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Be aware that altitude constraints in the flight plan are observed only when the descent is managed, i.e. when DES is displayed on the FMA. Any other vertical mode will disregard altitude constraints.
During a managed descent, with DES displayed on the FMA and the F-PLN accurately reflecting all altitude constraints, it is permissible to select the lowest ATC cleared altitude on the FCU and monitor that the correct profile is flown.
To avoid overshooting the computed descent path, 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 on the PFD and MCDU scratchpad. Consider selecting speed towards Green Dot and when cleared for descent, 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
FCTM REV 1 (6 JUN 05)
When DES is engaged and speed is managed, the AP/FD guides the aircraft along a 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 with managed speed operates within a speed range around a target speed to enable the aircraft to maintain the descent path.
If the aircraft gets high on the computed descent path, the speed will increase towards the upper limit of the speed range. If this increase in speed does not allow a descent constraint to be achieved, a message "EXTEND SPEEDBRAKES" is displayed on the PFD and MCDU scratchpad. A path intercept point ., which assumes half speedbrake extension, will be displayed on the ND descent track. When regaining the descent profile, retract the speedbrakes to prevent the A/THR applying thrust against speedbrakes. If the speedbrakes are not retracted, the "SPD BRK" message on the ECAM memo becomes amber and "RETRACT SPEEDBRAKES" is displayed on the PFD.

If the aircraft gets low on the computed descent path, the speed will decrease towards the lower limit of the speed range. When the lower speed limit is reached, the A/THR reverts to SPEED/MACH mode and applies thrust to maintain the descent path at this lower speed. A similar path intercept point . will be displayed on the ND.
The computed descent path remains unchanged if speed is selected. As the selected speed may differ from the speed used for the pre-computed descent path and the speed variation around target no longer applies, the aircraft may deviate from the descent path.
5.30.4

A330/A340 Climb, Cruise And Descent
REV 1 (6 JUN 05)
FCTM Descent
Selected Descent Mode
The selected AP/FD modes in descent are OP DES, V/S or less commonly FPA. During a selected descent with OP DES, V/S or FPA displayed on the FMA, successive altitude constraints must be set on the FCU. If under radar vectors, set the next ATC cleared altitude on the FCU.
OP DES is used if ATC gives radar vector or clears the aircraft direct to a given FL without any descent constraints. All FMGS descent altitude constraints will be ignored. V/S mode is normally used to recover from a below profile condition.
To initiate a selected descent, set the ATC cleared altitude on the FCU and pull the ALT selector at TOD. OP DES mode engages and is annunciated on the FMA. In OP DES, the A/THR commands THR IDLE and the speed is controlled by the elevators.
Speed may be either managed or selected. In managed speed, the descent speed is displayed as a magenta target only and there is no longer a speed target range. The computed descent path is ignored and consequently the speed will not vary around the target. The AP/FD does not consider any F-PLN descent altitude constraints and will fly an unrestricted descent down to the FCU selected altitude.
　
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