What happened
On November 19, 1999, an Air France Boeing 737-300, registration F-GRFA, was cleared for takeoff from runway 26R at Paris Charles de Gaulle. The flight, AFR 1336, was scheduled to proceed to Porto. The runway surface was wet following recent rainfall.
As the aircraft accelerated, the pilot applied thrust and engaged the autothrottle. During the initial acceleration, the aircraft began to veer to the right. The co-pilot attempted to correct the deviation using the rudder, but was unable to maintain the centerline. In an attempt to correct the trajectory, the commander reduced thrust on the left engine, which caused the aircraft to swing to the left.
In a subsequent attempt to restore the heading, the commander adjusted the thrust levers, but this created a significant thrust asymmetry in the opposite direction. The aircraft veered sharply to the left, causing the left landing gear to leave the paved surface and enter the muddy shoulder. The aircraft continued to slide until it came to a stop on the embankment. There were no fatalities among the 59 passengers and 6 crew members, though 6 passengers sustained injuries.
The investigation
The investigation focused on the sequence of thrust applications and the aircraft's handling during the takeoff roll. Investigators examined the engine performance, the flight data recorder (FDR), and the cockpit voice recorder (CVR).
Technical analysis of the engines revealed that both engines had ingested mud. The left engine's low-pressure turbine casing was damaged, and the high-pressure compressor showed damage requiring engine replacement. The right engine's turbine ventilation holes were obstructed by soil. The investigation also examined the nose gear, which had sustained significant damage, including a broken rim.
Findings
- The primary cause of the excursion was a significant thrust asymmetry caused by the pilot's manual adjustments to the thrust levers during the takeoff roll.
- The initial deviation to the right was driven by a 4-degree difference in the position of the thrust levers between the left and right engines.
- The pilot's attempt to correct the rightward veer by reducing left-engine thrust resulted in a leftward swing.
- The subsequent attempt to correct the leftward swing by increasing left-engine thrust and reducing right-engine thrust created a secondary, more severe thrust asymmetry that pulled the aircraft off the runway.
- The runway surface was wet, contributing to the difficulty in maintaining directional control during the asymmetric thrust events.