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
The MCDU checks every entry as you make it, and tells you when an entry was accepted but wrong.
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.
- Put a fix in the gap to plan the descent around, and close it. Type DJB and press 2L (the discontinuity line). DJB takes the line and the discontinuity moves down below it. Then clear the discontinuity: press CLR, then 3L.
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 3R (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 3L.
ECON is the fuel-optimal descent speed, but ATC often assigns their own: “descend at 280 knots.” Before the descent starts you overwrite the ECON speed on the MANAGED line, and the FMGC builds the descent profile on 280 and still flies it managed. CLR on 3L puts ECON back. The SELECTED line below stays blank: it only shows a speed once you are descending on a speed pulled on the FCU speed knob.
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.
The rest of Descent & Arrival
Managed Descent Rescue
PremiumadvancedYou are high on the descent profile and the FMGS is not helping. Read the vertical picture off the MCDU, find the real cause (the arrival was never selected, so there are no constraints to descend to), fix it, hang the crossing restriction on the fix, and read what managed DES is actually flying.
📍 KLAX → KLAS - high on profile in the descent⏱ ~8 min📊 10 stepsSelecting the Approach: ILS RWY 27L at Atlanta
PremiumintermediateOn the Airbus you do not type a landing runway. You choose an approach, and the runway comes with it - then the feeder route, then the arrival, in that order.
📍 KDCA → KATL - selecting the ILS RWY 27L approach⏱ ~9 min📊 10 stepsLate Runway Change on the Arrival
PremiumadvancedYou are inbound to San Francisco with the arrival programmed, and ATC changes the landing runway. Re-select the arrival, see what the re-selection does to your flight plan instead of assuming it cleaned up after itself, and put the plan back together so it flies forward from where you are rather than backwards through an arrival you have already left behind.
📍 KLAS → KSFO - ATC changes the landing runway on the arrival⏱ ~9 min📊 10 stepsThe Approach Numbers (PERF APPR)
PremiumintermediateThe approach briefing, in the box: QNH, wind, temperature, VAPP and minima for the ILS onto Heathrow's 27R.
📍 EGLL - approach numbers for an ILS RWY 27R⏱ ~8 min📊 7 stepsLondon City Numbers (PERF APPR)
PremiumintermediateA numbers drill for the simulator: PERF APPR for London City runway 09, where the landing distance is 1494 m and the minimum comes off the chart rather than a rule of thumb.
📍 EGLC: PERF APPR numbers for runway 09, simulator exercise⏱ ~6 min📊 7 steps
Background reading
- GuideMastering VNAV Descent: Stop Arriving Too High and Too Fast
How VNAV descent works in the PMDG 737 and Airbus A320 - PATH vs SPD, managed descent, altitude constraints, and how to recover when you're too high.
- GuideA320 Managed Descent Explained: DES Mode, the Yoyo, and Staying on Profile
How A320 managed descent actually works - DES vs OP DES, push vs pull, the V/DEV yoyo, MORE DRAG, EXPED, and why the aircraft won't descend.
- GuideA320 Managed vs Selected Modes - When to Push, When to Pull, and Why the Plane Ignores You
Push for managed, pull for selected - and what to do when the A320 ignores the push. FCU dots and dashes, managed speed, NAV, CLB/DES vs OP CLB/OP DES.