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a design which has demonstrated a propensity for APC and subsequent need for the pilot
to release the controls is a non-airworthy design for transport category aircraft.
The divergent motions of the A300 (with the most sensitive rudder control system of any
transport category aircraft) unexpectedly and unpredictably reacted to the environment
and to the pilot attempts to control it. These subsequent lateral accelerations occurred in
the brief period after the aircraft tangentially traversed the counter-rotating wake field
during the final seconds of the AA 587 flight. The forces imposed on the airplane by the
rudder doublets, commanded in response to the unwanted motion, exceeded the vertical
stabilizer structural limitations. The total system failure began with a faulty flight control
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design and culminated with a pilot struggling to control an aircraft that only moments
before had been a docile, stabilized platform. The aerodynamic loads built in excess of
ultimate load; separation of the vertical stabilizer occurred only 6.5 seconds after the
aircraft entered the vortices of the 747 (Official Docket Aircraft Performance Report 33).
One of the findings of the NRC study states:
“Adverse APC events are rare, unintentioned, and unexpected oscillations or divergences
of the pilot-aircraft system. APC events are fundamentally interactive and occur during
highly demanding tasks when environmental, pilot, or aircraft dynamic changes create or
trigger mismatches between actual and expected aircraft responses” (33).
National Research Council
In 1997, at the time the NRC published its report, they had identified ten possible pilotinvolved
oscillatory APC events in Airbus aircraft. The manufacturer acknowledged only
three as genuine—but even one is an acknowledgment of an unsatisfactory aircraft design
and a flawed certification system.
In addition to the reluctance of the manufacturer to admit or address latent deficiencies in
the A300 control design, analysis is further complicated because APC events have been
difficult for line pilots to detect and report. Test pilots are trained to recognize and
analyze APC anomalies; commercial line pilots are not. Further exacerbating discovery
of this latent hazard is the low fidelity of Digital Flight Data Recorders (DFDRs). The
low sampling rate of current DFDRs does not facilitate post-event analysis of high
frequency oscillatory APC events. The sampling rates of flight control parameters are
typically one to two hertz. These rates are designed to best record human versus
mechanical discrepancies. A much higher rate and unfiltered data is required to
accurately assess flight control motion. At the NTSB Public Hearing for Flight 587,
Investigator-In-Charge (IIC) Robert Benzon stated:
“In 1994, the Safety Board recommended to the FAA that such filtering be removed from
information sent to the flight recorders. And yet in 2001, this investigation was hampered
by totally unacceptable filtering of the data. In addition, the sampling rates of such data
are simply not adequate” (31).
Airbus clearly subscribes to the use of enhancements offered by the digital age with
modifications to the flight control design of their aircraft, including the A300-600. Flight
augmentation computers, flight envelope protection and even limited Fly-by-Wire (FBW)
technologies were modified into this derivative model in 1986. Verification through flight
testing of these new technologies is appropriate with respect to flight control changes.
Testing should be accomplished through proven evaluation methods, such as the FAA’s
Handling Qualities Rating Method (HQRM).
The Pilot In Command (PIC), assisted by the designated Pilot Flying (PF) were both
type-rated in the A300/310 aircraft. Type-rating training is developed in accordance with
FAR 121.401 using the aircraft information provided by the manufacturer. Pilots must
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demonstrate competent knowledge of the aircraft as well as complete an extensive
flight/simulator-based training course. Both pilots had completed this process; however,
no line pilot possessed critical knowledge of the limitations in the A300 design. The
critical peculiarities of the flight control design found in this investigation were not
shared with pilots or with American Airlines. Further confounding the understanding of
rudder system limitations was the Flight Manual “L/G [Landing Gear] Unsafe Indication”
procedure which dictated alternating sideslips. The overall sensitivity of the rudder
system was not addressed nor was a prohibition on rudder reversals placed in the FCOM
or flight manual. The manufacturer’s decision to omit information from the FCOM links
　
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