Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 41
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160 35
impact-related damage. One of the four also exhibited the following two unusual
characteristics on its internal mechanisms: (1) the pin that attaches the spring guide to the
valve slide was sheared, and (2) a portion of the bias spring (about one full coil) was
improperly positioned over the head portion of the spring guide. It could not be
determined whether these conditions existed before impact or whether they were impact
related. The Safety Board’s measurements of these components indicated that the inside
diameter of the servo valve cap into which the bias spring and spring guide fit was
0.872 inch and that the outside diameter of the spring guide at its widest point was
0.749 inch, leaving a clearance of 0.123 inch between the spring guide and the servo valve
cap. Measurements indicated that the bias spring wire had a diameter of 0.031 inch.
Impact marks and damage were observed on other components in this PCA; however,
there was no evidence of scraping, abrasion, or other marks on the improperly positioned
bias spring or adjacent surfaces that would indicate that these metal parts had jammed in
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the PCA.
Five of the elevators’ six bellcranks (all three right elevator and two of the left
elevator bellcrank assemblies) were recovered. Postaccident examination of the recovered
bellcrank assemblies revealed that all of the shear rivets in the recovered bellcrank
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assemblies were sheared,
with the sheared surfaces appearing consistent with shear
overstress. However, the rivets in some of the bellcrank assemblies sheared in a direction
opposite to others; shear rivets in the two recovered bellcrank assemblies from the left
elevator surface and in the inboard bellcrank assembly from the right elevator surface
were sheared as if the bellcrank arms were moving to a higher relative angle, whereas the
shear rivets in the middle and outboard bellcrank assemblies from the right elevator
surface were sheared as if the bellcrank arms were moving to a lower relative angle. Most
of the recovered elevator control linkages were found broken or otherwise damaged.
Examination of the fracture surfaces on the recovered pieces of wreckage revealed
that the fractures were consistent with failures generated by a high-speed impact. None of
the fracture surfaces examined exhibited any sign of preexisting fatigue or corrosion. No
evidence of foreign object impact damage or pre- or postimpact explosion or fire damage
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was observed.
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The right elevator center PCA was identified as such because of its location in recovered horizontal
stabilizer wreckage. The right elevator outboard PCA was identified as such by EgyptAir personnel, who
matched the PCA’s S/N to their maintenance documents for the accident airplane. The condition of the other
two recovered PCAs precluded identification of their location on the airplane.
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The Safety Board recognizes that a jam between two surfaces can occur without leaving any physical
evidence. However, as discussed in the Board’s report on the September 8, 1994, accident involving USAir
flight 427, tests conducted in connection with that accident investigation showed that physical evidence of a
jam was always observed after tests involving hardened steel chips jammed and/or sheared in a PCA.
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As previously discussed, the shear rivets are designed to fail when they are subjected to about 50 lbs
of force or more at the control column, the PCA is jammed, and the compressible links are bottomed out. In
addition, shear rivets may fail as a result of impact or recovery-related forces.
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For additional information, see Systems Group Chairman’s Factual Report and its appendixes and
addendum, Materials Laboratory Factual Report, and Structures Group Chairman’s Factual Report and its
appendixes and addendum.
NTSB/AAB-02/01
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