Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 61
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160 55
Most of the recovered elevator control linkages were broken; however, this type of
damage is typical following a high-speed water impact and underwater wreckage recovery
operations. The shear rivets in the recovered elevator bellcrank assemblies were sheared in
different directions; however, the Safety Board considers it likely that the rivets sheared as
a result of impact or recovery-related forces. Nonetheless, on the basis of the examination
of the structure alone, the absence or presence of a jammed or disconnected input linkage
or a jam in the servo valve in one of the accident airplane’s elevator PCAs could not be
established.
However, ground tests, studies, and calculations showed that each of the first three
failure scenarios would have resulted in airplane and flight control movements that were
inconsistent with the accident airplane’s elevator movements. Specifically, each of those
three failure scenarios would have caused the failed elevator surface to move to, and
remain at, a position consistent with a single functioning PCA operating at 100 percent of
its maximum force. The failed elevator surface would resist being backdriven with a force
equivalent to about 130 percent of a single functioning PCA and would not have
responded to nose-up flight control inputs. If one of these scenarios occurred at the
accident airplane’s indicated airspeed at the time of the initial dive (280 knots), the failed
elevator surface would have initially moved from its prefailure position (close to neutral)
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to about a 6º nose-down position.
However, the initial elevator movement (for both
elevator surfaces) on the accident airplane recorded during the accident sequence was to a
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nose-down position of only about 3.6° .
The Safety Board also compared the recorded elevator movements following the
initial upset to elevator movements resulting from the first three failure scenarios. As the
airplane’s speed increased after the initial upset, the maximum deflection value associated
with the three failure scenarios would have decreased in response to the increased
aerodynamic forces on that surface. However, subsequent movements of both elevator
surfaces on the accident airplane deviated repeatedly, for sustained periods of time in both
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the nose-up and nose-down directions,
from the maximum deflection values that the
failure scenarios would have produced, at times exceeding the maximum deflection values
by several degrees. As shown in figure 2, the elevator movement profile from the accident
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FDR and ground test data indicated that a single PCA failure would have resulted in much higher
offsets between the two elevator surfaces than were recorded during the accident flight and on the ground
before takeoff. During the accident flight, the slight offset that was recorded by the FDR was only 47 percent
of the offset that would be expected if a latent single PCA failure had occurred. On the ground before
takeoff, the recorded offset was only 27 percent of what would be expected if a latent single PCA failure had
occurred.
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As previously discussed, failure scenarios resulting in nose-up motions of the elevators were also
possible but were not considered relevant to this accident investigation.
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As discussed in the section titled, “Accident Sequence Study,” the initial deflection for the left
elevator surface was about 3.4º, and the initial deflection for the right elevator surface was about 3.8º. As
shown in the graphical representations of the recorded elevator positions, a position of about 6º can be easily
distinguished in the data from a position of either 3.4º or 3.8º.
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The failure scenarios would not preclude additional commanded nose-down movement of the failed
elevator surface. However, commanding additional nose-down movements would be inconsistent with an
attempt to recover the airplane.
NTSB/AAB-02/01
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