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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.
For this 2.5 ÒgÓ limit, there is a FAR requirement for at least 50% more ÒgÓ (3.8 g) available
before structural failure. 15 The fuel load and distribution can typically add to the structural
integrity an aircraft. The regulations require that an aircraft must have the 2.5 g and 50%
safety factor (3.8 g capability) for all fuel loads; from the lightest to the heaviest.
The Wequivalent (Weq) of the A-320 at a takeoff gross of 169,750 pounds pulling 2.5 ÒgsÓ is
424,375 pounds. Adding the 50% yields a Weq of 636,562 pounds.
Weq = nW
As weight is reduced, at the end of the flight, the ÒgÓ necessary to generate the same Weq
structural load is higher, and can be solved by rearranging the above equation.
n = Weq/W
For a descent weight of 130,000 pounds, ÒnÓ now becomes 4.9 Ògs.Ó Although this rather
basic analysis would appear to yield a higher g capability, there is unfortunately incomplete
engineering in this regard. The only required capability of the aircraft for any fuel load is the
3.8 g requirement. The 4.9 g capability is not currently supported by engineering analysis.16
15 FAR 25.303
16 Personal conversation Alain Garcia, Senior Vice-President Engineering Airbus Industrie, April 1998.
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Although aircraft structural integrity may be compromised by a g load in excess of 3.8 gs,
aircraft structural integrity is more severely compromised by terrain impact
Depending on dive angle, this increased ÒgÓ capability can result in an altitude loss of almost _
of the 2.5 ÒgÓ case. A 4.9 ÒgÓ recovery results in a significantly less altitude loss than a 2.5 ÒgÓ
recovery.
It is conceivable that a fully intact aircraft after an upset could impact the ground pulling 2.5
ÒgsÓ, where a little more ÒgÓ would have resulted in a successful recovery The pilot in
command needs to be given the choice between ground contact or the possibility of an overstressed
aircraft. This can be accomplished while retaining the ÒgÓ protection in the Airbus
FBW design, with the addition of an over-ride button to ÒdropÓ the protections, if the pilot in
commands deems such emergency action is necessary.
6.5.3 FBW Aircraft With ÒSoftÓ Protection Features
With this design, the pilot is granted full authority limited only by the constraints of the
aircraftÕs aerodynamic capability and the feel system. If ground contact becomes an overriding
safety consideration, the pilot is allowed to intentionally exceed the design limits.
In the B-777, the pilot is not warned when design limit of 2.5 ÒgsÓ is reached. This warning
could be implemented through the aircraftÕs feel system, or by the use of audio warnings.
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6.6 Flight Envelope Protection Conclusions
Both the hard and soft FBW flight control systems afford a higher degree of maneuverability
and performance over conventional hydromechanical systems. The soft flight control system
affords higher pitch rates and attainable g, and this increased capability results in shorter
exposure times below the entry altitude, but not necessarily less altitude lost during the
maneuver.
The most effective flight control system would be one that combines the best features
of both the current hard and soft flight control system designs. This desired flight
control system would then be one that allows the pilot to easily attain maximum
allowable aircraft performance (as with the design of current hard flight control
systems). However, if the pilot desired increased performance, the hard limits could be
over-ridden and full aerodynamic performance could be attained (as is the capability
with current design of soft flight control systems). In addition, a g limiting system
could be designed that takes into account current weight, Mach, airspeed, and CG, and
varies the hard limit accordingly.
7.0 FUTURE TRENDS
Airbus is proposing a 600-800 passenger aircraft, the A3XX. One of the possible additions to
the protection scheme is an automatic recovery in case of a GPWS terrain warning. This is a
　
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