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pilot priority over the other if both
are trying to fly at the same time.
Lights on the instrument panel illuminate
to indicate who has control.
This switch can also be used to
negate a malfunctioning sidestick.
Flight operations quality assurance
(FOQA) sprang up at the airlines
shortly after arrival of the sidestick-
equipped Airbus A320. One
of the first problems FOQA identified
was dual inputs—as when a
captain assists the FO with his
landing without telling him, which
can lead to significant confusion in
the cockpit. As a result, Airbus
added a voice annunciation of
“Dual input!” to the A320 fleet. In
the Falcon 7X an eccentric cam
inside the sidestick vibrates both
sidesticks when 2 pilots are on the
controls together. This tends to get
pilots’ attention more readily than
voice annunciation.
PROFESSIONAL PILOT / September 2004 95
Falcon Test Pilot Philippe Deleume in the right
seat of the 7X simulator at this year’s EBACE.
The Falcon 7X simulator at Saint-Cloud is linked to the “global test bench,” where a full complement
of electrical and hydraulic systems and flight computers will “fly” together as in the aircraft.
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 parlance)
and trim is fully automatic.
With no control deflection the FC
will maintain a 1G constant flightpath.
Sidestick inputs cause a
G/rate response. Pull or push the
sidestick and you get a G load proportional
to your deflection at higher
speeds, or a pitch rate proportional
to deflection at slower
speeds. Left/right sidestick gets you
a roll rate proportional to deflection.
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.
Like Airbus, Dassault Falcon
chose C* control laws, which give
flightpath stability but apparent
neutral speed stability. For example,
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 aircraft
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 requirement
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 representing
speed.) Which is better is essentially
a matter of pilot preference.
Hard vs soft limits
In the 7X simulator, Deleume let
me sample the protections programmed
into the 7X laws. From
level flight I pulled the nose up
sharply—it would go no steeper
than +35° up, the positive pitch
limit. This is a “hard limit” that the
pilot cannot exceed no matter what
he/she does. I held the nose there
while the speed bled off, keeping
the sidestick full aft against the
stop. When the angle of attack
(AOA) reached its maximum limit
(something just safely below
stalling AOA), the nose began to
lower smoothly in spite of the aft
stick, keeping the AOA at this maximum
safe value. This protection, a
hard AOA limit, would work well
in a windshear or CFIT recovery situation
　
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