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Operating Manual (FCOM) revisions, or Flight Manual limitation revisions.
Investigators also learned in the discovery process that, in an in-service event in 1997,
another AA Airbus 300-600 vertic al stabilizer had exceeded ultimate load. They
questioned why multiple overloads of the tail fin—a primary aircraft structure—occurred
without warning. To investigators, there appeared to be a certification “loophole” since
aircraft certificating authorities left these in-service events unaddressed. The inaction on
the part of either the manufacturer or the certificating authority violates U.S. transport
certification criterion as neither party acted to ensure the aircraft design met industry
standards. The breakdown in the integrity of the system that governs transport category
aircraft shocked the aviation industry as the investigation progressed, and the flight
community learned that critical operating limitations of the A300 had not been revealed
for over a decade. Investigators uncovered these startling facts months after the 587
accident while the flight crew had less than 6.5 seconds to analyze and react to the
aircraft as it became unrecoverable.
At the public hearing, the manufacturer testified that, unlike aircraft manufactured in the
United States, the A300-600 aircraft had not undergone formal testing for aircraft
handling characteristics. Aircraft manufactured in the U.S. must undergo testing that
evaluates aircraft handling characteristics in both benign and adverse flight conditions.
U.S. aircraft manufacturers employ a standard methodology such as those defined in
Advisory Circular (AC) 25-7A. Airbus used an internal rating system that they felt met
the requirements of FAR 25 but did not apply their internal rating system to gauge the
handling characteristics of the A300-600 in a wake vortices environment.
Facts uncovered during this investigation also highlighted a flaw which is inherently built
into the A300B4-605R rudder design. Airbus’ predecessor aircraft, the A300B2/B4
variant, is equipped with a rudder in which the pilot would exert control forces similar to
those found in other transport category aircraft. However, the modified A300B4-605R
exhibits an oversensitive rudder; it is 7.32 times more sensitive than the similarly- sized
Boeing 767 rudder control system. The FCOM provided by Airbus fails to notate the
extreme rudder control sensitivity difference, fails to outline restrictions placed upon
rudder usage, and fails to reveal how Airbus intended the pilot to use this system.
1 FAR Part 25.301 states, “(a) Strength requirements are specified in terms of limit loads
(the maximum loads to be expected in service) and ultimate loads (limit load multiplied
by prescribed factors of safety).”
3
A corollary effect of the A300B4-605R rudder pedal sensitivity is the propensity for a
pilot to become adversely coupled with the aircraft. This anomaly is known as adverse
Aircraft Pilot Coupling (APC) and is usually the result of a deficient flight control design.
An APC event causes a pilot’s rudder inputs to be out of sync with the motion of the
aircraft. In the case of Flight 587, the Pilot Flying—uninformed and unaware of the
hypersensitive rudder pedals—made appropriate and controlled rudder inputs in response
to the aircraft’s motion as it encountered wake vortices. An unintentional result was that
excessively high aerodynamic loads were placed on the vertical stabilizer which then
broke off the aircraft only 9 seconds after the first rudder pedal input.
A. PROBABLE CAUSE AND CONTRIBUTING FACTORS
The probable cause of this accident was an Aircraft Pilot Coupling (APC) event. This
APC occurrence was the result of a flawed design modification to the A300.
Additionally, the modification was not tested by an accepted Handling Qualities Rating
Method (HQRM). The APC event led to the development of excessive aerodynamic
loads and consequent structural failure of the vertical stabilizer in only 6.5 seconds.
Airbus was forewarned of this catastrophe by preceding in-service events and failed to
caution operators and regulators of this tendency.
The contributing factors of the accident are outlined below:
a. Airbus failed to identify the dramatic changes in a rudder control design that
radically deviated from other aircraft designs.
b. Airbus failed to use an objective standard for rating the aircraft handling
characteristics of the A300B4-600R flight control design, such as the FAA
Handling Quality Rating Method (HQRM), or the Cooper-Harper Pilot
Rating.
c. Airbus failed to publish limitations on the aircraft’s rudder design.
d. Airbus failed to properly educate operators about rudder system limitations.
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