Approach and Autothrottle
The flight was cleared for the ILS Runway 19 approach, with a circle-to-land on Runway 24, a 6,013-foot-long, 150-foot-wide asphalt runway. During the approach, at about 800 feet above ground level (agl), the auto throttle and autopilot were disengaged. However, just prior to touchdown, at about 35 feet agl, the auto throttle reengaged. The flightcrew did not recall reengaging the auto throttle and were unaware of its reengagement. According to the auto throttle computers, the reengagement was commanded through one of the Engage/Disengage paddle switches located on each power lever. The target airspeed set for the auto throttle system was 138 knots.
Landing and Runway Excursion
After touchdown, as the airplane decelerated below 138 knots, the auto throttle system gradually increased the power levers in an attempt to maintain the target airspeed. Because the power levers were not in the idle position, the ground spoilers and thrust reversers did not deploy. While the flightcrew pulled up on the thrust reverser levers, they may not have initially provided enough aft force on the power levers (15 to 32 lbs.) to override and disconnect the auto throttle system. The flight data recorder indicated that the autothrottle system disengaged 16 seconds after the weight-on-wheels switches were activated in ground mode. As the airplane neared the end of the runway, the pilot engaged the emergency brake, and the airplane departed the right side of the runway.
System Design and Environmental Conditions
The autothrottle Engage/Disengage paddle switches were not equipped with switch guards. Although the autothrottle system provided an audible tone when disengaged, it did not provide a tone when engaged. The reported wind about the time of the accident was from 290 degrees at 16 knots, gusting to 25 knots, with a peak wind from 300 degrees at 32 knots.