Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 62
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160 56
flight differs significantly throughout the accident sequence from the elevator movement
99
profile that would have resulted from any of these three failure scenarios,
indicating that
neither elevator surface on the accident airplane was limited by a mechanical failure but,
rather, that both surfaces were responding normally to flight control inputs. Therefore, the
first three failure scenarios are inconsistent with the elevator movements recorded after
the initial upset.
Similarly, the elevator movements that would have followed any variant of the
fourth failure scenario are also inconsistent with the accident airplane’s recorded elevator
movements after the initial upset. The Safety Board notes that, in one of the four
variations of this scenario (a jam in the aft portion of the elevator control cable combined
with a cable break forward of the jam), the initial elevator positions match those on the
accident airplane. However, this similarity between the failure scenario and the accident
airplane’s elevator movements lasts only a few seconds. For the remainder of that
variation of the scenario (and for the entire duration of the other three variations of this
failure scenario), the elevator positions are inconsistent with those of the accident
airplane.
In addition, if one of the first three failure scenarios had occurred, the nonfailed
surface would have responded immediately to any nose-up flight control inputs from
either control column and would have resulted in an increase in the magnitude of the
difference between the two elevator surface positions (because the failed surface would
remain at its failure-induced position). If the fourth failure scenario had occurred, both
elevator surfaces would have responded immediately to nose-up inputs from either control
column. However, the FDR data from the accident flight showed that there was no
significant nose-up elevator movement or difference between the two elevator surface
positions for the first 28 seconds of the accident sequence—until the captain returned to
100
the cockpit.
If a failure had actually occurred, this would indicate that no attempts were
made to recover the airplane for the first 28 seconds after the initial pitchdown. Further,
after the captain returned to the cockpit, both elevator surfaces began moving together in
the nose-up direction, indicating that neither surface was limited by a mechanical failure
but, rather, that both surfaces were responding normally to flight control inputs. Similarly,
later in the accident sequence, when the elevator split occurred, the right elevator
deflected well beyond the maximum position possible for a failed elevator surface in any
99
The Safety Board recognizes that there was some uncertainty in the aerodynamic hinge moment data
used to calculate the elevator movement profiles for the failure scenarios depicted in figure 2, especially at
Mach speeds greater than 0.91. However, for the initial elevator movement on the accident flight to match
the elevator deflections in response to the failure scenarios, the aerodynamic hinge moment data used to
calculate the failure scenario profiles would have to have been about 79 percent greater than assumed. The
Board considers this amount of error to be extremely unlikely, particularly because the initial elevator
movement occurred below Mach 0.91, where hinge moment data were validated by certification flight tests.
100
As previously mentioned, 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
(less than 1° ) differences between the left and right elevator surface positions were observed. Even where
these offsets were observed, the elevator surfaces always moved in the same direction about the same time.
However, beginning at 0150:21, the elevator surfaces moved in opposite directions and remained there until
the FDR ceased recording.
NTSB/AAB-02/01
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