Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 20
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Figure 2. Comparison of the elevator positions recorded by the accident airplane’s
FDR with failed and nonfailed elevator positions following a dual PCA failure.
The 767 autopilot system controls the airplane’s movement about the pitch axis by
using the elevators for dynamic control of the airplane’s pitch and the horizontal stabilizer
to trim out steady-state elevator deflections. When the autopilot is engaged and the
airplane is in a steady-state flight condition, the autopilot is designed to keep the elevators
near their neutral (or faired) position, using the elevators primarily for short-term dynamic
adjustments (such as those necessitated by atmospheric disturbances). The elevators are
also used for small trim adjustments, such as those necessitated by fuel consumption
during flight. As these small elevator adjustments accumulate over time, the elevator
deflections move further from their neutral (or faired) position. When the elevators’
deflections reach a threshold value, the autopilot “retrims” the horizontal stabilizer and the
elevator returns to a neutral (or faired) position. According to Boeing, when the autopilot
system is disconnected, the force applied by the autopilot actuator to the elevator control
system is removed, and, if the horizontal stabilizer has not been adjusted recently, small
NTSB/AAB-02/01
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