Approach Modes and the Common APPR Logic

Approach is the managed world's last mile — and its densest mode logic: two kinds of approach, three selection doors, two-stage capture geometry, and a script that dissolves itself at the missed-approach point. The AMM's classification frames everything: ILS-type approaches (the LAND mode family — guidance rides the LOC and G/S beams, autoland-capable) versus FM-type approaches (FINAL APP — guidance rides an FMS-computed theoretical profile, down to the minimum only, landing not included). LAND/FLARE/ROLL OUT are article 13; capability categories are article 12.

1. Three selection doors and the MMR chain

 ① ARRIVAL page (standard door, FM world)
 ② RADIO NAV page (manual LS tune inside the FM) → freq + course → both MMRs → deviations → FG
 ③ RMP NAV key (the back door when all FMs are dead)

Per FCOM DSC-22_30-40-80:

The flight crew selects the approach via the ARRIVAL page.

If all the FMCs are fault, the flight crew selects the NAV key on the RMP 1 or 2, in order to manually-tune the frequency, or select a GLS approach, or an ILS approach.

The RMP is the last word in more ways than one — per AMM 22-13-00, an ILS frequency tuned on the RMP forces an ILS approach regardless of what is selected on the MCDU. Whichever door is used, the result converges:

When an approach is selected on the ARRIVAL page, or manually-tuned on RADIO NAV page, or via the RMP, the approach frequency and course are sent to both MMRs. In case of discrepancy between both MMRs the NAV LS TUNING DISAGREE alert triggers.

And low down, the selection is nailed shut:

Note: Below 700 ft RA, when an approach mode is engaged, the flight guidance system freezes the LS selection in the MMR. As a result, the MMR does not take into account any change of the LS selection.

The AMM calls the mechanism ILS TUNE INHIBIT: with LOC/LAND armed or LOC-G/S-LAND TRACK active, the FMGEC freezes the tuned frequency's value and validity in the receivers. So "change the approach only above 700 ft" is not procedural advice — it is a hardware freeze. Below 700 ft your change edits the MCDU's display, not the receiver.

2. Selection conflict — two FG standards, two opposite answers

What if the ARRIVAL page says one approach and the RADIO NAV page is hand-tuned to another? The FCOM answers twice, one block per FG standard — and the answers are opposite:

If the flight crew manually-tunes an LS approach on the RADIO NAV page, the flight guidance system will disregard the manual tuning and guide the aircraft on the approach selected on the ARRIVAL page.

If the flight crew manually-tunes an LS approach on the RADIO NAV page, the flight guidance system will guide the aircraft on the manually-tuned approach.

Variant A: the plan wins. Variant B: the hand-tune wins. Both standards coexist within one fleet, and both annunciate the conflict as CHECK APPR SEL (FMA and MCDU scratchpad). The teaching rule that survives either standard: treat CHECK APPR SEL as "the selections are fighting" and resolve it at the source — set the approach you intend on the ARRIVAL page. Never bet on remembering which standard your airframe carries. A related message: when the ARRIVAL page and the manual tune name different LS facilities, the MCDU shows RWY/LS MISMATCH (the hand-tuned one is flown).

3. The LOC family — arming and two-stage capture

Per FCOM DSC-22_30-40-80:

L2 LOC can only arm when all of the following are applicable: ‐ HDG/TRACK or NAV is engaged, and ‐ The vertical mode is not SRS TO, nor SRS GA, and ‐ The aircraft is above 400 ft RA.

(Trigger: the APPR or LOC pushbutton, with an LS delivered by one of the three doors. Hardware side: at least one LS receiver working, and with both working, their tuned parameters must agree — a disagreement disarms.)

Modern FMS adds a pre-capture stage. Per FCOM DSC-22_30-40-80:

With the enhanced LOC Capture function, the aircraft uses the aircraft position and the FMS trajectory to provide guidance in order to pre-capture the LOC beam.

The pre-capture zone is available, when all of the following conditions are applicable: ‐ The LOC deviation is more than 2.3 dots, and ‐ The FMS is in GPS PRIMARY , and ‐L2 The difference between the track and the LOC course is between 25 ° and 115 °, and ‐ The roll order is such that LOC* will capture the LOC beam with a single turn.

Beyond 2.3 dots the beam is untrustworthy, so the aircraft steers on its own position and trajectory instead — better capture performance, immunity to false lobes, no-overshoot intercepts. The beam capture zone proper:

‐ The LOC deviation is less than 2.3 dots, and: L2 • The difference between the track and the LOC course is between 25 ° and 115 ° , and • The roll order is such that LOC* will capture the LOC beam with a single turn. ‐L1 The LOC deviation is less than 1.3 dot, and the difference between the track and the LOC course is less than 15 °.

Two gates: the wide gate (under 2.3 dots, 25–115° cut, one turn suffices) and the shallow gate (under 1.3 dots at less than 15°). A display subtlety that prevents a false "it didn't capture":

Note: During the LOC capture, the LOC deviation starts to move toward the center of the LS scale, on the PFD and ND, when the LOC deviation is less than 2 dots.

LOC* green with the diamond still parked at the edge simply means you are between 2.3 and 2 dots — scale range, not failure. NAV also lends a hand lining up the cut:

The flight guidance intercepts the LOC axis with a 20 ° track convergence, when all the following conditions are applicable: ‐ The LOC mode is armed, and ‐ The NAV mode is engaged, and ‐ The aircraft is within 20 NM of the destination runway, and ‐ The difference between the aircraft track and the runway course is less than 20 °.

— the anti-parallel-track feature: near the runway with a shallow angle, the FG banks to a 20° convergence so the capture actually happens. Established on the beam, LOC* becomes LOC. The family's reversions (article 10): a change of approach/frequency/course, a DIR TO RADIAL IN/OUT, or an MMR disagreement → back to HDG/TRACK, with the vertical dropping to V/S–FPA in sympathy; pressing APPR with the G/S family armed/engaged likewise knocks it back. (LOC B/C mirrors LOC — armed via the LOC pushbutton, back-beam, no glide.)

4. G/S — the vertical waits for the lateral

Per FCOM DSC-22_30-40-80:

G/S* engages when the aircraft reaches the capture zone of the G/S beam, and all of the following conditions are applicable: ‐ G/S is armed, and ‐ LOC* or LOC is engaged.

No lateral establishment, no vertical capture (a pin option can decouple this on some standards; the teaching baseline keeps lateral-first). The capture geometry:

‐ Passage far from the G/S beam: • The G/S deviation is less than 2 dots, and: • The pitch order is such that G/S* will capture the G/S beam with a single maneuver. ‐ Passage near the G/S beam: The G/S deviation is less than a threshold depending on the radio altitude. This threshold is between 0.1 and 1/3 dot.

G/S* is engaged for more than 15 s, and the G/S deviation becomes less than 1/3 dot.

(That last line is one of the quantified "established" criteria turning G/S* into G/S.) Two behaviours worth keeping from the section's preamble:

G/S or G/S* modes may engage at altitudes that are above the radio altimeter validity. In that case, the landing capability displayed on the FMA (CAT1) reflects the lack of radio altimeter validity until the radio altimeter becomes active. If the radio altimeters fail, or if the FGs don't receive any radio altimeter data, then LOC, G/S and both AP/FDs disengage and FDs re-engage on basic modes.

A high-altitude glide capture legitimately shows CAT1 until the radio altimeters come into range and the capability upgrades (article 12); and total RA loss takes the approach modes and both AP/FDs down — the flight-deck face of article 05's "one RA valid in approach" condition. And the TCAS interaction:

‐ The AP/FD TCAS mode engages upon a RA (if LOC is engaged, LOC remains engaged, APPR pb comes off and the LOC pb comes on)

An RA on approach splits the package: the vertical (G/S) yields to the TCAS mode, the lateral (LOC) stays — and the button lights re-arrange themselves to say so (APPR out, LOC in). Article 15 continues.

5. FINAL APP — the FM-type approach

Per FCOM DSC-22_30-40-80:

The FINAL APP mode includes the following modes: ‐ APP NAV mode for lateral guidance ‐ FINAL mode for vertical guidance.

Arming needs three things:

APP NAV and FINAL modes arms, when all the following conditions are applicable: ‐ The flight crew presses the APPR pb ‐ The active flight plan is valid (lateral and vertical profile) ‐ The approach selected in the ARRIVAL page is compatible with the FINAL APP function

(Which published approaches are FINAL APP-compatible is a normal-procedures table — article 34.) The lateral half holds no mystery:

APP NAV and NAV modes engage under the same conditions: ‐ If NAV is engaged, APP NAV engages immediately ‐ If HDG/TRACK is engaged, APP NAV engages when the intercept conditions are met

APP NAV is NAV in approach dress (article 09). Engagement of the full mode is a six-condition checklist:

FINAL APP mode engages when all the following conditions are applicable: ‐ The aircraft is in the capture zone of the vertical FMS profile

— plus, from the same list: NAV or APP NAV engaged; FINAL armed; FMS predictions available; APPROACH phase active; and the aircraft intercepting a descending leg of the vertical path. The capture zone is tightly drawn:

• Laterally: ±1.5 NM from the Final Descent Point. • Vertically: +250 ft above the Final Descent Point.

Two notes convert directly into line-flying wisdom:

Note: APPROACH phase may have to be manually activated on PERF APPR page if the approach starts at high altitude above aerodrome level (i.e. RNAV(RNP) approaches).

Note: If APPR pb is pressed earlier, FINAL APP mode may engage. As a consequence, resulting speed and altitude management in the FINAL APP may be inappropriate before FDP.

High-elevation RNAV(RNP) approaches routinely stall on the phase condition — activate APPROACH manually on PERF APPR, or FINAL APP sits one condition short forever. And pressing APPR too early can capture you into final-descent management before the FDP, with speeds and altitudes managed as if you were already on final. The ND tells you where you stand:

A blue arrow is displayed on NDs to indicate where the FINAL APP engagement conditions are met and where the final descent will begin automatically.

Blue arrow: all conditions met, the descent will start there by itself. White arrow: at least one condition missing — FINAL APP will not engage and the aircraft will not descend by itself. Two definitions anchor the geometry:

The FDP is the waypoint from which starts the FMS segment with coded FPA. For RNAV(RNP) approaches, this point may be indicated on the chart as FDP or “VIP”.

The FAF is the position from where the obstacle clearance is defined relative to an OCS. Obstacle clearance is only ensured if the aircraft is flying on the defined vertical flight path. Before the FAF, minimum altitudes are published with fixed MOC. The FDP is the point in the procedure at or before the FAF from which a constant vertical flight path is defined.

FDP owns the geometry (start of the constant-path segment); FAF owns the obstacle clearance (OCS reference). They may coincide — or not; the FDP is at or before the FAF, and clearance is only guaranteed while you fly the defined path.

Dissolution and interlocks: FINAL APP auto-disengages at the MAP — past the missed-approach point the script is over. A TCAS RA takes the vertical (FINAL drops, NAV remains, APPR light out); pressing the V/S knob drops FINAL APP to V/S with NAV kept. And the AMM generalises the pattern into a two-axis network: engaging V/S–FPA while FINAL DES is active drags the lateral back to HDG/TRACK; engaging HDG/TRACK while NAV is active drags the vertical back to V/S–FPA. The FM-type approach lives and dies as a pair — try to take back one axis and the system hands you both. A three-dimensional profile split in half is no longer a profile.

6. The speed dimension — the 700 ft memory and wind compensation

Two mechanisms from the AMM's approach chapter (not expanded in the FCOM, high teaching value; per AMM 22-13-00): in automatic approach the FMGEC memorises VAPP at 700 ft RA — lose the FM below 700 ft and the managed approach speed survives, the approach continues; lose it above 700 ft and speed control reverts to selected. And from the FM approach phase onward, the managed speed target carries a ground-speed compensation term (the difference between tower-reported and actual wind), so the approach ground speed never falls below the threshold value — a windshear-margin design, shown by the magenta speed bug on the PFD. This is the A330 documentation's rendering of the ground-speed-mini concept; article 17 and article 18 meet it again from the A/THR and FE sides.

7. Flying it

The two monitoring-card skeletons (developed in article 30): LOC G/S approach — APPR pushbutton → both APs engaged (article 05's dual-AP conditions cashing in) → check LOC/G-S armed → monitor capture → set the go-around altitude → deviation callouts at LOC/GLIDE ½ dot → at 350 ft check LAND (article 13). FINAL APP (RNAV(RNP)) approach — navaid deselection at descent preparation → GPS PRIMARY checked on both FMs → APPR pushbutton → check APP NAV armed/engaged and FINAL armed → at the FDP check FINAL APP engaged → deviation callouts at XTK > 0.1 NM / V-DEV > ½ dot.

From above the glide, the system will not rescue you. Per FCOM standard procedures:

If the aircraft is above the glide slope, the system will not capture the G/S automatically. The pilot must bring the aircraft onto the glide slope beam, and selects an appropriate V/S to intercept it.

(The other face of the near-capture threshold: from above, you are outside the 0.1–1/3 dot funnel.) Changing the approach has its own gate — per FCOM DSC-22_30-40-80, the flight crew can change the approach selection, only when the aircraft is above 700 ft RA — the freeze of section 1. And the MEL closes the menu: a failed APPR pushbutton leaves NAV/FPA, TRK/FPA, LOC/FPA and raw data, capability capped at CAT 1 (article 02, article 32).

[!warning]- Four misconceptions this article corrects (1) Below 700 ft RA an approach change does not happen — the LS selection is frozen in the MMR by hardware; your MCDU edit changes a display, not the receiver. (2) Whether the plan or the hand-tune wins a selection conflict depends on the FG standard — two FCOM variants give opposite answers; on CHECK APPR SEL, fix the ARRIVAL page rather than betting on memory. (3) LOC* engaged with the deviation diamond still parked at the scale edge is normal between 2.3 and 2 dots — the needle only starts moving inside 2 dots. (4) FINAL APP cannot be taken back one axis at a time — V/S during FINAL DES drags the lateral to HDG/TRACK too; the FM-type approach is a pair or nothing.

Self-test

[!note]- Q1. The three approach-selection doors — and does an FM approach on the MCDU survive an ILS frequency tuned on the RMP?

ARRIVAL page; RADIO NAV page manual tune; RMP NAV key (the all-FM-dead back door). No — an RMP-tuned ILS forces the ILS approach regardless of the MCDU selection.

[!note]- Q2. CHECK APPR SEL appears — what does each FG standard fly, and what is the standard-proof response?

One standard disregards the manual tune and flies the ARRIVAL-page approach; the other flies the manual tune. Standard-proof response: recognise a selection conflict and set the intended approach on the ARRIVAL page — don't rely on remembering which variant you have.

[!note]- Q3. The two LOC capture gates, and the four pre-capture conditions?

Gates: deviation < 2.3 dots with a 25–115° track-to-course angle and a single-turn solution; or deviation < 1.3 dots with the angle < 15°. Pre-capture: deviation > 2.3 dots, GPS PRIMARY, angle 25–115°, and a single-turn capture solution — guidance on FMS position instead of the beam.

[!note]- Q4. From how many dots does the LOC deviation actually start moving toward centre?

Two dots — between 2.3 and 2 the diamond sits at the scale edge even though LOC capture is already running.

[!note]- Q5. Why does G/S capture require LOC* or LOC engaged — and will the system capture the glide from above?

The vertical waits for the lateral: established on the localizer first (baseline standard). From above, no — the near-capture threshold (0.1–1/3 dot, RA-dependent) is unreachable from that side; select a V/S and fly back onto the beam yourself.

[!note]- Q6. The six FINAL APP engagement conditions — and which one do high-elevation RNAV(RNP) approaches usually lack?

In the capture zone (±1.5 NM/+250 ft of the FDP); NAV or APP NAV engaged; FINAL armed; predictions available; APPROACH phase active; intercepting a descending leg. The phase condition — activate APPROACH manually on the PERF APPR page.

[!note]- Q7. What does a white ND arrow tell you — and how do FDP and FAF divide their jobs?

White: at least one engagement condition is missing — FINAL APP will not engage and no automatic descent will start there (blue means all conditions met). FDP: geometry — start of the coded constant-FPA segment. FAF: obstacle clearance relative to the OCS. FDP is at or before the FAF; they may or may not coincide.

[!note]- Q8. An RA during FINAL APP: what happens to the lateral, the vertical, and the APPR light?

The vertical FINAL yields to the AP/FD TCAS mode; NAV remains engaged; the APPR pushbutton light goes out (on an ILS the same split keeps LOC and lights the LOC pushbutton).

[!note]- Q9. Can you change the approach below 700 ft RA — and what is the mechanism?

No — with an approach mode engaged below 700 ft, the FG freezes the LS selection in the MMRs (ILS TUNE INHIBIT); changes no longer reach the receivers.

Key takeaways

References

Approach selection, the two FG-standard conflict variants, the RMP back door, the MMR chain and the 700 ft freeze per FCOM DSC-22_30-40-80 (general section); LOC family arming, enhanced pre-capture, capture gates, display note, convergence and reversions per the LOC section; G/S dependencies, capture geometry, high-altitude capture, RA validity and TCAS split per the G/S section; FINAL APP composition, arming/engagement conditions, capture zone, phase and early-APPR notes, arrow language and FDP/FAF definitions per the FINAL APP section (all FCOM DSC-22_30-40-80). RMP-forces-ILS, ILS TUNE INHIBIT, the VAPP 700 ft memory, ground-speed compensation and the two-axis exit network per AMM 22-13-00. Glide-from-above technique per FCOM standard procedures; monitoring-card skeletons per a representative operator QRH. GLS/SLS and the F-LOC/F-G/S branches are quoted as printed; those functions are not installed on the baseline airframe of this series (article 01).

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.