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a failure to recognize a latent hazard (sensitive rudder) with a supervisory failure to
mitigate future risk. With this higher-risk design, it was only a matter of time before the
necessary preconditions were met for the airplane to experience a non-recoverable
adverse APC event.
A training program could not have been developed because the manufacturer offered no
information about the limitations or peculiarities of the rudder system. Neither pilot was
trained to experience this unexpected and unpredictable flight characteristic nor the
aircraft gyrations experienced in the accident sequence. It was far beyond the pilots’
practical experience, surprising the PF and leaving the PIC unaware of the peril they
faced as the PF struggled to maintain control of the aircraft.
The pilots experienced “Surprise and Startle” factor, a known Human Performance
reaction, when the aircraft entered the wake field, and the resultant unanticipated motion
of the airplane began. The airplane reaction to the counter-rotating wake vortices first
tended to overbank AA 587 from a steady-state left turn. The PF attempted to counter the
undesired airplane motions. At this point in the mishap sequence, the reaction of the
airplane departed from the rational expectatio ns or practical experiences of either pilot.
The PF’s inability to precisely make an input to the rudder system to stop the oscillatory
lateral motions of AA 587 forced the aircraft to yaw excessively which created
aerodynamic side loads on the airplane structure. The oversensitive rudder, needing only
22 pounds of force to initiate, with an extremely short stroke of only 1.2 inches of travel
to command full rudder, was a mere flex of the PF’s foot. The tactile feel and travel
distance of the aircraft’s rudder pedal was out of balance with line pilot expectations of
the rudder control system.
“Artificial trim and feel systems which produce controllers with too small a displacement
and light force gradients may also lead to severe overcontrol.”
FAA Advisory Circular 25-7A
Attempts to counter or dampen this motion, an aerodynamic phenomenon known for
decades and addressed by yaw damper systems, were now being exacerbated again by the
system design. The A300 yaw damper actuator has a maximum deflection authority of ±
10°, with a maximum rate of ± 39° per second. The rudder actuator has a maximum rate
of 60° per second, ± 5°. Because the rudder authority is significantly higher than that of
the yaw damper, it can suppress and/or override the yaw damper which negates one of the
yaw damper’s primary functions.
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No evidence exists to suggest that the actions by the PF were anything other than a
judicious response to correct an unwanted flight attitude. However, this unexpected
condition degraded as the PF struggled to maintain control of the aircraft. Responding
quickly to the three-dimensional motions caused by the wake of a preceding super- heavy
airplane, the PF used as much authority of the FCS as his years of experience handling
aircraft had taught him to use. He reacted to maintain airplane control, up to and
including, the full command authority of his FCS. Commercial line pilots operate
airplanes in this manner: using performance feedback to control their input rather than
physically looking at a control and cognitively applying a fixed distance stroke. The
pilots were unaware of the latent deficiencies in the modified rudder FCS:
· The extremely light rudder pedal force
· Shorter rudder pedal travel as speed increases
· Excessive degrees of rudder travel per pound of force
· The absence of limiting or protecting systems
The manufacturer failed to warn pilots of these design characteristics and the need to
avoid cyclical motion of the rudder controls. The PF was functioning within the
guidelines written in various procedures, policies, and practices as he struggled to
maintain airplane control. In doing so, he coupled with a hidden flaw in the rudder design
and exceeded an unstated limitation.
Safety Recommendation 5
The NTSB should recommend installation of unfiltered, high sample
rate DFDRs in all transport category aircraft. Further, the NTSB
should recommend that a detailed analysis of operational data from
those aircraft with reports of unexpected performance variances be
made available as part of Service Difficulty Reports (SDRs) required
of aircraft manufacturers.
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Safety Recommendation 6
Safety Recommendation 7
The NTSB should recommend that the French DGAC require Airbus
to develop formal training for all operators of the A310 and A300-600
　
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