This can vary depending upon the situation.
If your flying offline, it can be set to the the ILS intercept altitude, the Airbus will follow the decent along with any constraints of the STAR and hold altitudes as required on its way down, just be sure to check the FMGC to the charts.
If your flying online - it can vary by region what instructions you recieve for the decent. If its a high traffic area you will most likely be given one or more stop altitudes but this will will come with a restriction (for example to reach a VOR by a certain altitude). You can also be told to "decend via the STAR" which means you would enter the last pubished altitude and follow it down, allowing the aircraft to again follow the contraints.
The Airbus has essentially 3 modes of climb and decent avaiable to the pilot:
1) V/S - This is more commonly used to override the decent by the PIC by either expediting or reducing the vertical speed - used most when given instructions by ATC or in the case of TCAS
2) DES - This mode allows the aircraft to follow the decent profile of the selected arrival, following altitude constraints as published - needs to be monitored by the crew to ensure the next published altitude and hold altitude are both displayed above the altitude tape on the PFD. Used when told to "Decend via the STAR"
3) Open DES - This will allow the aircraft to decend at its best rate with idle thrust - it will not follow any contraints as published within the FMGC and will go down to one altitude. Use when given altitude constraints or waypoint restrictions as it will get you down to the required altitude.
Below is a good exerp for reading on this topic from the following link: http://www.smartcockpit.com/docs/Airbus_Descent_Monitoring_1.pdf
T/D The top of descent displayed on the FPLN page (T/D) and on the ND is a position that the FMGS calculates, assuming that the aircraft will begin its descent in DES mode with Managed speed, and that the system will guide the aircraft along the FPLN track and the descent profile computed is with all the vertical FPLN data (ALT CSTR, MANAGED MACH/SPEED, SPD CSTR, SPD LIMIT and inserted winds) to reach VAPP at 1,000 feet AGL.Note: The ND does not display the top of descent arrow when HDG (or TRACK) mode is engaged.
AIRBUS DESCENT MONITORINGPage 5of 29V1.1©June2007Level Off (Climb/Descent)The level off arrow(climb),(descent) gives the point on the flight plan where the aircraft will reach the FCU selected altitude.Continue (Climb/Descent)The start of climbor continue descentshows the point on the flight plan where the aircraft is predicted to leave the current flight level.Intercept PointThe intercept symbolshows the point on the flight plan where the aircraft will intercept the FMGC computed vertical descent profile. It is displayed in white if DES mode is not armed and in blue when DES mode is armed.It can appear if the aircraft is either above or below the FMGC computed descent profile.If Above The FMGC ProfileIf the aircraft is above the descent profile and in Managed Speed, the speed will increase toward the upper limit of the Managed Speed target range. If the speed reaches the upper limit, the aircraft will maintain the speed but will deviate above the profile (A/THR at IDLE).The ND Intercept Point assumes the aircraft will return to the profile using:Idle thrust,Half speedbrake extension, andECON speed plus a margin (until intercepting the profile).When this symbol reaches the next ALT CSTR waypoint “EXTEND SPD BRK” appears on the PFD and MCDU indicating that speedbrakes must be extended in order to match the next altitude constraint. This is an advisory message.Note: When DES mode is engaged, the speedbrake extension will not necessarily increase the descent rate. It does so only if the aircraft is abovethe profile. If the aircraft is on or below the profile the AP will maintain the aircraft on profile (or intercept the profile from below) and the A/THR will add thrust to keep the aircraft within the speed target range.If Below The FMGC ProfileThe system maintains the target speed (Managed or Selected speed) with the A/THR in SPEED/MACH mode, and at:1,000 ft/min rate of descent (if the aircraft is flying an IDLE segment), or500 ft/min rate of descent (if the aircraft is flying a GEOMETRIC segment),until it reaches the constraint altitude or intercepts the profile.Arrow Symbol Colours (T/C, T/D, Level Off, Continue, Intercept Point)The variousarrow symbols are coloured blue if using a Managed mode or it is armed (CLB or DES), magentaif a constraint or whiteif using a Selected mode (OP CLB, OP DES, V/S or FPA).The T/D arrow is always white as there is no automatic descent (see initiating descent).Speed ChangeThe speed change waypointrepresents the point(s) of the flight plan where the speed has to change (e.g. SPD LIM).
AIRBUS DESCENT MONITORINGPage 6of 29V1.1©June2007Decel The decelerate waypointrepresents the point of the flight plan where the aircraft is predicted to decelerate for approach. It is displayed in magenta when in Managed speed and NAV or APPR mode is engaged. Displayed in white when in Selected speed mode or HDG/TRK mode. Automatic decelerations occur only when displayed in magenta.Energy CircleThe energy circleis indicated by an arc drawn 20°. The radius corresponds to the required distance to land from present position. The energy circle computed by the FMGC is available in ROSE NAV and ARC modes only.Waypoint Altitude ConstraintsThe constraint waypoint (a circle around the waypoint) is displayed at flight plan waypoints where an altitude constraint (speed constraints are not displayed on the ND with circles) is defined:Magenta when the ALT CSTR is predicted to be satisfied.Amber when the ALT CSTR is predicted to be missed.White when the ALT CSTR is not taken into account by the guidance and the NAV mode is engaged (e.g. OP DES).Not displayed when in any other lateral mode except NAV (e.g. HDG).DESCENT MODE (DES)This mode would have to provide the most confusion for pilots when they are trying to monitor the Airbus during descent. If you can understand what the FMGS is trying to achieve and how this translates into AP/FD and A/THR commands when in the DES mode then you can rightfully call yourself an Airbus pilot...if you can’t, then you’re a passenger.There are two kinds of airplanes —those you fly and those that fly you. You must have a distinct understanding at the very start as to who is the boss.(Ernest K. Gann)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 (speedand altitude constraints and SPD LIM), and it is based upon the Managed descent speed profile. It does not take holding patterns into consideration.Initiating descent(DES Mode)The aircraft will not start its descent automatically when reaching the topof descent (T/D). In order to initiate the descent, you set the ATC lower clearance altitude on the FCU, then pushthe 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 aircraftdescends 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.D
AIRBUS DESCENT MONITORINGPage 7of 29V1.1©June2007Descent Profile Segments Internally, the computer divides the descent path into various segments, depending on the relative positions of the constraints. It starts at top of descent (T/D) by setting up an “Idle” segment that takes the aircraft down to the first constraint, and followsthis with “Geometric” segments between constraints.The descent profile has several segments:Repressurizationsegment. When necessary, this produces a repressurization rate for the cabin during descent. It is a function of the destination airport altitude and the selected cabin rate (defaulted to − 350 feet/min but this can be modified). Don’t worry about this segment. Practically it has little bearing on the vertical descent.Idlepath segment. The AP/FD controls the speed and A/THR stays at idle thrust. Guidance computes this profile from T/D or the end of the repressurization segment to the first vertical constraint that cannot be flown at idle thrust.Geometricpath segment(s). The AP/FD controls the vertical path, and A/THR controls the speed. These segments take the aircraft from the first constraint that cannot be flown at idle thrust (and any subsequent constraints) to the Decel Point. Note that there may be more than one Geometric segment and the various Geometric segments may be at different descent angles to satisfy the various altitude or speed constraints at the waypoints contained in the Geometric segments.When DES mode is engaged, NAV mode is engaged (or another way to look at it is, you won’t be able to engage DES mode unless you are in NAV mode), and the system takes into account all altitude and speed constraints.Profile MonitoringThe key parameter for monitoring the descent is the vertical deviation (VDEV) displayed on the PFD and on the MCDU PROG page, which indicates whether the aircraft is on, above, or below the FM calculated vertical descent profile.Well that’s what the books say. Ensure that you mentally compute your own vertical descent profile and compare it with the FM calculated vertical descent profile. Blindly trust VDEV at your own peril!It’s not a matter of IFyou’ll get caught out, but WHENyou’ll get caught out (and by how much)!
AIRBUS DESCENT MONITORINGPage 8of 29V1.1©June2007If the aircraft is on the descent profileThe aircraft is considered to be on the vertical profile when it is within 50 feet of it. VDEV isclose to zero, and the system predicts that it will match constraints until the aircraft levels off at the next FCU altitude.The A/THR adjusts the thrust for the particular segment. The first FMA column may display THR IDLE or SPEED/MACH (depending on whether the Managed Speed reaches the upper or lower limit of the Managed Speed target range) but will normally display THR DES.If the aircraft is above the descent profileVDEV is down on the PFD and positive on the PROG page.The A/THR sets THR IDLE and if in Managed Speed the AP increases speed up to the maximum of the Managed Speed target range by calling for down elevator. If the aircraft reaches the upper limit of the Managed Speed target range, the aircraft diverges from the FM calculated vertical descent profile and maintains the upper limit speed.If the aircraft is below the descent profile:VDEV is up on the PFD and negative on the PROG page. The system maintains the target speed (Managed or Selected Speed).The A/THR is in SPEED (MACH) mode with a 1,000 ft/min rate of descent (if the aircraft is flying an Idle segment), or 500 ft/min (if the aircraft is flying a Geometric segment) until the profile is regained.