Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 63
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160 57
of the first three failure scenarios. However, the elevator movements during the split are
well within the limits of a pilot-commanded movement.
After reviewing all of the inconsistencies between the effects of the four potential
failure scenarios evaluated in depth, the actual behavior of the airplane, and the
controllability of the airplane in the event of such failures, the Safety Board determined
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that none of these failure scenarios occurred during the accident sequence.
Therefore,
these four failure scenarios can be ruled out along with all of the other potential failure
scenarios considered during this investigation.
The Safety Board also conducted simulations in which pilots from Boeing,
EgyptAir, the Federal Aviation Administration (FAA), and the Board evaluated the
controllability of the airplane following an initial upset that might have been caused by
any of these failure scenarios. During these simulations, the pilots were consistently able
to regain control of the airplane and return it to straight and level flight using normal
piloting techniques, and the airplane could be trimmed to hands-off level flight. In fact, the
767’s redundant actuation system is designed to allow pilots to overcome dual failures
such as these.
Even though increased control forces were necessary, recovery could be
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accomplished by a single pilot using either the left or right control column.
Further, the
simulations also demonstrated that the airplane could climb to about 25,000 feet msl with
the engines shut down, even with the speedbrakes extended. The simulation also
documented that the engines could have been promptly restarted and (assuming there were
no opposing pilot inputs) that the airplane could have been recovered during the climb
after the recorders stopped recording. Although the Safety Board recognizes that the
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simulator did not duplicate the accident airplane’s actual flight conditions in every way,
such limitations are not uncommon in simulations, and the Board takes those limitations
into account when evaluating simulator results. In this case, the Board determined that the
differences were not significant and did not affect the validity of the results of the
simulations.
Immediately after the airplane’s initial nose-down dive, the relief first officer
would have felt an immediate uncomfortable sensation as the airplane’s load factor
decreased to near 0 Gs. He should also have noted sudden changes in the airplane’s pitch
attitude, pitch rate, airspeed, and altitude. In response to these obvious cues, the relief first
officer did not attempt to counter the dive by commanding nose-up elevator, a largely
intuitive pilot response to initiate a recovery.
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Further, although the first three failure scenarios evaluated in depth involved simultaneous dual PCA
failures at the start of the accident sequence, as previously discussed, it is also clear from the FDR data that
no latent jam of a single PCA occurred before the accident sequence.
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As a former chief flight instructor with 5,191 hours in the 767, the relief first officer should have been
readily able to regain control of the airplane.
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For more information about the limitations of Boeing’s simulator, see the section titled, “Potential
Causes for Elevator Movements During the Accident Sequence.”
NTSB/AAB-02/01
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