Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 152
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160 Flight 990’s elevator system. Unless the NTSB can exclude non-impact causes for this observed
damage, it is not correct to state that there is no evidence of a mechanical defect, and it is not
appropriate to assert a probable cause based on purported flight control inputs by the RFO.
32.
(Page 63) The shear rivets in the recovered elevator bellcrank assemblies were
sheared in different directions; however, “the Safety Board considers it likely that the rivets
sheared as a result of impact or recovery-related forces.” The NTSB continues to ignore the
history of the bellcrank shear rivets since this accident. The unexplained rivet failures have
resulted in two FAA Airworthiness Directives, new airline inspection procedures for the rivets,
and possible design changes of the system by Boeing. In addition, there was an unexplained
elevator jam on an American Airlines Boeing 767 in March 2001. It is irresponsible for the
NTSB to summarily dismiss this issue with a “considers it likely” assessment in the accident
analysis. The existence of uncontroverted evidence of deformed rivets, suggesting a defect in the
Boeing 767 elevator system, makes opinion “evidence” highly speculative and inappropriate.
There is significant physical evidence that suggests that this type of failure was a factor in the
accident.
As a relevant factor, however, the issue is not so much whether sheared rivets caused the
uncommanded dive, but whether such shearing -- potentially evidenced by the opposing shear
patterns on the recovered bellcranks -- is indicative of as yet unidentified problems elsewhere in
the elevator control system. If nothing else, this investigation has shown that the Boeing 767
elevator system is extremely complex and that there is relatively little certainty as to its precise
operation under various failure scenarios. Consequently, the mere existence of examples of rivet
shearing and deformation, the cause of which is unknown, should be adequate evidence of the
possibility of a mechanical malfunction relating to Flight 990. This possibility is all the more
likely where, as here, the rivets in question are designed and intended to give way in the event of
a PCA jam. It is hardly farfetched or speculative to suggest that the rivet defects that have been
observed are attributable to intermittent PCA jams, which are sufficiently transient as to leave no
direct evidence and are sufficiently random as to defy predictability. The ECAA also notes that
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