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In emergency situations, howev-er, there are advantages to hard limits. In 1999 members of an ALPA committee studied CFIT avoidance maneuvers using actual aircraft—a Boeing 777 (soft protec-tions) and an Airbus A330 (hard protections). In a pull-up from slow speed the A330’s measured loss of altitude was less than the 777’s. The test pilots felt that the 777’s soft
Flightpath stability
In normal flight the pilot does not trim—thus there is no trim switch on the sidestick. Instead, switches on the center pedestal can be used during manual reversion or if the FC defaults to direct law. In normal control law the 7X is a flightpath stable airplane (C* in engineer par-lance) and trim is fully automatic. With no control deflection the FC will maintain a 1G constant flight-path. Sidestick inputs cause a G/rate response. Pull or push the sidestick and you get a G load pro-portional to your deflection at high-er speeds, or a pitch rate propor-tional to deflection at slower speeds. Left/right sidestick gets you a roll rate proportional to deflec-tion. Maximum sidestick deflection on the 7X gives an impressive roll rate of 40° per second, compared to the A320’s 15°/sec max.
Kinesthetic cues are, naturally, missing in the fixed simulator, but rates of pitch and roll response and control stick forces seemed just right. This being the world of FBW, stability was perfect. Stability and maneuverability are not mutually exclusive—you can have both in whatever combination you want. Just write the laws that way.
Computer-generated view of Falcon 7X’s EASy cockpit. Left seat position shows sidestick, four 14-in flat panels, pitch trim switches in center console and pull-out tray.
Like Airbus, Dassault Falcon chose C* control laws, which give flightpath stability but apparent neutral speed stability. For exam-ple, let’s say you are flying straight and level at a steady 250 knots. Take your hand off the sidestick and pull the throttles to idle. In a traditional aircraft the nose will fall as the speed bleeds off and the air-craft seeks its trimmed airspeed—a speed stable response. In an FBW aircraft, such as the A319CJ or 7X, the flightpath remains constant (level in this case) as speed bleeds off and the FC raises the nose to maintain the flightpath constant—a flightpath stable response. The FC automatically trims the stabilizer.
FAR25.173 discusses the require-ment for static longitudinal stability (speed stability.) Certification of FBW aircraft falls under special conditions of a different section of the FAR. The speed stability requirement is in effect waived, in part due to the protections FBW provides which compensate for it.
Not everyone has chosen this approach. Boeing made the 777 speed stable and therefore not flightpath stable. The 777 control laws are termed C*U (U represent-ing speed.) Which is better is essen-tially a matter of pilot preference.
96 PROFESSIONAL PILOT / September 2004
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本文链接地址：Introducing full FBW for business aviation 公务航空电传系统(4)
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