What happened
On September 13, 1993, an Air France Boeing 747-428 B, registration F-GITA, was performing a scheduled passenger flight from Los Angeles to Tahiti. During the final approach to runway 22 at Tahiti Faaa airport, the aircraft was flying in night conditions with good weather.
As the aircraft descended, the crew monitored the flight path using VOR-DME and PAPI indications. At approximately 500 feet, the flight mode annunciator indicated a change to Thrust Reference/V NAV Speed, which caused engine power to increase across all four engines. This resulted in the aircraft flying slightly above the glide path and increasing its speed to 18/0 knots at 150 feet. The co-pilot noted that the thrust levers tended to move forward and that he had to manually hold them in the idle position, despite being unable to disconnect the autothrottle.
The aircraft touched down at 21:05 local time with a speed of 168 knots. Two seconds prior to touchdown, the number 1 engine (outer left) began to increase power, eventually stabilizing at 107% N1 in positive thrust. This asymmetric thrust prevented the deployment of spoilers and caused the automatic braking system to disarm. As the aircraft attempted to decelerate using reverse thrust on engines 2, 3, and 4, the asymmetric power caused the aircraft to veer right. The plane exited the runway at the end of the pavement, with the nose entering the lagoon waters.
The investigation
The investigation focused on the behavior of the automatic flight systems, specifically the VNAV mode and the autothropper during the approach. Investigators examined the flight management system (FMS) and the flight mode annunciator (FMA) transitions. The investigation also reviewed the cockpit voice recorder (CVR) and flight data recorder (FDR) to analyze the crew's actions and the engine's power fluctuations. Technical examinations were conducted on the engines, the landing gear, and the braking systems to determine why the spoilers failed to deploy and why the engine thrust increased unexpectedly.