What happened
Between February and September 1996, an Airbus A300 B4, registration OH-LAA, experienced three separate thrust control malfunctions during takeoff phases on charter flights.
In the first instance, occurring in Ovda, Israel, on February 29, the right thrust lever stopped at 106% N1 while the left lever continued to move backward toward idle. The crew manually stabilized the engines to 98% N1 to continue the flight.
Two subsequent incidents occurred in September 199/6. In Antalya, Turkey, on September 4, the left thrust lever drifted toward idle, dropping to 60% power during climb-out, requiring manual intervention by the crew. On September 9, in Iraklion, Greece, the left engine power dropped rapidly to 80% while the right engine remained at 113% takeoff power. In both cases, the crew manually adjusted the levers to maintain the required thrust.
The investigation
Investigators examined the aircraft's thrust control system, including the Thrust Control Computer (TCC), wiring, and mechanical linkages. While the TCC showed a zero-point shift in its servo amplifier output voltage, this was not considered the primary cause.
Technical inspections of the thrust control cables and mechanisms revealed that the slip mechanism in the left automatic thrust control unit was jammed. Furthermore, the slip mechanism in the right unit was operating at the very lower limit of its allowable tolerance. The investigation also reviewed maintenance records and correspondence with Airbus Industrie regarding friction tolerances and maintenance requirements.
Findings
The investigation established that the malfunctions were caused by a combination of mechanical factors. The right thrust control mechanism failed to move due to friction levels that were near the upper limit of allowable tolerances. Because the right engine's thrust did not decrease as commanded, the automatic control system continued to execute the reduction command, which resulted in the left engine's thrust lever moving toward idle.
Ultimately, the improper functioning of the slip mechanism—specifically its operation at the lower limit of the friction tolerance—was the primary cause of the uncommanded thrust changes.