Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 22

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for each of the warning functions listed above: one warning signal uses software logic that
is powered by normal power (which would be inhibited by a loss of normal power or a
computer failure), and the other uses hardware logic that is powered by 28-volt alternating
current standby power.
767 Autothrottle Information
The 767’s thrust management system provides autothrottle control based on
selected modes, existing conditions, and engine limitations. The autothrottle can be
operated independently of or with the autopilot system. The autothrottle servomotor
generator is connected to the throttle levers through a clutch pack assembly, which, when
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overridden,
allows the pilots to make manual thrust inputs when the autothrottle is
engaged. Movement of the throttle levers aft of the autothrottle commanded position for a
given flight condition would require a manual force of about 9 lbs at the throttle levers to
override the autothrottle servomotor clutch.
When the autothrottle function is engaged, it controls throttle lever movement. The
maximum autothrottle commanded throttle lever movement rate for a normally
functioning autothrottle system is 10.5° per second. Manual throttle lever inputs can
exceed this rate; for example, the accident airplane’s FDR recorded throttle lever
movement at a rate of 25° per second at the beginning of the accident sequence. The
minimum throttle lever position that the autothrottle can command varies as a function of
the airplane’s speed and the autothrottle mode selected. For the accident airplane’s flight
conditions and the selected autothrottle mode at the beginning of the accident sequence,
this position would have been 40° to 50° . The FDR recorded a throttle lever position of
about 33° at the beginning of the accident sequence.
Reported Autopilot Anomalies in the Accident Airplane
During interviews conducted at the request of the Egyptian Government on
February 21, 2001, an EgyptAir captain who had flown the accident airplane from Newark
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International Airport (EWR), Newark, New Jersey,
to LAX on October 30, 1999,
reported that he had experienced difficulties with the autopilot during a portion of that
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flight.
The captain told investigators that the autopilot was “hunting” for the glideslope
at 8,000 to 10,000 feet msl during the approach to LAX and that, because he was
uncomfortable with the autopilot’s performance, he disconnected it. The captain reported
that his three subsequent attempts to reengage the autopilot to intercept the glideslope in
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A manual force of about 9 pounds (lbs) is required to override the clutch.
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The October 30, 1999, EgyptAir flight from Cairo was scheduled to land at JFK but diverted to EWR
because of weather.
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During interviews conducted 3 days after the accident, the captain that had flown the airplane from
EWR to LAX on October 30, 1999, described several noncritical anomalies (a deactivated thrust reverser, an
intermittent air conditioning pack “inoperative” light, a full aft lavatory holding tank, and the autopilot
anomaly previously mentioned) but stated that the airplane was “almost perfect.” The first officer of the
flight to LAX did not describe the autopilot anomaly.
NTSB/AAB-02/01

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