Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 24
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FAA Service Difficulty Reports (SDR)
and accident and incident data from all
operators flying 767s between 1990 and 2000 were also reviewed by investigators.
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Although some elevator-related SDRs were noted,
there were no documented
maintenance trends or anomalies that were relevant to the circumstances of this accident.
767 Bellcrank Anomalies
On March 8, 2000, Boeing personnel reported to the Safety Board that Boeing had
been informed of an air carrier incident involving a 767 in which failed bellcrank shear
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rivets were found in the left inboard and left center elevator PCA bellcrank assemblies.
The bellcrank shear rivets are designed to shear if an elevator PCA jam occurs, the
compressible links between the bellcrank assemblies and the PCA input arms are
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bottomed out,
and a force of about 50 lbs is applied to the control column. Research and
testing indicated that sheared rivets in a bellcrank assembly could result in an elevator
PCA disconnect. Such a failure is discussed briefly later in this section and in detail in the
section titled, “Potential Causes for Elevator Movements During the Accident Sequence.”
Boeing and the FAA conducted additional tests and research to further investigate
why the rivets failed and what the possible repercussions of such a failure would be,
including metallurgical examination of high-time bellcranks, material properties testing
on old and new bellcranks, review of bellcrank failure rate data obtained from
767 operators, and examination of maintenance procedures to determine whether changes
in procedures and/or intervals were warranted. The Safety Board monitored the FAA’s and
Boeing’s tests and research into the bellcrank shear rivet failures.
The research conducted by Boeing and the FAA revealed that single bellcrank
shear rivet failures had occurred on other 767s, some of which might not have been
detected during the single hydraulic system maintenance check that is to be conducted by
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767 operators every 400 flight hours.
On August 17, 2000, Boeing issued Service
Bulletin 767-27A0166, which described methods by which failed bellcrank shear rivets
that might not be detected during the single hydraulic system maintenance check could be
identified. Subsequently, the FAA issued AD 00-17-05, effective September 11, 2000,
which required all 767 operators to perform a one-time functional check of one shear rivet
in all six elevator PCA bellcrank assemblies within 30 days, reworking or replacing the
bellcrank assembly if needed. AD 00-17-05 indicates the following:
[F]ailure of two [of the three] bellcrank assemblies on one side can result in that
single elevator surface [but not both surfaces] moving to a hardover position
independent of pilot command resulting in a significant pitch upset recoverable by
the crew. Failure of [all] three bellcrank assemblies on one side can cause an
elevator hardover that may result in loss of controllability of the airplane…the
FAA has received no factual information that indicates that this incident is related
to [the EgyptAir flight 990] accident….The cause of that accident is still under
investigation.
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Through its SDR program, the FAA collects information about mechanical failures from reports
submitted by aircraft operators or maintenance facilities, as required by regulations.
NTSB/AAB-02/01
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