initial messages suggested engine-related faults.
- There was no specific procedure in the Quick Reference Handbook (QRH) to guide the crew in addressing abnormal display unit operation when no 'INVALID DATA' message was present.