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Standard Operating Procedures that are not in keeping with the benefits derived from
system improvements and advancements, such as FBW flight control systems.
VI. Conclusions/Recommendations
From the data gathered in the evaluation, there was not a distinct advantage of the B777 soft
limits vs the A320/330 hard limits for CFIT recovery open loop performance.
However, closed loop evaluations showed that the pilots could achieve more consistent
performance results as well as achieve target pull out parameters more quickly in the
A320/330 than the B777. Even with the B777 soft limit features, pilots were able to use
abrupt pitch inputs without fear of overstress or stall. Both aircraft types offered better
handling during CFIT recoveries than conventional aircraft since the FBW design features
allowed the pilot more precise control of pitch rate and g onset rate than with conventional
flight controls.
RECOMMENDATION: Operators of fleets with a mix of conventional and FBW aircraft
should reevaluate the benefits of a fleet standard CFIT recovery procedure vs. a FBW aircraft
specific procedure that would provide such aircraft with better performance.
The A320/330 full aft stick CFIT recovery vs 3 deg/sec pull gave better and more consistent
performance without any increase in risk of exceeding envelope parameters. No additional or
specific pilot training was necessary to perform the full aft stick recovery technique since the
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FBW design provides excellent pitch rate and g control as well as excellent envelope protection
for stall, overstress, or overspeed.
RECOMMENDATION: A320/A330/A340 operators should use the manufacturers
recommended full aft stick CFIT recovery procedure.
The evaluation pilotsÕ found that the enhanced flight path control precision and envelope
protection features available through FBW design were highly desirable.
RECOMMENDATION: Incorporation of similar FBW design features is highly desirable in
future designs.
The evaluation team preferred the flight envelope limiting features (Òsoft limitsÓ) of the B777
design to a Òhard limitÓ design. This was a subjective judgement based on the premise that
there may be situations unforeseen by the designers where the pilot might need to achieve full
aerodynamic capability as opposed to being software/control law limited.
Another approach may be to incorporate Òhard limitsÓ with a pilot override capability such as
an Òinstinctive cut-outÓ switch. Or alternately, the CFIT recovery capability on the 777 could
be enhanced if the aircraftÕs Primary Flight Computers (PFC) were design to recognize
aggressive pilot inputs as a desire for maximum aircraft performance. The PFCs would then
provide maximum pitch rate consistent with AOA or g limits (depending on airspeed). If the
resultant aircraft performance was not sufficient, the pilot could then pull to the full
aerodynamic capability of the aircraft. Additionally, automatic speed brake retraction, in the
event of a go around or CFIT escape maneuver, should be provided in the 777 design. This
system although somewhat complex mechanically, can be implemented since the PFCs will
control any undesired pitch excursions.
RECOMMENDATION: Further research and development should be conducted to optimize
flight envelope protection control laws and design features with emphasis on providing pilot
override authority.
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PILOT AUTHORITY AND AIRCRAFT PROTECTIONS(24)
