Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 50
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160 44
Calculations showed that under the conditions of the accident flight when the
initial descent began, the degree of deflection for the nonfailed surface would
be the same as during the ground tests (about 4º).
•
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
80
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.
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 elevator surface.
•
During the ground tests, the failed elevator surface was driven to its full
nose-down position and would not respond to nose-up flight control inputs
from either control column. A study of the elevator control system indicated
that if this scenario occurred in flight, it would result in an initial deflection of
the failed surface to a position consistent with a single functioning elevator
operating at 100 percent of its maximum force (as limited by aerodynamic
blowdown forces); the failed elevator surface would resist being backdriven
with a force equivalent to about 130 percent of a single functioning PCA. As
discussed in connection with the previous failure scenarios, calculations
showed that, under the conditions of the accident flight, this position would
initially have been about 6º nose-down elevator deflection, and the degree of
deflection would be reduced as the airplane’s speed increased above 290 knots.
•
During the ground tests, the nonfailed elevator surface moved to about
2.1º nose-down deflection in the same direction as the failed surface. A study
of the elevator control system’s force balance and calculations of the effect of
this failure under the conditions of the accident flight indicated that the
nonfailed surface would move to a position corresponding to 15 lbs of feel
force. Calculations showed that under the conditions of the accident flight
when the initial descent began, the degree of deflection for the nonfailed
surface would be the same as during the ground tests (about 2.1º).
79
Feel force is the amount of force generated by the aircraft’s feel-and-centering unit. In normal
operation, the feel force is a function of control column deflection and aircraft flight condition. If a jam of
the input linkages or servo valves in two of the three PCAs on a single elevator surface occurred, the
airplane’s feel-and-centering unit would provide a force to oppose the forces needed to deflect the
compressible links on the input side of the failed elevator PCAs.
80
To avoid shearing the test airplane’s bellcrank rivets during the ground tests evaluating the two failure
scenarios that involved jammed PCA linkages and/or servo valves, full travel of the elevator surface was not
commanded. However, a study of the elevator control system indicated that full travel of the nonfailed
surface could have been achieved under these two failure scenarios, if commanded.
NTSB/AAB-02/01
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