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if the pilot required max
AOA. He need only hold the sidestick
firmly against the aft stop and
let the computer do the work.
In a steep downward pitch, the
7X would allow only –25°, the
negative pitch limit, no matter how
hard I pushed.
According to Deleume, Dassault
had chosen not to set a limit in
bank angle. I rolled the 7X past 90°
to sample this, enjoying the
40°/second available rate. I then
rolled to about 80° of bank and
pulled the sidestick rapidly back to
the stop. The G indication displayed
on the HUD showed slightly
over 3 G. Falcon 7X FCs will allow
3.0–3.5 G with full aft sidestick.
Like the pitch and AOA limits, this
is a hard limit.
Hard limits are not universal in
civil FBW aircraft. Boeing chose
soft limits for the 777, which pilots
can exceed with more effort.
Boeing argues that the pilot should
always be in control in case of situations
where, for example, he/she
needs to exceed the design load
limit in a pull-up to avoid hitting a
mountain.
In emergency situations, however,
there are advantages to hard
limits. In 1999 members of an
ALPA committee studied CFIT
avoidance maneuvers using actual
aircraft—a Boeing 777 (soft protections)
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
96 PROFESSIONAL PILOT / September 2004
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.
limits had allowed them to overshoot
optimum AOA, resulting in a
“mush” effect and greater altitude
loss. In high-speed pull-ups 777
pilots had to respect G limits, while
in the A330 they merely pulled the
sidestick to the aft stop and let the
computer give them 2.5 G.
The ALPA committee approved of
what it called “the carefree handling
afforded by hard limits” but
added, “We would like the pilot to
retain the authority to override limits
and protections.”
Unfortunately, that isn’t possible—
it’s simply an either/or choice
between hard and soft limits.
The 3.0–3.5 G limit of the 7X is a
significantly harder pull-up than
Airbus control laws allow (2.5 G).
While 3.5 G is a stroll in the park
to a fighter pilot or an aerobatic
competitor, a typical business jet
pilot will have had zero experience
pulling Gs and would probably not
obtain that load in an emergency
unless he panicked (and thus risked
pulling the wings off).
In a roll, Airbus FBW aircraft are
hard limited to 67° bank. The 7X
has no bank limit, so in terms of
maneuverability its hard limits are
quite a different proposition—the
Falcon 7X has 90° of roll in just
over 2 seconds and 3.5 G available,
giving it high maneuverability
and hard limit protection at the
same time.
The throttles for the 7X move with
thrust changes, whether the autothrottle
is on or off. Boeing too
backdrives the 777’s throttles with
its autothrottle engaged, so the pilot
can always see or feel thrust selection
in moving the throttles. On the
other hand, even in the A380, Airbus
continues with throttles that
remain stationary (in the climb
detent) with the autothrottle operating.
ALPA and IFALPA have criticized
this system.
Redundancy means safety
Dassault designed the 7X with 3
main flight computers (MFCs), each
of which is dual-channel (A and B),
and 3 secondary flight computers
(SFCs), each with one channel (C).
It has programmed channels A, B
and C independently to guard
against common faults.
In normal operation MFC number
1 is flying the airplane. Its channel
A generates the control commands,
while its channel B performs parallel
calculations. Comparator software
checks A against B outputs to
be sure they agree within tolerance.
If MFC 1 fails, loses power or
declares itself faulty, MFC 2 will fly.
If MFC 2 subsequently fails, MFC 3
flies. If all 3 MFCs fail, the SFCs fly
the airplane. SFCs are single-channel
but they cross-monitor each
other. If one SFC fails the other 2
continue—1 flying, 1 monitoring. If
a second SFC fails, the lone
remaining SFC will not fly because
it has no monitor.
　
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