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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 FBW design provides excellent pitch rate and g control as
well as excellent envelope protection for stall, overstress, or overspeed. From the data
gathered in the evaluation, there was not a distinct advantage of the B777 soft limits vs
the A330 hard limits for CFIT recovery open loop performance.
· Closed loop performance showed that the pilots could achieve more consistent
performance results as well as achieve target pull out parameters more quickly in the
A330 than the B777.
· Flight test results indicated that an aggressive pull up in the B777-300 to a pitch
attitude of 17.5 degrees generally yielded better CFIT recovery performance than the
recommended 3 deg/sec recovery procedure.
· The evaluation pilotsÕ found that the enhanced flight path control precision and
envelope protection features available through FBW design were highly desirable.
· 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.
6.4.6 Recommendations
· Airbus FBW operators should use the manufacturers recommended full aft stick CFIT
recovery procedure. This may seem obvious, but until this report, none of the US
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operators were following the Airbus recommended procedure. None felt that it was
prudent to do so.
· The manufacturerÕs B777 CFIT recovery procedure should be utilized.
· Future FBW designs should consider
¨ Protected flight envelope limits with
¨ Envelope protections over-ride
· Incorporation of FBW design features is highly desirable in future designs.
· Further research and development should be conducted to optimize flight envelope
protection control laws and design features with emphasis on providing pilot override
authority.
6.5 High Speed Upset Recovery
6.5.1 Aircraft With Conventional Flight Controls
For an aircraft with conventional flight controls, pilot authority again is unlimited in the
traditional sense, but aircraft protection is also minimal. There is a ramp-up of flight control
forces, but no indication when a limit is reached or exceeded. In this case, the pilot is usually
given the authority to demand the full aerodynamic capability of the aircraft (limited only by
the feel system). However, since commercial airliners usually do not have ÒgÓ meters installed,
the pilot does not have the ability to maneuver the aircraft with the highest degree of safety.
Concern for over stressing the aircraft can actually cause a delay in recovery or cause a suboptimal
recovery to be flown.
Recovery from a high speed upset is straight forward, but dangerous. If flight controls are
abruptly applied, the aircraft can be readily overstressed, with possible disastrous results.
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FIGURE 6.1
Figure 6.1 shows a simulator evaluation of a rather severe upset in a B727. The recovery g
level peaked at around 3.5 to 4 gs. The altitude lost during the recovery was over 5,000 feet,
with a recovery at less than 5,000 feet AGL. If the g available had been limited to 2.5 gs, the
altitude loss during the recovery would have been significantly greater and the possibility of
terrain contact would have been significantly increased.
6.5.2 FBW Aircraft With ÒHardÓ Protection Features
There have been accidents and incidents where more than the design limit of 2.5 ÒgsÓ was
needed or used to effect aircraft recovery, from an upset. 14
There are portions of the flight envelope where the A-320 is aerodynamically capable of
pulling more than 2.5 ÒgsÓ. Flight computer software prevents the aircraft from over g by
limiting flight control deflections even when commanded by the pilot (protection limit). The
portions of the A-320 flight envelope where more than 2.5 g is aerodynamically possible are
shown in figure 6.2.
14 China Airlines, B747-SP, 2/19/1985, 300 miles NW of San Francisco.
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FIGURE 6.2
The ÒgÓ limit of an aircraft is typically determined for the maximum takeoff weight, flaps up.
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PILOT AUTHORITY AND AIRCRAFT PROTECTIONS(12)
