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magenta on the FMA to indicate the presence of a
constraint. As soon as the constrained waypoint is
passed, the target altitude becomes the FCU
altitude, unless another constraint is defined.
Managed Descent
The managed descent should be used only
when the aircraft is at cruise altitude. It can be
engaged only if the NAV mode is activated
(managed lateral mode). To engage the
managed descent, an altitude lower than the
current altitude must be selected on the FCU,
and the ALT knob must be pushed.
When the managed descent is initiated, the
aircraft will try to descend using idle thrust as
long as possible to save fuel. It will also respect
the constraints, especially the speed limitation
below the limitation altitude (usually, the speed
limit is 250 kt below 10,000 ft). In fact, the FMGC
computes a ideal descent path it will do
everything possible to maintain the aircraft on
this path.
As soon as you initiate the descent, the FMA
looks like this :
The thrust is reduced to idle, and a magenta
circle appears on the altitude tape. It
represents the vertical deviation between the
current altitude and the computed descent
path. In managed descent mode, the FMGC
will adjust the vertical speed to minimize the
vertical deviation. This will be done by
adjusting the descent speed by +/-20 knots
around the managed descent speed. This
interval is shown on the speed tape by 2 half
triangles showing the minimum and maximum
speed the aircraft can take to manage the
descent.
The best option is to initiate the descent when
you reach the Top of Descent point, displayed
on the Navigation Display, unless the ATC
commands you to descend at another time...
The top of descent point, computed by the FMGC,
is shown with a white down arrow (1)
For any reason, you may initiate the descent
before or after the computed top of descent point.
In this case, the FMGC will do its best to put the
aircraft back on the computed descent profile. If
you descend before, the FMGC will command a
slow descent (at 1.000 feet per minute) until it
intercepts the computed descent path. If you
descent after, the FMGC will initiate a idle descent
with a high rate of descent while keeping the
airspeed within the possible range (+/-20 knots
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around the descent target speed). If the aircraft is
very high above the descent profile, the FMGC
may be unable to intercept the path because it
would need a descent speed higher than the
maximum authorized speed. If this happens, the
only solution is to extend the speed brakes so that
the angle of descent increases with the same
airspeed.
The example below shows a managed descent
where the aircraft is above the computed
descent path :
During a managed descent, the PFD displays
speed and altitude information relative to the descent.
1. The descent target speed is shown with a
magenta = sign.
2. The FMGC can adjust the speed up to the
maximum managed descent speed, which
is the target speed + 20 knots.
3. If the aircraft has to slow down the aircraft,
it can adjust the speed down to the
minimum managed descent speed, which
is the target speed - 20 knots.
4. The descent path indicator (magenta
circle) shows the vertical deviation with the
computed descent profile.
On this example, the aircraft is above the
descent path (the magenta circle is below the
altitude yellow line), which is why the FMGC
commands a speed higher than the descent
target speed (300 knots) in order to increase
the angle of descent while keeping idle thrust.
The managed descent speed will not exceed
320 knots. If this speed is still too low to
intercept the descent path, you can extend the
speed brakes, but you should be aware that it
will result in a VERY high descent rate.
During the managed descent, additional
information is computed by the FMGC and
displayed on the ND with the pseudowaypoints.
These waypoints are computed by
the FMGC and added in the flight plan when all
the necessary information is entered by the
crew.
The pseudo-waypoints are the following:
• Speed Limit :
It is displayed as a magenta filled circle. It
shows where the aircraft will accelerate or
decelerate to reach a new target speed. On
this example, this waypoint is positioned
where the aircraft crosses 10,000 feet and
it will accelerate from 250 to 300 knots.
• Top of Climb (1) and Top of Descent (2) :
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