Flight Plan II — Vertical Elements, Constraints, and Wind

The lateral plan is the road; the vertical elements are the script — when the phase changes, how high and how fast each fix should be crossed, and which way the air is moving. This article covers the FMS phase machine (the official table that articles 07, 14 and 17 kept citing), the constraint system, RTA, steps, and the four-page wind organisation. (Phase table and step rules follow the airframe group modelled in this series; the parallel group's figures are flagged.)

1. Three data classes — and the phase machine

The FMS sorts its vertical inputs three ways (FCOM DSC-22_20-10-30): strategic (cost index, cruise FL and steps, ZFW, ZFWCG, block fuel — change one and the whole flight recomputes), meteorological (phase winds, destination QNH/temperature, cruise temperatures, tropopause), tactical (phase-switching conditions, speed profiles, vertical limits). The phase machine, condensed:

The cruise exit deserves its exact words. Per FCOM DSC-22_20-10-30:

CRUISE ECON CRZ MACH The flight phase changes from Cruise to Descent, when the flight crew orders a descent below the cruise flight level while one of the following occurs: ‐ The flight plan has no step descent ahead of the aircraft, and the distance to the destination airport (along the flight plan) is less than 200 NM, or ‐ All engines are operative and the selected altitude target is below the highest of the following: • FL 50 • The highest descent altitude constraint.

(The parallel group uses an FL 200 / 40-NM-to-T/D branch.) A descent ordered far from destination is treated as a step descent and stays in the cruise phase — article 08's early-descent family, phase edition. And the life-saving footnote: per the same section, if the Descent or the Approach phase is inadvertently activated..., the flight crew may reselect a CRZ FL on the PROG page to reactivate the Cruise phase — a slipped ACTIVATE APPR needs no FM reset, just a re-entered cruise level (article 07's ALT CRZ identity, cashed). DONE, by contrast, is a shredder: entered 30 s after touchdown, it clears all flight data — which is why "prepare the next leg in SEC" exists (article 19).

2. Constraints — who honours them, how they are judged, when they are removed

Per FCOM DSC-22_20-10-30:

An altitude constraint is considered as missed if the system predicts more than 250 ft of difference between the constraint value and the predicted aircraft altitude. Altitude constraints are observed in CLB, or DES, or APP NAV-FINAL APP modes.

250 ft of predicted miss turns the F-PLN star amber; and only CLB, DES and APP NAV-FINAL honour constraints at all — the OP modes ignore them (article 07's mute button, in the constraint clause itself). The fine print: window constraints (between two altitudes) come only from the database — a pilot cannot type one; a constraint at or above the cruise FL is deleted automatically (windows lose just their upper bound) with the message CSTR DEL ABOVE CRZ FL; a separate family is merely disregarded (e.g. constraints above the last step altitude) with ALT CSTR ABOVE CRZ FL — deletion and disregard are two verdicts with two messages. Two early-descent behaviours worth knowing: engaging DES before T/D with a constraint at cruise level → ALT CST engages with DES armed until the fix; and diving below a constraint altitude early on OP DES/V-S then re-engaging DES → the FMS levels the aircraft off until the fix is sequenced — paying back the "cross at the constraint" debt first.

Speed constraints have three admission tickets:

Speed constraints are managed by the FMS when: ‐ The NAV mode is engaged, and ‐ The speed target is managed, and ‐ The speed constraint is equal or above the maneuvering speed. In all other cases, the speed constraints are disregarded.

NAV engaged, speed managed, and the value no lower than the manoeuvring speed — miss any one and the constraint is ignored (the speed row of article 09's chain; a constraint below F/S/green dot is refused outright). And keep SPD LIM distinct: a speed limit is "maximum below an altitude" (one for climb, one for descent — the 250/10 000 shape); a constraint hangs on a fix, a limit hangs on an altitude band.

3. RTA — the time constraint's temperament

Per FCOM DSC-22_20-10-30:

The FM computes a new managed speed profile from the aircraft position to the constrained waypoint, in order to match the 30 s difference (ΔT) between the time predicted at the constrained waypoint and the Required Time of Arrival (RTA). ... The RTA function uses a speed range between Green Dot and VMO -10 (or MMO - 0.02).

Within 30 seconds counts as on-time; the FM re-profiles managed speed inside green dot to VMO−10 (MMO−0.02), reverting to ECON past the fix (descent-fix RTAs excepted). Its temperament: no speed changes in the climb phase (the pursuit starts at cruise entry); a descent-fix RTA entered within 40 NM of T/D is not recomputed; formats HHMMSS with +/− prefixes (at / at-or-after / at-or-before); the star magenta = achievable within 30 s, amber = judged missed; automatic deletion in four cases — engine-out, hold entry, go-around, an RTA entered at another fix (only one lives at a time) — each announced RTA DELETED; and RTA excludes OPT STEP proposals — insert the step first, then the RTA. Preflight's companion is ETT (estimated takeoff time) on the RTA page, the prediction clock's origin: past the ETT the scratchpad reminds CLK IS TAKE OFF TIME, the real clock takes over at takeoff — and entering an RTA first makes the FM back-compute a magenta ETT: to be there on time, lift off by this.

4. STEP — the stair rules, and the stair that won't climb itself

Per FCOM DSC-22_20-10-30:

Rules ‐ The minimum step size is 500 ft ‐ A S/C cannot follow a S/D ‐ A STEP is automatically cleared: • If the S/C (S/D) is sequenced without any level change commanded by the flight crew • If the flight crew achieves a LAT REV which deletes the associated waypoint from the F-PLN • By EO condition. ... ‐ A STEP entry is ignored if the remaining CRZ distance is less than approximately 50 NM

Up to four geographic steps; minimum 500 ft (parallel group: 1000 ft); no climb-step after a descent-step; auto-cleared by an untouched sequencing, by deleting its host fix, or by engine-out; ignored inside the last ~50 NM of cruise. And the trap in the middle of that quote, expanded:

When reaching the step point, the steps must be initiated by the flight crew by selecting the new CRZ FL and pushing the FCU altitude selector knob. If sequenced without any flight crew action, the step is automatically deleted. If the flight crew initiates the step: ‐ The CRZ FL is automatically reassigned to its new value ‐ The guidance is THR CLB/CLB for a step climb, or THR DES/DES with V/S = -1 000 ft/min for a step descent.

The step does not fly itself. At the point, you wind the new level and push the ALT knob; sail past untouched and the step silently deletes itself — the aircraft cruises on with no warning at all. Initiated, the cruise FL re-assigns automatically, and a step descent flies article 08's −1000 ft/min. OPT STEP proposes a step only when it saves at least 100 kg or 1 min (otherwise NO OPTIMAL), searching from 20 NM after T/C to 20 NM before the next step or 300 NM before T/D, needing plan/CRZ FL/CI/GW/CG as inputs; a proposal counts only once inserted, whereupon it becomes a fixed geographic point (re-laid equidistant from the aircraft after lateral re-routes), with UPDATE re-proposing when the parameters move. The message family: ABOVE MAX (over REC MAX), IGNORED (before T/C, after T/D, or within 50 NM of it), STEP AHEAD (within 20 NM — the alarm clock), NOT ALLOWED (a fifth step, the FROM point, pseudo-waypoints, stacked same-fix steps).

5. Wind and temperature — the four-page system

TRIP WIND (the ground first-guess): one head/tail component for the whole trip, feeding only the preliminary ground predictions — per FCOM DSC-22_20-10-30, once a CLIMB, CRZ or DESCENT WIND is entered, the system ignores the TRIP WIND — the real pages retire it. The division of labour:

‐ For climb phase : By inserting either the HISTORY WIND data (as recorded during the last descent), or by inserting winds (at up to 5 altitudes or flight levels) on the CLIMB WIND page. ‐ For cruise phase : By inserting winds (at up to 4 flight levels) at various CRZ waypoints on the CRZ WIND pages. The 4 levels are the same for all the cruise waypoints... ‐ For descent phase : By inserting winds (at up to 5 altitudes or flight levels) on the DES WIND page. ‐ For the ALTN F-PLN, an average wind may be entered on the DES WIND page for alternate cruise flight level.

Climb: five levels, or the HISTORY WIND recorded on the last descent (insertable only as a whole page, not editable — yesterday's descent: fine for a return to the same field, suspect across days and airports). Cruise: four levels per waypoint — with the four level numbers shared across the entire route: change one level value and every cruise waypoint's wind at that level is lost. Descent: five levels. Alternate: one average. Then the master switch and the change threshold:

Then, when wind update is complete for all these waypoints: PRESS the WIND UPDATE prompt. The wind profile is updated and predictions are re-computed only when this prompt is pressed. The flight crew should modify the entered winds and temperatures in flight, if a significant difference is expected (more than 30 kt or 30 ° for the wind data and more than 5 °C for the temperature).

Type all you like — nothing recomputes until WIND UPDATE is pressed; and in flight the manuals set the worth-the-trouble bar at 30 kt / 30° / 5 °C. The colour semantics and the hidden stream:

The CRZ WIND pages display the propagated values in small blue font, and the flight crew (or AOC) entries in large blue font. … When in flight, the FMGEC considers the real, measured wind up to 100 NM ahead of the aircraft, to permanently update the wind profile. This updated wind profile is used to compute the predictions and the performance data, but it is not displayed to the flight crew.

Large blue = someone entered it; small blue = the system propagated it (an entry spreads downstream to the next entered fix). And a third stream you never see: the measured wind, blended 100 NM ahead, permanently correcting the profile inside the predictions — the F-PLN B page shows interpolated forecasts, never the computed truth. Phase locks: CLIMB WIND freezes (green, NOT ALLOWED) once the climb activates; DES WIND likewise in descent/approach/go-around — with one deliberate exception: the PERF APPR tower wind stays editable through descent, approach and go-around (it is ground-speed mini's ration — article 17 — and it two-way links with the DES WIND page's ground line). All wind pages are true-north referenced (article 20's polar echo); the alternate profile is built in three segments (climb = average of cruise and destination winds; cruise = the entry, else destination wind, else zero; descent = interpolation to FL100).

CRZ TEMP (reached via CRZ WIND's next page): SAT/altitude pairs independent of the wind levels and independent per waypoint; propagated values in the same small blue — not clearable but overwritable, and overwriting any one propagated value converts that waypoint's other propagated temperatures into "manual" status — the one-step mechanism for correcting a single fix's ISA deviation. CONSTANT MACH segments: a fixed-Mach stretch between two cruise fixes (ATC Mach assignments, oceanic procedures) — one in the active plan (plus its TMPY), one in SEC, none in the alternate; default start = first available downstream fix, default end = T/D; deletion is silent, no confirmation.

6. Operating the script

OPT FL polluted by stale wind — the manuals' own advice: after a level change, the PROG page's OPT FL still leans on the propagated winds of the level you left; per FCOM DSC-22_20-10-30, if the propagated winds at the lower altitudes are significantly different from the actual winds, enter the wind at these altitudes, or if not available, the wind measured at the current CRZ FL (article 25's input end).

The wind pipeline (synthesis): copy the flight plan's trip wind at INIT B → real forecasts into the four pages (or AOC uplink — article 28) → WIND UPDATE → amend airborne only past the 30/30/5 bar → before descent, DES WIND plus the PERF APPR tower wind. Interfaces: DES WIND feeds the descent profile (article 08); cruise winds and temperatures feed ECON and OPT (article 25); star colours and page furniture belong to article 26.

[!warning]- Five misconceptions this article corrects (1) An inadvertent approach-phase activation needs no FM reset — re-enter the CRZ FL on PROG and the cruise phase returns. (2) "Missed constraint" is a prediction verdict (250 ft), not an accusation after the fact — and OP modes never honoured constraints in the first place. (3) A step does not fly itself — sail past the point untouched and it deletes silently; the STEP AHEAD message is an alarm clock, not a chauffeur. (4) Typing winds changes nothing until WIND UPDATE is pressed — and the profile actually used blends 100 NM of measured wind you never see displayed. (5) The four cruise wind levels are one route-wide set — editing a level number costs that level's wind at every cruise waypoint.

Self-test

[!note]- Q1. The three data classes with an example each — and what does changing the cost index touch?

Strategic (CI, CRZ FL/steps, ZFW, block fuel), meteorological (phase winds, destination QNH/temp, tropopause), tactical (phase switching, speed profiles, vertical limits). CI is strategic: the entire speed/prediction chain recomputes.

[!note]- Q2. The two PREFLIGHT→TAKEOFF conditions — and the APPROACH→DONE clock?

SRS TO engaged plus either N1 > 85 % (EPR ≥ 1.25) or ground speed > 90 kt. DONE arrives 30 s after touchdown — and shreds the flight data.

[!note]- Q3. The official cure for a slipped ACTIVATE APPR?

Re-select a cruise flight level on the PROG page — the cruise phase reactivates. No reset.

[!note]- Q4. The missed-constraint judgement, the three honouring modes, and the two removal messages?

Predicted deviation over 250 ft → missed (amber star). Honoured only in CLB, DES, APP NAV-FINAL APP. CSTR DEL ABOVE CRZ FL = deleted (at/above cruise level); ALT CSTR ABOVE CRZ FL = disregarded — two verdicts, two messages.

[!note]- Q5. The three admission tickets for a speed constraint — and is one below green dot executed?

NAV engaged, managed speed, constraint at or above the manoeuvring speed. No — below F/S/green dot it is refused.

[!note]- Q6. RTA's on-time window and speed range — the four automatic deletions — and does it chase time in the climb?

±30 s, green dot to VMO−10/MMO−0.02. Deleted by engine-out, hold entry, go-around, or an RTA at another fix. No — the pursuit begins at cruise.

[!note]- Q7. The minimum step (this group), the consequence of inaction at the point, and OPT STEP's thresholds and search window?

500 ft. Untouched sequencing silently deletes the step and the aircraft cruises on. OPT STEP proposes only if it saves ≥ 100 kg or 1 min, searching from T/C+20 NM to 20 NM before the next step or 300 NM before T/D — and counts only when inserted.

[!note]- Q8. The four wind pages' level counts — and why is the cruise page's level set "pull one thread, move the whole garment"?

Climb 5 (or HISTORY WIND whole-page), cruise 4, descent 5, alternate 1 average. The four cruise levels are shared route-wide: change one level number and that level's entries vanish at every cruise waypoint.

[!note]- Q9. What does WIND UPDATE do — and the three thresholds for bothering to amend in flight?

Only its press rebuilds the wind profile and recomputes predictions. Amend for differences beyond 30 kt, 30° or 5 °C.

[!note]- Q10. Large blue, small blue, green — whose words are they? And where is the 100 NM measured-wind blend displayed?

Large blue: crew/AOC entries. Small blue: system propagation. Green: locked by phase activation (history). The measured-wind blend is displayed nowhere — it lives only inside the predictions.

[!note]- Q11. What happens when you overwrite one propagated value on CRZ TEMP?

That waypoint's other propagated temperatures all convert to manual status — the deliberate one-step way to re-datum a single fix's ISA deviation.

Key takeaways

References

Data classes, the phase-machine table and its cruise-exit clause, the inadvertent-activation cure, constraint judgements and removals, speed-constraint conditions, RTA rules and ETT, and the STEP/OPT STEP rule set per FCOM DSC-22_20-10-30 (vertical-elements section, airframe-group variant modelled here); TRIP WIND, the four wind pages, WIND UPDATE and thresholds, propagation colours, the 100 NM measured-wind blend, phase locks, alternate wind construction and CONSTANT MACH per the wind section of the same chapter; the CRZ TEMP page per FCOM DSC-22_20-20-10. The pipeline narrative is an integrative synthesis.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.