Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 60
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160 54
“Potential Causes for Elevator Movements During the Accident Sequence,” the
investigation ruled out all but four possible anomalies and failure scenarios as potential
factors in the accident because they diverged too far from what was reflected on the
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accident flight’s FDR to warrant further consideration.
Analysis showed that the effects
of four failure scenarios (each of which involves dual failures) bore some resemblance to
some portions of the accident flight’s FDR data. Specifically, initially it appeared that each
of these failure scenarios could potentially cause nose-down elevator movements or a split
elevator condition that might resemble those recorded on the accident flight’s FDR. Those
four failure scenarios were (1) disconnection of the input linkages to two of the three
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PCAs on the right elevator surface,
(2) a jam of the input linkages or servo valves in two
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of the three PCAs on the right elevator surface,
(3) a jam of the input linkage or servo
valve in one PCA and the disconnection of the input linkage to another PCA on the right
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elevator surface,
and (4) a jam in the elevator flight control cable connecting the
right-side control column to the right aft quadrant assembly combined with a break in the
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same cable.
Therefore, the wreckage from the accident airplane was examined for
possible evidence of PCA anomalies, and the predicted elevator movements resulting
from these failure scenarios were evaluated and compared with the data from the accident
flight.
As previously mentioned, one of the recovered PCAs was found with a pin (that
attached the spring guide to the servo valve slide) sheared and one coil of the bias spring
improperly positioned over the head portion of the spring guide. However, there were no
marks on any of the surfaces or any deformation of the spring coil to suggest that the
spring coil had become jammed between the servo cap and the spring guide, as would be
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expected if such a jam had occurred.
Moreover, investigators measured the clearances
between these components and determined that those clearances were large enough that
even if a coil of the bias spring had become misplaced between the spring guide and the
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servo valve cap, no jam would have resulted.
Further, the FDR data preceding the
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accident sequence do not show any evidence of a single jammed PCA.
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For additional information, see Systems Group Chairman’s Factual Report and its appendixes.
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This would result in the failed surface moving to a nose-down deflection of about 6º and the nonfailed
surface remaining at its prefailure position.
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This would result in the failed surface moving to a nose-down deflection of about 6º and the nonfailed
surface moving to a nose-down deflection of about 4º.
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This would result in the failed surface moving to a nose-down deflection of about 6º and the nonfailed
surface moving to a nose-down deflection of about 2.1º.
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This would result in the left elevator moving to a nose-down deflection of between 1.2º and 3.9º and
the right elevator moving to a nose-down deflection of between 1.4º and 5.0º (depending on which variation
of this scenario is being tested).
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As previously mentioned, the Safety Board recognizes that a jam between two surfaces can occur
without leaving any physical evidence. However, as discussed in the Board’s report on the accident
involving USAir flight 427, physical evidence of a jam would be expected if a hardened steel component
(such as the bias spring in this case) were to become jammed between two surfaces because such evidence
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was always observed after tests involving hardened steel chips jammed/sheared in a PCA
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Even if two coils of the spring had somehow become displaced into the space between the spring
guide and servo valve cap wall, there would still have been sufficient clearance to avoid a jam.
NTSB/AAB-02/01
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