Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 47
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160 41
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indicated a split between the left and right elevator surfaces
The investigation attempted
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to determine if any mechanical failures could have caused these elevator movements.
The Systems Group reviewed numerous potential failure scenarios to evaluate
whether any of them might have been capable of causing the elevator surface movements
recorded on the FDR during the accident sequence, including failures associated with the
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elevator system’s flight control cables,
failures associated with elevator surface PCAs,
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and other system-related failures.
On the basis of the results of failure modes and effects
analyses, the Safety Board ruled out all but four of these potential failure scenarios
because they failed to reflect the accident flight’s elevator movements in obvious and
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significant ways.
For example, it was determined that neither an autopilot malfunction
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nor EMI
would have caused any elevator movements during the accident sequence.
Although some of the other scenarios could have caused some elevator movements, the
nature and degree of those movements differed so greatly from the elevator movements
recorded during the accident flight that they did not warrant further consideration.
However, the failure modes and effects analyses showed that the following four
elevator failure scenarios (each of which involves two failures) warranted further study
because they could potentially cause nose-down elevator movements or a split elevator
condition that might resemble some portions of the data recorded on the accident flight’s
FDR:
1. Disconnection of the input linkages to two of the three PCAs on the right
elevator surface. This failure scenario could be caused by the failure of any of
the components that comprise the actuator input linkage system, including the
bellcranks.
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For additional information, see Systems Group Chairman’s Factual Report and its appendixes and
addendums, Flight Data Recorder Group Chairman’s Factual Report and its attachments, Cockpit Voice
Recorder Group Chairman’s Factual Report and Sound Spectrum Study, and the Aircraft Performance
Group Chairman’s Aircraft Performance Study and its attachments and addendum.
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The cable-related failures considered included a single failed elevator body cable; a failed slave cable;
a failed component or other object falling on elevator cables; a cable tension regulator failure; an aft pressure
bulkhead failure, resulting in cable displacement; and a cable break combined with a jam in the same cable.
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The elevator PCA-related failure scenarios considered included an input rod/cable jammed at an
offset position (position jam), an input arm for a single PCA jammed at an offset position to command a
specific control surface rate of movement (rate jam), failure of the bellcrank assemblies on all three of the
PCAs on a single elevator surface, jam of the input linkage or servo valve of one PCA with a high breakout
force compressible link (a high breakout force compressible link would allow more force to be transmitted
to the input linkages of the nonfailed side before compressing and negating the jammed PCA),
disconnection of the input linkages to two of the three PCAs on a single elevator surface, a jam of the input
linkage or servo valve on one PCA and the disconnection of the input linkage to another PCA on a single
elevator surface, and a jam of the input linkages or servo valves in two of the three PCAs on a single elevator
surface.
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The other system-related failures considered included erroneous stick nudger activation; air in the
hydraulic system and elevated return pressure; hydraulic system failure to one surface; elevator position
transducer disconnect, resulting in erroneous indications on the FDR of an elevator surface offset or split; a
single linkage disconnect downstream of feel unit; a failure of the elevator feel unit’s attachment to aircraft
structure; electromagnetic interference (EMI); and an autopilot malfunction such as a servo jam, resulting in
a hardover autopilot output.
NTSB/AAB-02/01
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