What happened
On September 13, 2011, an Airbus A300-600ST, registration F-GSTA, operated by Airbus Transport International, was performing a non-scheduled cargo flight from Toulouse-Blagnac to Chester, UK. During the takeoff roll under visual meteorological conditions, the crew applied the Go levers to advance the thrust levers to the TOGA position.
Following rotation and the retraction of the landing gear, the commander noticed that the flight mode annunciator (FMA) displayed "THR L" and that the "SRS" mode was no longer visible. Simultaneously, the flight director 1 (FD1) indicated a pitch-down command. While the co-pilot's display appeared consistent with the flight path, the discrepancy between the two displays led the crew to take corrective action. The co-pilot disengaged the "THR L" mode and requested the engagement of the auto-thrust in "SPD" mode. This caused an immediate reduction in thrust as the system attempted to stabilize the airspeed at the target V2 speed. The crew manually intervened by disengaging the auto-thrust and continuing the climb via manual piloting. No injuries or damage occurred.
The investigation
The investigation utilized Flight Parameter Recorder (QAR) data and crew testimony to reconstruct the flight sequence. Analysis of the QAR revealed that the "THR L" mode was active from the moment the Go levers were moved. The investigation established that the takeoff mode (TAKE OFF) failed to engage, resulting in the activation of basic modes (V/S-HDG) instead of the expected takeoff modes (SRS-RWY).
Technicians examined the Thrust Control Computer 1 (TCC1), which had correctly received the Go lever signal, but found that Flight Control Computer 1 (FCC1) had not. While inspections of the wiring and connectors between the Go levers and FCC1 showed no visible defects and the fault could not be reproduced, the investigators identified a likely intermittent wiring failure.
Findings
- The primary cause of the incident was the crew's failure to detect the non-engagement of the takeoff mode during the FMA verification process during the takeoff roll.
- The detection was made difficult because the FMA indications for the co-pilot were visually similar to the expected modes, and the commander's verification was performed routinely without identifying the discrepancy.
- The crew did not notice that the thrust levers were advancing at a rate of 8°/s, which is significantly faster than the normal 3°/s rate seen in other modes.
- There was no specific procedure in the flight manual for handling a failure of the takeoff mode to engage or for managing divergent FMA indications.
- An erratic wiring failure between the Go levers and FCC1 likely prevented the takeoff mode from engaging.