The descent profi le computed by the Flight Management System (FMS) is a very efficient and useful tool to help the flight crew in managing the aircraft energy during the descent and approach phases.
Descent Profile Computation
The FMS can compute an accurate and optimized descent profi le, provided the descent winds have been entered in the FMS during the descent preparation, and provided the PERF and IDLE factors are tuned according to the actual aircraft performance.
IMPACT OF THE WIND ON THE DESCENT PATH
The descent path computed by the FMS uses the forecasted wind entered in the DESCENT WIND page. However, in flight, actual conditions may vary from the predicted ones. As a consequence, the difference between the predicted descent wind and the actual wind (Δwind) affects the aircraft’s behavior. If the speed target is maintained (as in OP DES mode), the aircraft tends to leave the FMS computed idle path (fig.2).
Managed Descent ( DES )
The DES mode is only available when the aircraft fl ies on the FMS lateral fl ight plan, i.e. when the aircraft uses the NAV horizontal guidance mode.
(fig.3) Speed range principle during the idle segment of a managed descent.
In DES mode with managed speed the elevators adjust the pitch to enable the aircraft to stay on the computed path and the A/THR commands idle thrust.
The AFS allows the aircraft speed to vary in a range of +/- 20 knots around the managed speed target (+5 kt or -20 kt in the case of a speed constraint), limited to VMAX -5kt to stay on path:
On the geometric segment, the A/THR adapts thrust to maintain the managed speed target.
The use of speedbrakes in DES mode must be limited to the situation where there is either a strong tailwind or much less tailwind than expected, and the aircraft diverges from the profile. The flight crew should increase drag by extending the speed brakes (fig.3).
As a visual clue the ND displays the intercept point at which the aircraft will reach the profile with half speed brakes extended. If the flight crew does not extend the speed brakes the interception point will continuously move forward along the flight plan. If the interception point gets closer to an altitude constraint, a ‘‘ MORE DRAG ’’ or ‘‘ EXTEND SPD BRK ’’ message is displayed on the FMA and on the MCDU scratchpad/MFD.
Selected Descent ( OP DES and V/S )
In OP DES mode, the AFS commands idle thrust and the elevators adjust the pitch to maintain the target speed (managed or selected).
(fig.4) Adjustment of the selected speed to modify the descent path.
The flight crew can use the V/S mode during descent to get accurate guidance to recover the intended flight path by adjusting the V/S using the V/S selector.
In V/S mode, the AFS adjusts pitch and thrust to maintain the selected vertical speed and the target speed.
Tools for Energy Management during Descent
When in NAV lateral mode, the flight crew uses the ‘‘yoyo’’ indication to estimate its position relative to the FMS computed path. The Vertical deviation (V/DEV) value is provided on the FMS PROG page (A320/A330/A340) (fig.5) or PERF DES page (A380/A350).
(fig.5) example of V/DEV indication on the PFD and on the FMS PROG page (A320)
When in HDG or TRK lateral mode, the ND displays the energy circle, and when the aircraft is within 180 NM of its destination. It provides a visual cue of the minimum required distance to land, i.e. the distance required to descend in a straight line from the current aircraft position at its current speed down to the altitude of the destination airport at approach speed. The descent profile used to compute the distance takes into account speed limits, the wind, a deceleration level off segment and a 3° final approach segment (fig.6). In other words, if the destination airport is inside the energy circle, the flight crew needs to lose some energy by extending the speed brakes and/or modifying the aircraft’s trajectory, and/or increasing speed during descent.
(fig.6) Energy Circle Computation Principle
Another useful tool to use during descent is the level-off arrow provided by the FMS. It provides an indication to the flight crew of where the aircraft will reach the altitude selected on the FCU (fig.7). A blending of actual wind conditions and the values for winds entered in the FMS are used to improve the accuracy of the computation. If in selected descent, the fl ight crew can adjust the speed of the aircraft to adapt the descent path or V/S to the situation.
(fig.7) Level-off Arrow Computation Principle
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