Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 25

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Because the FAA received reports that the one-time functional check required by
AD 00-17-05 revealed failed shear rivets on several 767-300 airplanes, on March 5, 2001,
the FAA issued AD 01-04-09, effective March 20, 2001, which required all 767 operators
to perform repetitive functional testing of the elevator control system to determine
whether the elevator PCAs are properly rigged and accomplish followup actions
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(including depth penetration inspection of the shear rivets),
as necessary. AD 01-04-09
required operators to perform the repetitive testing of the elevator control system at least
every 400 flight hours, beginning within 90 days of the AD’s effective date. Although the
cause of the bellcrank shear rivet failures has not yet been determined, Boeing and the
FAA are continuing to study the issue.
One of the mechanical failure conditions evaluated by the Safety Board during the
EgyptAir flight 990 investigation involved disconnection of the input linkages to two of
the three PCAs on one elevator surface. This failure condition could be caused by the
failure of any of the components that comprise the elevator PCAs’ input linkage systems,
including the bellcranks. As further discussed in the section titled, “Potential Causes for
Elevator Movements During the Accident Sequence,” the Board’s tests and simulations
indicated that the nonfailed elevator and the airplane are controllable from either control
column with a dual PCA disconnect on one elevator surface. Those tests showed that
neither a dual disconnection nor a triple disconnection (such as would result from a triple
bellcrank failure) on one elevator surface would produce elevator deflections that matched
the FDR data from the accident sequence.
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The SDRs included two reports of anomalous elevator behavior on the same United Airlines 767, the
first incident occurred on September 12, 1994, and the second on June 20, 1996. Both incidents involved
“stiff” or “frozen” elevator flight controls, and, in both cases, the pilots regained control of the elevator by
applying higher-than-normal pressure on the control column. Postincident examination of the elevator
system components revealed no discrepancies.
The Safety Board is also aware of the following two similar, more recent incidents:
1) On March 27, 2001, an American Airlines 767 experienced elevator control difficulties during
an approach to land. The pilots landed safely using horizontal stabilizer trim for pitch control
and reported that as they taxied to the gate, they “broke [the elevator] free” by applying a
higher-than-normal force on the control column. Postincident examination revealed no
discrepancies in the elevator’s mechanical flight control rigging, PCAs, pushrods, bellcranks,
or shear rivets; however, during postincident examination, investigators observed water
dripping directly on elevator system components in the empennage.
2) On April 23, 2001, the pilots of another 767 experienced elevator control binding during the
approach to land. The pilots applied additional force to the control column, and the elevator
binding released. Postincident examination revealed no evidence of mechanical anomalies;
however, investigators observed an accumulation of water and ice in the empennage around
the elevator system components.
Additional tests indicated that water could freeze on the elevator components and create the effects
described by these flight crews and observed in the FDR data of the two recent incidents. (FDR data were
not available for the two earlier incidents.) The Safety Board compared the FDR data from the two recent
incidents with that from EgyptAir flight 990 and found no similarities. Boeing and the FAA are evaluating
possible corrective actions related to preventing or limiting water from entering the 767 empennage,
freezing at altitude, and impinging on elevator system components.
NTSB/AAB-02/01

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