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routine or normal maximum available thrust may be set to a particular predetermined lower
value for operational considerations, a method to over-ride this restriction and obtain
maximum thrust available, consistent with aerodynamic and controllability issues, must be
provided.
6.0 FLIGHT ENVELOPE PROTECTIONS
The design goal of flight envelope protections is to protect the pilot and the aircraft from
exceeding the structural or aerodynamic flight envelope. The protection features afforded by
FBW designs allow the pilot an enhanced ability to aggressively maneuver the aircraft, without
fear of exceeding the flight envelope. Additionally, performance margins with respect to gross
weight, c.g., and wing configuration can be maintained. This ability to safely and aggressively
maneuver the aircraft enhances pilot capability to control the flight path and performance of
the aircraft.
It is assumed that the primary reason for a flight envelope exceedance is human error. As Dr.
Billings states in his book, Aviation Automation, it is a myth to think that you can design out
human error 6 . [Myth 2: Technology can help us supplant the unreliable human.]. You may
attempt to design out pilot error, but you may inadvertently replace it with designer error.
6 Billings, Aviation Automation. Page 53
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6.1 Design Error
Design error can become a factor when the pilot is limited to the designerÕs perception of the
safe flight envelope. Preventing the pilot from exercising ultimate control supposes that all
possible contingencies in all possible environments have been correctly and adequately
addressed. The danger of this philosophy was recently demonstrated when all cockpit
displays were temporarily lost during an aircraft upset In designing the A300, Airbus
developed a complex roll rate-of-change monitor that triggers the reset and self-test function if
that rate of change exceeds 40 degrees/sec. The design assumption was that no A300 in normal
or emergency operations would exceed that trigger rate.7
6.2 Flight Envelope Protection Design Philosophies
Currently, there are three types of flight control systems used in commercial transport
aircraft. The first and most prevalent is the conventional hydraulic/mechanical flight control
system. The next two are both fly-by-wire (FBW) flight control systems, each incorporating
a different design philosophy to limit the flight envelops.
6.2.1 Conventional Flight Envelope Protections
Aircraft with conventional flight control systems use a number of aural, visual, and tactile
systems to warn the pilot of actual or pending flight envelope exceedances. Except for the
case of some stick pusher designs, these systems can be ignored or over-ridden by the pilot.
This design is perceived as offering the pilot the ultimate in control, although aircraft
protection features are at a minimum.
Aircraft in the past have been designed with a set of ÒnormalÓ limits which have a buffer to the
Ònever exceed limitsÓ to allow for the inevitable lack of precision which can cause an overshoot
of the normal limits.
6.2.2 Hard Flight Envelope Protections
Airbus incorporates ÒhardÓ limits in the design of their FBW flight control system. Hard
limits prevent the pilot from exceeding the flight envelope of the aircraft. That is, the aircraft
is not allowed to be stalled, over-banked, over-stressed, or over-sped. In other words, the
designed aircraft envelope is maintained and protected.
6.2.3 Soft Flight Envelope Protections
Boeing incorporates ÒsoftÓ limits in the design of their FBW flight control system. Soft limits
ÒsuggestÓ and warn when a limit is being approached by increased control feel (stick forces)
7 N TSB Warns of Display Reset Proble m , Aviation Week and Space Technology, Feb. 9, p.76
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and by introducing aural and visual warnings. With soft limits the pilot is warned, but then
allowed to stall, over-bank, over-stress or over-speed the aircraft, if necessary or desired.
Sensor malfunctions may make a case for soft limits or at least pilot overrides. On the surface
it would seem that preventing the pilot from stalling the aircraft, is a desirable function of a
protection system. However, there have been cases of multiple sensor malfunctions
(lightening strikes welding both AOA vanes8), where incorrect information was passed to the
aircraft fight computer. These malfunctions have resulted in false stall warnings. If the pilot
in command is unable to over-ride a malfunctioning system, in this case a stall warning system
(it could be any system however), aircraft control may be lost. When a warning system can
　
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