This mode is one of the more dificult to understand and explain! The DES mode provides Managed vertical guidance along an FM computed vertical descent profile. The system computes this flight path backwards from the Decel point up to the top of descent (T/D) at the cruise flight level with respect to the speed and altitude constraints. The Decel point is where the guidance begins the deceleration to VAPP, to be reached at 1,000 feet above touch down on the final descent path.
The descent profile takes into account wind data and data from the lateral and vertical flight plans (speed and altitude constraints and SPD LIM), and it is based upon the Managed descent speed profile. It does not take holding patterns into consideration.
The aircraft will not start its descent automatically when reaching the top of descent (T/D). In order to initiate the descent, you set the ATC lower clearance altitude on the FCU, then push the ALT selector knob. The aircraft will then descend immediately:
- If the descent is initiated before the FM computed T/D, the aircraft descends at a constant V/S converging on the descent path from below. The A/THR operational mode will be SPEED/MACH.
- If the descent is initiated after the FM computed T/D, the aircraft descends at idle thrust and attempts to converge on the descent path from above. The A/THR operational mode will be THR IDLE.
- If the descent is initiated at the FM computed T/D, the aircraft is descending on the FM calculated vertical descent profile. The A/THR operational mode will be THR DES.
Hope this answer your question!