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? The speed constraint or limit when applicable.
GUIDANCE AND MONITORING
Applicable to: ALL
INTRODUCTION
To carry out the descent, the crew can use either the managed descent mode (DES) or the selected descent modes (OP DES or V/S). Both descent modes can be flown either with selected speed or managed speed.
The modes and monitoring means are actually linked.
The managed  DES mode guides the aircraft along the FMS pre-computed descent profile, as long as it flies along the lateral F-PLN: i.e. DES mode is available if NAV is engaged. As a general rule when DES mode is used, the descent is monitored using VDEV called "yoyo" on PFD, or its digital value on the PROG page, as well as the level arrow on the ND.
The selected  OP DES or  V/S modes are used when HDG is selected or when ALT CSTR may be disregarded or for various tactical purposes. As a general rule when OP DES or V/S modes are used, the descent is monitored using the Energy Circle, (displayed if HDG or TRK modes and
indicating the required distance to descend, decelerate and land from present position) and the level arrow on the ND. When the aircraft is not far away from the lateral F-PLN (small XTK), the yoyo on PFD is also a good indicator.
MANAGED DESCENT MODE
The managed descent profile from high altitude is approximately 2.5 °.
As an estimation of the distance to touchdown is required to enable descent profile monitoring, it is important to ensure that the MCDU F-PLN plan page reflects the expected approach routing. Any gross errors noted in the descent profile are usually a result of incorrect routing entered in the MCDU or non-sequencing of F-PLN waypoints, giving a false distance to touchdown.
DESCENT INITIATION
To initiate a managed descent, the pilot will set the ATC cleared altitude on the FCU and push the ALT selector. DES mode engages and is annunciated on the FMA. If an early descent were
required by ATC, DES mode would give 1 000 ft/min rate of descent, until regaining the computed profile.
 

  NORMAL OPERATIONS
DESCENT
A318/A319/A320/A321
FLIGHT CREW TRAINING MANUAL 

To avoid overshooting the computed descent path, it is preferable to push the FCU ALT selector
a few miles prior to the calculated TOD. This method will ensure a controlled entry into the descent and is particularly useful in situations of high cruise Mach number or strong upper winds. If the descent is delayed, a "DECELERATE" message appears in white on the PFD and in amber on the MCDU. Speed should be reduced towards green dot, and when cleared for descent, the pilot will push for DES and push for managed speed. The speed reduction prior to descent will enable the aircraft to recover the computed profile more quickly as it accelerates to the managed descent speed.
DESCENT PROFILE
When DES with managed speed is engaged, the AP/FD guides the aircraft along the pre-computed descent path determined by a number of factors such as altitude constraints, wind and descent speed. However, as the actual conditions may differ from those planned, the DES mode operates within a 20 kt speed range around the managed target speed to maintain the descent path.
　
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本文鏈接地址：A318/A319/A320/A321 機組訓練手冊 FCTM Flight Crew Training Manual(82)