Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 51
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160 45
•
During the ground tests, either control column could be used to control the
nonfailed elevator surface and to command that surface in either the nose-up or
the nose-down direction. The Safety Board’s study of the elevator control
system indicated that under the accident flight conditions, inputs from either
control column would have resulted in corresponding movement of the
nonfailed elevator surface.
4. A jam in the elevator control cable connecting the right-side control column to
the right aft quadrant assembly combined with a break in the same cable.
•
During the ground tests, the left elevator surface moved to nose-down
deflections of 1.2º to 3.9º and the right elevator surface moved to nose-down
deflections of 1.4º to 5.0º, depending on the scenario tested. Analysis of the
elevator system indicated that if such failures occurred in flight, the resultant
elevator surface positions would not have varied as a result of changes in the
aerodynamic forces acting on the elevator in the same manner as the previous
three failures because all three PCAs would still be functioning properly.
•
During the ground tests for all break/jam combinations, either control column
could be used to control both elevator surfaces. Testing showed that a pull from
either control column of 25 lbs would result in sufficient movement of both
elevators in a nose-up direction to be evident on the FDR. A pull from either
control column of 50 to 100 lbs would result in sufficient movement of both
elevators in a nose-up direction to either reverse or significantly slow the
airplane’s nose-down dive.
Pilots from Boeing, EgyptAir, the FAA, and the Safety Board evaluated the
controllability of the airplane following the first three of these failure scenarios in
Boeing’s fixed-base engineering simulator. The simulations assumed that the right
elevator was affected by the failure scenario being evaluated and duplicated the airplane’s
response to the occurrence of that scenario. As previously mentioned, the simulator
reflected the flight characteristics of the 767 to the maximum extent possible. Although all
flight conditions (for example, airspeed, altitude, roll attitude, load factor, and pitch
attitude) were calculated correctly in the simulator, the fixed-base simulator could not
duplicate the physical sensations that would have resulted from these flight conditions.
For example, the load factors that would be produced under actual flight conditions were
not produced in Boeing’s fixed-base simulator nor were the actual attitude changes that
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would have occurred in flight.
In addition, the simulator did not model the override
mechanisms between the two control columns.
Simulations of the three failure scenarios showed that it was possible to regain
control of the airplane using either control column and return it to straight and level flight
using normal piloting techniques and that the airplane could be trimmed to hands-off level
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flight after each of the three failure scenarios occurred.
Further, for all three failure
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The simulator’s cockpit displays did replicate the visual cues (cockpit instrument displays and
out-the-window presentation) that would have been present during actual flight.
NTSB/AAB-02/01
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