Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 48

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2. A jam of the input linkages or servo valves in two of the three PCAs on the
right elevator surface. In order for this failure scenario to occur, the internal
slides of the affected servo valve would first have to be moved (by manual or
autopilot input) to an offset position and then jam. Although such jams could
theoretically occur in either direction, all tests and simulations involving
jammed elevator PCAs were intentionally configured to produce nose-down
(rather than nose-up) elevator input.
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.
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 same
cable. (Four variants of this scenario were studied. For additional information,
see the Systems Group Chairman’s Factual Report and its addendum regarding
the cable break/jam and PCA jam with high breakout force [compressible link]
ground testing.)
For further evaluation, the Systems Group conducted ground tests on an
instrumented 767 to record the elevator system’s response to each of these failure
scenarios. During the ground tests, the test airplane’s systems were configured to simulate
the accident airplane’s altitude and airspeed. The Systems Group also studied the effect
that each failure scenario would have on the elevator control system and calculated the
effect on the elevators that each scenario would have had at the specific conditions of the
accident flight at the time of the initial pitchdown. The results of the tests, studies, and
calculations were as follows:
1. Disconnection of the input linkages to two of the three PCAs 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
PCA operating at 100 percent of its maximum force (as limited by
aerodynamic blowdown forces); the failed elevator surface would resist being
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For additional information, see Systems Group Chairman’s Factual Report and its appendixes.
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EMI is electromagnetic radiation that is emitted and/or received by an electronic device and adversely
affects the performance of that device or other devices.
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An autopilot malfunction was ruled out as a potential cause of the elevator movements because the
autopilot was disconnected before the beginning of the nose-down elevator movements. Even assuming that
the autopilot was engaged during the accident sequence, the elevator movements recorded by the FDR
exceeded the maximum inputs that could be commanded by the autopilot. EMI was ruled out because
elevator surface movements are not electrically actuated (and, therefore, are not susceptible to the effects of
EMI) except through the autopilot. (When the autopilot is not engaged, elevator surface movements on the
767 are mechanically signaled and hydraulically actuated.)
NTSB/AAB-02/01

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