Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 43
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160 37
Accident Sequence Study
The Safety Board used the FDR, radar, winds aloft, and 767 performance data to
determine the accident airplane’s motions and performance during the accident sequence.
These data (and associated calculations) indicated the following:
•
Aside from the very slight movement of both elevators (the left elevator moved
from a 0.7° to about a 0.5° nose-up deflection, and the right elevator moved
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from a 0.35° nose-up deflection to about a 0.3° nose-down deflection)
and
the airplane’s corresponding slight nose-down pitch change, which were
recorded within the first second after autopilot disconnect at 0149:45, and a
very slow (0.5º per second) left roll rate, the airplane remained essentially in
level flight about FL 330 for about 8 seconds after the autopilot was
disconnected.
•
At 0149:53, the left and right throttles were retarded to the aft idle stop
(equivalent to a throttle lever angle of about 33º) at a rate of about 25º per
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second.
About 1 second after the start of the throttle movement, the FDR
recorded slight motion in the inboard ailerons, the left elevator surface moved
to about a 3.4º TED position, and the right elevator surface moved to about a
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3.8º TED position.
•
At 0149:54, the airplane began to pitch nose down, reaching a pitch attitude of
about 40º nose down at 0150:15. During the dive, the wings remained within
about 10º of level and the heading remained about 80º, increasing to about 85º
between 0150:20 and 0150:33.
•
Between 0150:05 and 0150:06, the FDR recorded additional movements in the
inboard ailerons, and the left and right elevators moved an additional 1.5º TED
to about 5.5º TED. Before this time, the load factor had been about 0.2 G; after
this time, the load factor decreased to about -0.1 G. Between 0150:06 and
0150:10, the FDR began to record “Low Engine Oil Pressure” signals for both
engines; the FDR recorded these signals until after the load factor increased to
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above 0 G between 0150:17 and 0150:21.
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Throughout the FDR data for the accident airplane (including data recorded during uneventful
portions of the accident flight and during previous flights and ground operations), small (generally less
than 1° ) differences between the left and right elevator surface positions were observed. The elevator
surface movements were consistent (that is, moved in the same direction about the same time) where these
offsets were observed.
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As previously mentioned, at the accident airplane’s flight conditions at the beginning of the accident
sequence, the minimum autothrottle commanded throttle lever position would have been between 40º and
50º; this value would have decreased as the airplane’s airspeed increased. The maximum autothrottle
commanded throttle lever movement rate for a normally functioning autothrottle system is 10.5º per second.
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Elevator movement in the TED direction would result in a decrease in the airplane’s lift and load
factor and an increase in the airplane’s nose-down attitude.
NTSB/AAB-02/01
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