Aviation Investigation Report A98H0003

1.12.7  Examination of Flight Controls

The flight control system actuators were examined; there was no indication of a pre-impact fault within the actuator assemblies that would have affected their normal operation. (STI1-67) Based on this examination, the flaps were extended to 15 degrees; however, the slats were not extended as would be expected with flap extension. (STI1-68) Electrical power is not required for flap extension; however, it is needed for slat operation. The failure of the slats to extend was likely the result of an interruption in electrical power to the slat control valves owing to the effects of the in-flight fire.

With a flap extension of 15 degrees, the outboard ailerons would have become active. The spoilers (speed brakes) (STI1-69) were retracted at the time of impact; however, it is unknown whether they were deployed at any time during the descent below 10 000 feet after the loss of the FDR. Impact markings on the elevator, (STI1-70) rudder, (STI1-71) and aileron actuators, (STI1-72) along with the as-recovered positions of the horizontal stabilizer jack screws indicate the following possible aircraft flight control configuration at the time of impact: 2 to 3 degrees up-elevator; 3 degrees left rudder; and ailerons and horizontal stabilizer in a neutral position. (STI1-73)

The FDR recording, just prior to the loss of the FDR, showed no anomalies with the flight control systems. The last valid recordings indicated that the autopilot had disengaged and the upper and lower yaw damper A control was lost (the upper and lower yaw damper B control was still available). When the FDR recording stopped, the aircraft was in a "clean" configuration, with the flaps, slats, and landing gear retracted.

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