Managed Descent Rescue

KLAX→KLAS is 205 nm — a short sector where the descent is most of the flight. There is very little cruise in which to notice a bad vertical plan, which is exactly why this leg catches people out.

Without a cruise level the FMGS has no vertical profile at all. With one, it computes a descent path backwards from the arrival — so the quality of that path depends entirely on what it finds in the flight plan ahead of you.

PROG is where you confirm what the FMGS thinks it is doing vertically — the cruise level it is planning to, and its recommended maximum. It will not tell you that you are above the path; that deviation lives on the PFD and ND on the real aircraft. PROG confirms the plan. The flight plan tells you whether the plan is worth anything.

Here is the diagnosis: the plan reads KLAX, a discontinuity, KLAS. No arrival, no fixes, no crossing constraints. The FMGS has nothing to descend TO except the destination, so it builds a lazy path aimed far past the real restrictions. That is why you are high — not because managed descent is broken, but because you never gave it any constraints to honour.

INSERT drops the CHOWW arrival’s fixes into the plan ahead of KLAS, and the approach you picked is what decides which of them: CHOWW is coded with a different branch for each landing direction, so an 01 approach and a 26 approach give you genuinely different last twenty miles. The descent is now planned through real waypoints instead of straight at the airport. This is the step that actually rescues the descent — everything after it is refinement.

VERT REV is where constraints get attached. Inserting a STAR gives the FMGS the lateral track; it does not by itself give this trainer the crossing altitudes, so you hang them on the fix yourself — which is also what you do on the real jet whenever ATC issues a restriction that is not the published one.

VANDL is published to be crossed at 9,000 ft. This one number is what the whole descent path is now built backwards from: the FMGS works from the deceleration point, back through every constraint, to top of descent. Change 9,000 and the top of descent moves with it.

This is the DRAG REQUIRED half of the problem. When the FMGS works out it cannot make a constraint even at idle thrust, it posts MORE DRAG on the PFD and the MCDU, and it stays there while speedbrake is still needed — the airframe cannot descend steeply and slow down at the same time. Your hands-on response is speedbrake; your MCDU response is to make sure the FMGS knows the speed it has to make, so it plans the deceleration instead of discovering it late. (250 kt is the US limit below 10,000 ft and VANDL is a 9,000 ft fix — it is a speed limit, not an approach speed. Note this trainer does not post MORE DRAG; neither does the A32NX yet.)

ACT MODE reads MANAGED. That is the whole DES vs OP DES decision: managed DES flies the FMGC-computed path and honours the constraints you just entered. OP DES ignores them — idle thrust, pitch holding the speed target, straight down to whatever altitude the FCU is set to. OP DES is a legitimate tool when ATC has cleared you all the way down and you want the descent expedited; with a 9,000 ft crossing still ahead of you it is exactly what busts it. Being high is not, by itself, a reason to reach for it. On the real aircraft this is a push of the FCU ALT knob for managed and a pull for OP DES — there is no MCDU key for it, which is why this step is a read.

SPD CSTR 250 and ALT CSTR 9000, both magenta, both still on VANDL. Magenta means a crew constraint the FMGS is planning to. The descent path is now anchored to something real, and managed DES has a profile worth following — which is the whole difference between a rescued descent and a rushed one.