Aircraft Accident Brief Ntsb/aab-02/01 (Pb2002-910401): Egypt Air Flight 990, Boeing 767-366er, Su-Gap - National Transportation Safety Board Page 69
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160 63
(at 0150:21), both elevator surfaces passed through their neutral positions into nose-up
deflections. However, less than 1 second later, the right surface reversed its motion and
moved back in the nose-down direction, and the left surface continued to move in the
nose-up direction.
According to Boeing’s tests and research, with the elevator PCAs operating
normally, the accident airplane’s elevators would have only been minimally affected by
the aerodynamic forces that would have resulted from the small sideslip angle, roll rates,
and the Mach numbers that existed during the accident sequence. Therefore, it follows that
the elevator split recorded by the FDR was the result of flight control inputs to each
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elevator surface and not the result of aerodynamic forces on those surfaces.
(In contrast,
Boeing indicated that an outboard aileron split recorded between 0150:27 and 0150:32
could be explained by the aerodynamic effects of the small sideslip angles and roll rates
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calculated to have been present at that time.)
Testing confirmed that the left and right elevator surfaces could be moved in
different directions by differential column movements from the relief first officer and
captain in the cockpit. As intended by the elevator control system design, the elevators
would split, each surface following the movements of the control column on its side (the
left elevator moving in response to the left column movement, and the right elevator
moving in response to the right column movement). The opposing control column inputs
likely existed during the 7 to 8 seconds before the elevator split (when both elevators were
moving in a trailing-edge-up direction); however, the elevator split would not occur until
the difference between the two control column forces was great enough to engage the
override mechanism. Tests conducted in a 767 simulator and airplane (on the ground)
demonstrated that pilots with heights and weights similar to those of the command captain
and relief first officer could apply enough force on the control column to produce and
maintain the split elevator condition recorded by the FDR.
The captain’s actions just after the elevator split began were consistent with an
attempt to recover the airplane and the relief first officer’s were not. In rapid sequence,
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During the Safety Board’s tests and simulations, a pilot similar in height and weight to the EgyptAir
flight 990 command captain was physically able to move from the aft cockpit into the captain’s seat, to brace
himself against the control console or floor structure, and to apply enough back pressure on the control
column to match the physical pulling forces computed to have been required to generate the split elevator
condition recorded by the FDR. However, the pilot stated that it was physically difficult or uncomfortable
for him to manipulate the control column while kneeling on the floor or standing behind the captain’s seat
and suggested that, given his build and his need to manipulate the controls, the captain of EgyptAir
flight 990 would almost certainly have attempted to enter his seat immediately upon his return to the
cockpit.
As previously discussed, the simulator did not duplicate the accident airplane’s actual flight
conditions in every way; for example, the simulator did not duplicate the negative G loads recorded by the
FDR. However, once the captain was normally seated and effectively braced, these forces should not have
substantially affected the maximum fore-and-aft forces he could generate. Further, the G loads on the
accident airplane did not remain negative for long; FDR data show that the G loads increased to greater than
1/2 G within 2 to 3 seconds of the start of the recovery.
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For additional information, see Boeing’s April 16, 2001, letter in the public docket for this accident.
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For additional information, see Boeing’s April 12, 2001, letter in the public docket for this accident.
NTSB/AAB-02/01
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