Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 21
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elevator movements result. Boeing representatives indicated that the following
circumstances could result in elevator movements at the time of autopilot disconnect:
•
Differences between the neutral position recognized by the autopilot and the
actual neutral position of the elevator feel-and-centering unit would result in
the autopilot actuator holding a force that would be released when the system
is disconnected.
•
The autopilot may have moved the elevators since it last trimmed the stabilizer,
placing the elevators at a position other than their neutral position at the time of
disconnect. When the autopilot is disconnected, the elevators would return to
the neutral position commanded by the feel-and-centering unit. (During
steady-state flight conditions, this situation occurs because of the effect of fuel
consumption on the airplane’s center of gravity.) According to Boeing, “this
type of elevator motion upon autopilot disconnect is inherent in the operation
of the autopilot system.”
•
Pilot forces on the control column at the time of manual autopilot disconnect
can affect the movement of the elevator. (The autopilot can be disconnected
manually by double-clicking the control yoke-mounted autopilot disconnect
switch.)
•
Mechanical aspects of the elevator control system (including friction, the
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effects of compliance in the system,
variations among individual autopilot
actuator units, and variations in the centering detent force) can cause elevator
movement at the time of autopilot disconnect.
Boeing’s 767 Maintenance Manual indicates that if the autopilot disconnects
because of a system failure, the following cockpit warnings and annunciations would
occur:
•
the red autopilot disconnect warning light illuminates,
•
the red master warning light illuminates,
•
the engine indication and crew alerting system computer displays an autopilot
disconnect message, and
•
a siren alert sounds.
Although these autopilot disconnect warnings and annunciations are also
generated when the autopilot is disconnected by pressing the autopilot manual disconnect
switch on the control wheel, pressing the manual disconnect switch a second time within
0.5 second resets, and thus cancels, the system’s disconnect warnings and annunciations
before they are displayed to the flight crew. The 767 autopilot warnings and annunciations
system contains multiple redundancies. For example, two warning signals are generated
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Compliance in the elevator system can occur as a result of cable stretch; yield, give, or elastic
deformation in linkages (that does not damage the linkages but allows additional motion); and variations in
tolerance buildups throughout the system.
NTSB/AAB-02/01
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