What happened
On December 3, 2002, an Airbus A300-600 operating a scheduled flight from Munich to Frankfurt experienced a severe flight control disturbance during its climb. While flying at approximately 17,000 feet with autopilot 2 engaged, the crew noticed the aircraft's airspeed was increasing toward the maximum operating limit (VMO). In an attempt to manage the speed, the crew reduced the preset speed and increased the rate of climb. However, the aircraft nose began to dip, prompting the pilot to disconnect the autopilot.
Upon taking manual control, the pilot found the aircraft was heavily nose-heavy and trimmed in a way that required significant physical force to maneuver. The aircraft was unable to maintain its climb, resulting in a loss of 1,000 feet in altitude and a peak airspeed of 351 KIAS. During the recovery, vertical accelerations between 0.2g and 1.5g occurred, causing a flight attendant to fall and sustain minor injuries.
The investigation
The BFU investigation revealed that the aircraft was operating with a deferred maintenance item. A previous fault in the pitch trim system 1 (PTS 1) had been deferred under the Minimum Equipment List (MEL). While the crew had noted a similar nose-down trim tendency on previous flights that day, a ground test of the second pitch trim system (PTS 2) had failed to replicate the issue, leading technicians to clear the fault as resolved.
Technical analysis of the aircraft's Flight Data Recorder (FDR) showed that the trimable horizontal stabilizer (THS) had been moving continuously at 0.01°/s toward a nose-down position. Further inspection of the hardware uncovered a broken wire in the connection to the Flight Augmentation Computer 2 (FAC2). Subsequent simulator testing at the manufacturer's facility confirmed that a software discrepancy in the "Theta Trim" modification caused the FAC2 to trigger uncommanded trim movements once a specific airspeed was exceeded.