Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 19

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the forces produced by the two failed PCAs would overpower the opposing force
produced by the one nonfailed PCA. The resultant initial force on the elevator surface in
the failed direction would be equivalent to a single functioning PCA operating at
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100 percent of its maximum force. The failed PCAs would resist the backdriving force
with a force equivalent to about 130 percent of a single functioning PCA. The high
internal pressures required for activation of the PCAs’ pressure relief valves allow the
PCAs’ pistons to resist the aerodynamic backdriving movement with more force than
normal operating pressures would allow. Therefore, if a dual PCA failure occurred in
flight, the elevator would initially move to a position consistent with a single functioning
PCA operating at 100 percent of its maximum force, balanced against the aerodynamic
forces affecting the elevator surface. As the airspeed increases, the failed elevator surface
would remain at this initial position until the backdriving forces exceeded those of a single
PCA operating at 130 percent of its normal capability, at which point the deflection of the
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failed elevator surface would decrease.
(Figure 2 is a comparison of the elevator
positions recorded by the accident airplane’s FDR with failed and nonfailed elevator
positions following a dual PCA failure.)
767 Autoflight Systems Information
The 767 autoflight systems include the autopilot/flight director, yaw damper,
automatic stabilizer trim, Mach trim, maintenance monitoring, instrument landing system
deviation monitor, and thrust management systems. The thrust management system
includes autothrottle control.
767 Autopilot Information
The 767 autopilot/flight director system consists (in part) of three separate
autopilot systems that can be used singly or in combination to provide automatic control
of the ailerons, elevator, stabilizer, and rudder control systems when operating in selected
flight modes. Any one of the three autopilot systems can control the airplane in the normal
climb, cruise, descent, and approach modes.
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The Safety Board thoroughly examined the dual elevator PCA failure scenarios during its
investigation of this accident. For more information, see the section titled, “Potential Causes for Elevator
Movements During the Accident Sequence.”
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The term “backdriving” refers to the effect of aerodynamic forces that act on the elevator surface and
move the surface in the direction opposite to that being commanded (by the two failed PCAs, in the case of
a dual elevator PCA failure). This backdriving force increases as an airplane’s airspeed increases.
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For additional information, see the section of this report titled, “Potential Causes for Elevator
Movements During the Accident Sequence.”
NTSB/AAB-02/01

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