Flight Plan I — Structure and Lateral Revisions

Position solved (article 20), now lay the road. This article covers the flight plan's skeleton (ACTIVE/TMPY/SEC) and the whole lateral revision family — DIR TO, airways, holds, offsets, diversions. Vertical elements (constraints, winds) are article 22; transition geometry is article 23. (This article follows the FMS standard of the airframe group modelled in this series; the parallel group's differences appear as configuration notes.)

1. Three plans, two volumes

Per FCOM DSC-22_20-10-30:

The FMGES can contain two different flight plans: ‐ The ACTIVE flight plan, which is the basis for: • Lateral and vertical guidance • MCDU and ND display • Radio navigation autotuning • Performance predictions • Fuel planning. ‐ The SECONDARY flight plan which the flight crew may use: • To prepare and store a second departure procedure before takeoff • To plan a diversion • To prepare the next flight leg • To compare predictions or evaluations.

ACTIVE is not "a map" — it feeds five downstream consumers (guidance, displays, autotuning per article 20, predictions, fuel), the system's single source of truth. Each plan binds two volumes: primary (origin → destination → missed approach) plus alternate (destination → alternate). Capacity on the group modelled here: three plans (ACTIVE/SEC/TMPY) of 250 legs each — under 215 for the primary, under 35 for the alternate — with F-PLN FULL raised when a revision would overflow (the parallel group: five elements of 200 legs). And the database's memory-saving construction explains the ARRIVAL page's four columns: DEPARTURE = SID + en-route transition; ARRIVAL = approach VIA + STAR + en-route transition — SIDs and STARs are shared trunks, transitions and VIAs the couplings.

2. The leg language — discontinuities, sequencing, and legs you cannot build

Per FCOM DSC-22_20-10-30:

The flight plan has a discontinuity if any two waypoints do not have a leg defined between them. The computer assumes that the aircraft will fly a direct leg between the two waypoints that define the discontinuity. Note: When the aircraft enters a flight plan discontinuity, the NAV mode automatically switches to the HDG/TRK mode.

Predictions pretend there is a bridge (a direct leg); guidance admits there isn't — enter the discontinuity and NAV drops to HDG/TRK (article 09's radar-vector SID, generalised). Sequencing rules (this group): in HDG/TRK, a waypoint sequences once behind the aircraft within 5 NM; the destination airport is special — it also needs, within 3 s, XTK < 0.5 NM and a track split under 30° (brushing past the field must not count as arriving); NAV mode applies the same rule without the distance term (on the leg is enough). (Parallel group: XTK < 7 NM with the angle under 90°, plus a 5 NM term at approach fixes.)

The leg registry: DME arcs, holds, course-to-fix, radius-to-fix, heading legs and MANUAL legs are all database-procedure exclusives — the crew cannot build them; a pilot's hands make only straight legs between geographic fixes. MANUAL legs deserve their own card:

A MANUAL leg stays on a constant TRK or HDG and has no termination point... When the aircraft is flying a MANUAL leg, the NAV mode remains engaged and predictions assume that the aircraft will fly a direct leg from its present position to the next waypoint (DIR TO)... Note: 1. In NAV mode, a MANUAL leg is sequenced only by performing a DIR TO. 2. The use of the descent mode (DES) on a MANUAL leg is not recommended.

A straight line with no ending — NAV stays engaged, only a DIR TO turns the page, and DES on it is discouraged (no geometric endpoint, no computable profile — article 08's family of unbuildable geometry).

3. TMPY — the yellow draft

On this group, every lateral revision passes through a temporary flight plan (the parallel group inserts waypoint entries and DIR TOs immediately). Its character:

The flight crew can perform several revisions in the same temporary flight plan before inserting it. The TMPY F-PLN is a copy of the active F-PLN, but corrected by the lateral or vertical revisions in progress. The UNDO function is available when there is more than one pending revision in the temporary F-PLN. It can only be used once by the crew to cancel the last revision in the temporary F-PLN. The aircraft continues to follow the active F-PLN, until the temporary revision is inserted. The temporary F-PLN is continuously sequenced as the active F-PLN, until it diverges from the active F-PLN.

Stack several revisions and insert once; UNDO exists but fires once, on the last revision only; the aircraft flies ACTIVE while the draft hangs; and the draft is alive — sequenced along with the active plan until the two diverge. House rules: revisions show yellow (missed-approach and alternate segments stay blue — article 03's colour); one TMPY at a time; no TMPY while a DIR TO is in process (DIR TO IN PROCESS); TMPY predictions update continuously on this group (the parallel group's TMPY has none); and the pre-insertion check escalates with complexity — glance at the parameters, look at the ND, or page through the draft leg by leg.

4. The lateral revision family

Airways: up to five consecutive airways per revision; two airways with no common fix but an intersection get a system-built point named X-plus-airway (XUB25); none → NO INTERSECTION FOUND; endpoints off the airway → AWY/WPT MISMATCH. One trap: when duplicate-name fixes lie on the airway itself, no DUPLICATE NAMES page appears — the database's first entry is taken silently; on long airways, check the reasonableness yourself.

Waypoint entry — five identities: ident; latitude/longitude; place-bearing-distance (PBD); place-bearing/place-bearing (PBX); place-distance along the plan (PD). Off-database creations are registered as PBD01–20 and kin (article 04's stored elements). With NAV engaged you cannot change the TO waypoint by insertion — the active leg is only ever changed with a DIR TO. Along-track points (AMB/-040) create no discontinuity and may not sit on the FROM point; lat/long grid points belong to cruise only and are deleted at sequencing, never stored.

FIX INFO: intersections of a radial, a circle or an abeam with the flight plan — the radial drawn as a blue dashed line, the intersection auto-named (POI120, D040POI), delivered with time/along-path distance/altitude predictions. The tool for "report crossing the 120 radial".

NEW DEST: the revolutionary revision — everything after the revise point is deleted (old destination and missed approach included), one discontinuity left for you to re-road.

Holds — three species:

HOLD TO FIX (HF) The holding pattern is always part of an arrival or departure procedure. The aircraft flies it once and then exits the holding pattern automatically at the fix. … HOLD TO ALTITUDE (HA) ... The aircraft flies the hold until it reaches the specified altitude. Then it exits the hold automatically at the fix. … HOLD WITH MANUAL TERMINATION (HM) ... This type of holding pattern is exited according to the decision of the flight crew, and not automatically.

HF flies one circuit and leaves; HA climbs out; both are database goods. HM is yours — DATABASE HOLD when the library has one, COMPUTED HOLD invented otherwise, every parameter editable. HM's prediction philosophy is the counter-intuition: an inserted hold is not pre-charged into the predictions — only a deceleration point appears (a segment capped at 20 NM); entering the circuit starts the metering, one lap booked at a time, renewed at each fix passage. LAST EXIT is fuel policy turned into a clock: the latest departure time/fuel that still reaches destination, flies the missed approach and arrives at the alternate with reserves intact. Default holding speed = min(max-endurance ≈ green dot to green dot +10, ICAO limit, any speed constraint); on selected speed the aircraft won't slow itself — SET HOLD SPD flashes instead. Holds inside a managed descent obey three iron rules: the hold never enters the descent profile; a constraint attaches only to the entry fix; circuit guidance is −1000 ft/min but will not bust a downstream constraint (level-off at the constraint altitude) — the full source of article 08's holding line. IMM EXIT has two moods: pressed before the fix — no entry, carry on; pressed in the circuit — shortcut back to the fix and exit at passage (RESUME HOLD repents). Entries (direct/teardrop/parallel) are system-chosen; on the teardrop/parallel boundary either answer is correct, and a reciprocal joining is always parallel.

OFFSET: an integer 1–50 NM, laid from present position to the first waypoint with a hold or the last fix before the runway; the offset line solid green, the original dashed; not available on approach legs; cancelled by clearing the value, by DIR TO, or automatically. Traps: a hold inserted at PPOS flies on the offset line, but a hold anywhere else deletes the offset first; too close to the offset's endpoint → ENTRY OUT OF RANGE.

Alternate and ENABLE ALTN: the alternate changes via INIT A without a TMPY, via LAT REV with one; OTHER ALTN candidates can be compared and promoted. Alternate predictions rest on six assumptions — landing weight at destination, a distance-banded cruise level (FL100/220/310), cost index 0, one constant wind (the alternate line of the DES WIND page), constant ΔISA, along-route distance — conservative estimates, not precision. ENABLE ALTN grafts the alternate volume after the revise point (with a discontinuity) and pops USING COST INDEX 0 — go set a real CI on PERF and a cruise level on PROG afterwards.

5. DIR TO — the master key's three teeth

‐ DIR TO defines a direct leg from present position to a specified waypoint. NAV mode engages simultaneously to the DIR TO selection. When the pilot uses DIR TO, the present position (PPOS) becomes the 'FROM' waypoint and the active flight plan shows it as T-P (turn point). ‐ The DIR TO/ABEAM function computes the abeam waypoints along the direct leg... ‐ The DIR TO/INTCPT function allows to define a specified RADIAL INBOUND or OUTBOUND an inserted waypoint... NAV mode is armed simultaneously to the DIR TO/INTCPT selection.

Tooth one: PPOS becomes T-P, NAV engages immediately (article 09's universal key). Tooth two: the old fixes projected onto the new line (lat/long points renamed AB-XXXXX). Tooth three: a radial intercept — NAV arms, HDG guides you to the cut. INTCPT details: a target inside the plan defaults its inbound radial to the inbound track (outbound track if a discontinuity precedes); angles over 140° draw no transition, over 160° compute no intercept (NO NAV INTERCEPT); a DIR TO/INTCPT also cancels any offset; and which tooth is the default is an AMI policy choice (article 04). Its rescue role is official:

‐ When the pilot encounters a flight plan discontinuity, or if a major reset occurs, the flight plan page displays 'PPOS-F-PLAN DISCONTINUITY', and the pilot looses managed guidance in both the lateral and vertical plans. The autopilot or flight director reverts to the basic HDG V/S (or TRK FPA) modes... ‐ In both of these cases, the only way to get back to a standard flight plan is to perform a 'DIR TO' a designated waypoint.

Dead-end road (discontinuity or a major reset — article 29): the only way back is DIR TO. (If the resulting leg is so long the ND reports MAP PARTLY DISPLAYED, insert a midpoint.) And OVERFLY (Δ): force the turn to begin over the fix rather than before it — for visual and procedural "station passage" requirements; press again to cancel.

6. Changing the departure or the approach — what survives

Changing the approach while flying the arrival: inside the old/new procedures' common section, manual lateral revisions (holds included, TO-point edits included) are washed away (a deleted hold reports MANUAL HOLD DELETED) while vertical constraints survive; a TO point outside the common section keeps the current leg plus a discontinuity with the whole new procedure stacked behind; deep into the arrival, NO STAR / NO VIA strings just the final segment. Changing the departure: the whole procedure is replaced — but any waypoints after a previous manual revision have been re-registered as en-route points and survive, tacked on after the new SID. So the runway-change mantra: lateral washed, vertical kept; departure re-boned — check for the old tail.

7. POSITION UPDATE AT — the last resort

Use this facility with extreme caution: It is apt to be inaccurate, because it relies on the pilot's estimation of when a designated position has been reached. … If the update error is large, the system will reject any radio updating because its internal 'reasonableness test' will reject the various NAVAIDs. … When the GPS PRIMARY is operative, the 'POSITION UPDATE AT' is inhibited.

A manual position update trusts the second your thumb hits the key — and a bad update is self-sealing: the reasonableness test then rejects every navaid that disagrees with the error. With GPS PRIMARY operative the function is inhibited outright. The manuals leave it four legitimate scenes: no plan on the ND on the ground; CHECK A/C POSITION with an obvious map slew; CHECK IRS/FM POSITION; FM/IR POSITION DISAGREE.

8. The diversion toolbox — and SEC's two lives

Five tools: ETP, CLOSEST AIRPORTS, SEC F-PLN, ENABLE ALTN, NEW DEST. Division of labour (synthesis): ETP — the equal-time point between two reference airports with their own winds blended with route forecasts; it lives on the ND, not the F-PLN page (pair it with a time marker). CLOSEST AIRPORTS — the nearest four plus one manual pick; predictions assume managed cruise and a continuous descent, engine-out state included automatically; on a very different selected speed the predictions are abandoned in favour of raw bearing/distance; page two freezes the list. Ocean craft: PROG 4R on MCDU 1 holds the current alternate, MCDU 2 the next — two continuous bearing/distance watches, rolled forward at each ETP.

SEC's prediction boundary is the fourth counter-intuition:

‐ When the flight crew initializes the secondary flight plan with the SEC INIT function: The FMGES computes the secondary flight plan predictions as if the aircraft were on ground before engine start. The FMGES computes these predictions one time and does not update them... ‐ When the flight crew initializes the secondary flight plan with the COPY ACTIVE function: The FMGES computes the secondary flight plan predictions as for the active flight plan. However: • The FMGES stops the update and the display of the predictions if the first leg of the active flight plan is no longer the same as the active leg (i.e. if both flight plans diverge).

Two births, two lives: the SEC INIT edition is computed once as if parked before engine start — a still photograph for comparisons; the COPY ACTIVE edition lives with the aircraft until the plans diverge (updates also stop at the earlier T/D, and post-divergence TO-point differences can corrupt its distance and fuel/time figures — the ACTIVE plan is unaffected, including after activation). ACTIVATE SEC has two admission conditions: in HDG/TRK, or in NAV with both plans sharing the same active leg — no one-key swap while flying a different leg. SEC draws white on the ND, carries all vertical elements except history wind (article 22), and its five official uses are the ones in section 1's quote plus the ground re-initialisation for a changed flight.

9. Operating the road

The diversion three-step (refined in article 30): plan in SEC (ground version: a SID fix + NEW DEST + approach + SEC PERF APPR with QNH/wind/config; cruise version: pick the field with CLOSEST AIRPORTS/ETP, rebuild SEC's destination) → at decision, ACTIVATE SEC + DIR TO; no plan prepared → LAT REV at the TO point + NEW DEST; late in cruise, in descent, or after a go-around → ENABLE ALTN + DIR TO. And the manuals' own deadline: the ALTN flight plan shall be finalized whenever the landing runway is known.

Discontinuity discipline (this article's three threads tied): clear the discontinuities your edits create; entering one costs NAV and leaves a PPOS dead-end; the only exit is DIR TO. And the yellow rule: a hanging TMPY changes nothing — yellow doesn't drive; insertion does.

[!warning]- Five misconceptions this article corrects (1) A discontinuity is not cosmetic — predictions pretend a direct leg, but guidance drops NAV to HDG/TRK at the gap; clear it or be ready on the heading. (2) The TMPY is not a frozen snapshot — it sequences along with the active plan until they diverge, and UNDO works exactly once. (3) An inserted hold does not appear in the fuel predictions — metering starts only when you enter the circuit, one lap at a time; LAST EXIT is the fuel-policy deadline. (4) Changing runway/approach does not wash everything — lateral edits in the common section are cleared while vertical constraints survive; a changed departure can keep an old tail of re-registered fixes. (5) SEC predictions are not always live — the SEC INIT edition never updates, and the COPY ACTIVE edition dies at divergence.

Self-test

[!note]- Q1. The ACTIVE plan's five downstream consumers — and the capacity split on this group?

Lateral/vertical guidance; MCDU and ND display; radio autotuning; performance predictions; fuel planning. Three plans of 250 legs each — under 215 primary, under 35 alternate.

[!note]- Q2. At a discontinuity, what do predictions assume and what does guidance actually do?

Predictions assume a direct leg across the gap; guidance drops NAV to HDG/TRK on entry — the bridge exists only on paper.

[!note]- Q3. The sequencing rule on this group — and why is the destination airport special?

HDG/TRK: sequenced once behind the aircraft within 5 NM (NAV: same without the distance term). The destination also requires XTK < 0.5 NM and track split < 30° for 3 s — brushing past the field must not register as arrival.

[!note]- Q4. Three properties of a MANUAL leg — and why no DES on it?

Constant track/heading with no termination point; NAV remains engaged; only a DIR TO sequences it. No endpoint means no computable descent geometry — DES is not recommended.

[!note]- Q5. How many times does UNDO work — and can a TMPY coexist with a DIR TO in process?

Once, cancelling only the last pending revision. No — DIR TO IN PROCESS blocks a new temporary plan.

[!note]- Q6. The five waypoint-entry formats — and how do you change the TO waypoint in NAV?

Ident; lat/long; place-bearing-distance; place-bearing/place-bearing; place-distance. The active leg is changed only by DIR TO — never by insertion.

[!note]- Q7. For an HM hold: when do predictions start counting it, what is LAST EXIT computed against, what is the circuit guidance in DES, and can a constraint sit on the exit?

Only on entering the circuit, one lap at a time (before that, just a ≤20 NM deceleration segment). LAST EXIT = latest time/fuel that still makes destination + missed approach + alternate with reserves. Circuit guidance −1000 ft/min, never busting a downstream constraint. Constraints attach to the entry fix only.

[!note]- Q8. OFFSET's range and automatic endpoint — and is it available on approach legs?

1–50 NM (integers), from present position to the first hold-bearing waypoint or the last fix before the runway. Not on approach legs. Cleared value, DIR TO, or reaching the endpoint cancels it.

[!note]- Q9. On an approach change, what happens to holds, TO-point edits, and vertical constraints in the common section?

Manual lateral revisions — holds and TO-point edits included — are washed (MANUAL HOLD DELETED); vertical constraints survive. Lateral washed, vertical kept.

[!note]- Q10. The four legitimate scenes for POSITION UPDATE AT — and its status under GPS PRIMARY?

Ground with no plan on the ND; CHECK A/C POSITION with obvious map slew; CHECK IRS/FM POSITION; FM/IR POSITION DISAGREE. Inhibited entirely while GPS PRIMARY is operative — and a bad update locks out radio corrections via the reasonableness test.

[!note]- Q11. The prediction difference between SEC's two births — and ACTIVATE SEC's two admission conditions?

SEC INIT: computed once as if on ground before start, never updated. COPY ACTIVE: live like the active plan until the plans diverge (then frozen, and possibly corrupted for distance/fuel). Activation: in HDG/TRK, or in NAV with the same active leg in both plans.

Key takeaways

References

Plan structure, downstream consumers, volumes and capacities, database construction, leg types, discontinuities and sequencing per FCOM DSC-22_20-10-30 (structure sections); TMPY behaviour, airways, waypoint formats, FIX INFO, NEW DEST, the three hold species with prediction and exit rules, OFFSET, alternate assumptions, ENABLE ALTN, DIR TO's three functions and rescue role, OVERFLY, POSITION UPDATE AT, and departure/approach-change survival rules per the lateral-revision sections; the diversion toolbox and ocean technique per the diversion section; SEC lives and activation conditions per the secondary-plan section (all FCOM DSC-22_20-10-30, the airframe-group variant modelled here — the parallel group's figures are flagged inline). The three-step diversion pattern 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.