Descent Setup — The Crossing Constraint
PremiumadvancedThe A320 builds its descent backwards from a crossing constraint. Add the fix, hang the constraint on it, then read the profile the FMGS computes.
What this scenario covers
A managed descent is not a fixed rate. The FMGS computes a path from the constraints in the flight plan and the descent speed schedule, working back from the runway, and the top of descent it draws is wherever that path intersects the cruise level.
This scenario constructs one: add a fix, apply a crossing altitude to it through vertical revision, then read the resulting ECON descent on PERF DES and give the FMGC an ATC-imposed descent speed to fly.
How it applies in flight ops
Understanding that the path is computed backwards explains the behaviour crews see every day: add a constraint and the top of descent moves; delete one and the aircraft suddenly has more room. Nothing about the aeroplane changed — the geometry did.
It also explains why an early or late descent clearance matters so much. Held high past the computed top of descent, the aircraft is now above its path and needs drag; sent down early, it is below the path and burns fuel level at low altitude.
Fly this one with Premium
Everything about this scenario is on this page, steps included — reading it costs nothing and always will. What Premium adds is doing it: the MCDU validates every entry as you make it, tells you when an entry was accepted but wrong, and saves your progress across devices.
Step-by-step: Descent Setup — The Crossing Constraint
- Set the route. Type KORD/KCLE into the scratchpad and press 1R.
The descent profile is built backwards from the arrival, so the FMGC needs a route with a fix to plan against. KORD→KCLE is the same Chicago–Cleveland leg the 737 descent drill uses — short enough that top of descent comes quickly after level-off.
- Set the cruise level. Type 340 and press 6L.
The FMGC needs a cruise altitude to compute where the descent starts (top of descent). Without CRZ FL there is no vertical profile and the PERF DES page stays empty.
- Press F-PLN to open the flight plan.
The lateral flight plan is where crossing constraints live — on the A320 the descent path is built to satisfy the altitude constraints you attach to waypoints here, so first you need a fix between origin and destination.
- Close the gap so there is a fix to plan the descent around. Type DJB and press 2L (the discontinuity line).
DJB (Dryer) sits on jet route J60 into Cleveland — a real fix aircraft cross on the way in. Inserting it gives the descent a named point to be planned around; without an en-route waypoint there is nothing to constrain.
- Open the vertical revision for DJB. Press 2R — the RIGHT key next to DJB.
A lateral key (left) revises the route; a vertical key (right) revises the profile. VERT REV is where you attach speed and altitude constraints to a waypoint — the building blocks of the descent path.
- Enter the crossing altitude. Type FL100 and press 1R (ALT CSTR).
This is the anchor of the whole descent. The FMGC now computes a path that leaves cruise at the right point and arrives at DJB exactly at 10,000 ft — the A320 equivalent of an end-of-descent altitude. Change this and the top-of-descent point moves.
- Open the descent page. Press PERF, then NEXT PHASE (6R) until the DES page shows.
ACT MODE reads MANAGED: the FMGC will fly its computed ECON descent — shown as a Mach/speed like .78/310 — to meet your DJB constraint. Managed descent means the aircraft follows the path and speed the FMGC planned, not one you dialled up.
- Give the FMGC a descent speed. Type 280 and press 4L.
ECON is the fuel-optimal descent speed, but ATC often assigns their own — “descend at 280 knots.” On the real jet a selected speed comes from pulling the FCU speed knob; this trainer is MCDU-only, so you enter it here. Either way the FMGC now targets 280 in the descent instead of ECON.
What to take away
- The descent path is computed backwards from constraints, not forwards from now.
- Adding or removing a constraint moves the top of descent.
- PERF DES shows the managed profile and takes an ATC descent speed.
- Above the path needs drag; below the path costs fuel. Both start with the profile.