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not be over-ridden, the warning system is now in command of the aircraft. A false stall
warning could result in the pilot not being able to select a slow enough speed to effect a safe
landing.
6.2.4 Flight Envelope Protection Overrides
Even with the soft protection features on the FBW Boeing 777, the Primary Flight Computers
can be shut off by the activation of a guarded switch on the overhead panel. This places the
flight control system in a direct inceptor to control surface activation mode. There is no
similar provision in the Airbus FBW design to readily place the aircraft in the direct mode.
The soft flight control limits in the Boeing FBW design are implemented in part by increased
control forces on the yoke. This design philosophy allows both protection and ease of
override when necessary. Some unpublished Boeing and Calspan research suggests that force
limit, or the ramp-up of force gradients, do not work well on small displacement controllers to
implement a soft protection scheme. Airbus may have had no choice but to go with hard
limits on their side stick design.
6.2.5 Flight Envelope Authority
In addition to the design questions, there is also a philosophical question that needs attention.
That question regards FAR 91.3, which states, ÒThe pilot in command of an aircraft is directly
responsible for, and is the final authority as to, the operation of that aircraft.Ó As Dr. Billings
states in his book, ÒDoes a pilot whose control authority is limited by software encoded in the
flight control computer have full authority to do whatever is necessary to avoid an imminent
collision, or ground contact? An editorial in Flight International (ÒHard limits, soft options,Ó
1990) states:
ÒThere is, however, another approach available: to develop a ÒsofterÓ fly-bywire
system which allows the aircraft to go to higher limits than before but with
a progressive degradation of flying qualities as those higher limits are
approached. It is this latter philosophy which was adopted by the Soviets with
fighters like the MiG-29 and Sukhoi Su-27. It is not, as MikoyanÕs chief test
8 Personal conversation, John Cashman, Director, Boeing Flight Test,
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pilotÉadmits, Ónecessarily a philosophy which an air force will prefer,
however: Although thisÉ.approach requires greater effortsÉit guarantees a
significant increase in the overall quality of the aircraft-pilot combination. This
method also allows a pilot to use his intellect and initiative to their fullest
extent.Ó
The American counterpart, the F-16, was designed with ÒhardÓ limits. The original General
Dynamics flight envelope algorithms did not predict a deep stall mode later encountered in
flight. Therefore the design of the FBW flight control system did not allow for sufficient flight
control authority necessary for pilot recovery from the deep stall mode. This necessitated the
last pilot option - ejection from the aircraft. Subsequently a system for manual over-ride of the
flight control computer has been incorporated.
Fighter aircraft are designed with flight control systems requiring near linear inputs yielding
controlled and predictable flight characteristics up to the edge of their design envelope. Fuel
considerations, freight and optimal passenger load considerations often dictate that commercial
transport aircraft likewise operate near their design limits. In commercial transport aviation,
the edge of the envelope is also often approached during high altitude flight where the spread
between low speed and high-speed buffet is at a minimum. In the early days of aviation, this
was referred to as the Òcoffin corner.Ó
As Dr. Billings states in his book: ÒAlthough civil aircraft do not face the threat posed to a
fighter aircraft under attack, if its maneuverability is limited, their pilots do on occasion have
to take violent or corrective action, and they may on rare occasions need control or power
authority up to (or even beyond) normal structural and engine limits to cope with very serious
problems. 9Ò The issue is whether the pilot, who is legally, morally and ultimately
responsible for safe mission completion should be permitted to operate up to or even beyond
airplane limits, when he or she determines that a dire emergency requires such operation.
To illustrate the interrelation of protection features and pilot authority, two maneuvers critical
to the safety of flight, where the differences between a conventional and a FBW aircraft design
protection features are the most pronounced, will be discussed. The first maneuver is the
Controlled Flight into Terrain or CFIT avoidance maneuver. The second is the high speed
　
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