Lateral Modes

The lateral axis has a simpler cast than the vertical — but every handover ("who is steering, and when does it change") hides a trap. This article covers HDG/TRK, the preset function, NAV, and the two takeoff stewards RWY and RWY TRK. The LOC family belongs to article 11, GA TRK to article 14, and leg-transition geometry to article 23.

The design philosophy mirrors the vertical axis: when the managed world (NAV) fails, the fall is always onto the selected basic modes (HDG/TRK) — with the takeoff segment adding two transitional stewards to hand the aircraft safely into NAV or into your hands.

1. HDG/TRK — the lateral basic modes

Per FCOM DSC-22_30-40-30:

HDG and TRK are selected lateral modes. They are the basic modes of lateral guidance.

The main entry is pulling the HDG/TRK knob, with the by-now familiar no-go zones:

‐ The aircraft is in flight, for at least 5 s after liftoff, or ‐ Not in LAND, FLARE, or ROLL OUT, or ‐ Not in GA TRK below 100 ft RA.

The landing script and the first segment of go-around are protected from a manual lateral grab — exactly symmetric with V/S's restrictions in article 08. Other entries (same section): a DIRECT TO RADIAL IN/OUT inserted while in NAV, loss of FINAL in APP NAV, re-engaging AP/FD after all were off (basic modes HDG+V/S — article 05), arming LOC after FINAL APP, deselecting APPR/LOC above 400 ft RA, and deselecting LOC on a LOC-only approach.

The presentation pair switches as a set. Per FCOM DSC-22_30-40-30:

At the aircraft power-up, the HDG-V/S / TRK-FPA selection is set to HDG-V/S by default.

When TRK-FPA is selected: ‐ TRK appears on the HDG/TRK window of the FCU ‐ FPA appears on the V/S / FPA window of the FCU ‐ HDG and V/S mode(s) cannot be engaged ‐ TRACK and/or FPA modes can be engaged ‐ Both PFDs display the FPV in green.

No HDG+FPA mix-and-match — the pair is one selection, the window values convert to equivalents at the switch (article 02), and the FPV that appears is a statement of path, not a command (article 05 — the command symbol in this presentation is the FPD).

2. Preset — storing an intention in the window

Per FCOM DSC-22_30-40-30:

If the flight crew decides not to fly the flight plan after takeoff or go-around, the flight crew can turn the HDG/TRK knob on the FCU to preset a heading or a track. The set value remains displayed in the FCU HDG/TRK window until the flight crew pulls the knob.

If the flight crew presets the HDG/TRK before takeoff and up to 30 ft RA, NAV disarms. The preset value remains displayed in the HDG/TRK window (even when RWY TRK engages at 30 ft RA)

After go-around engagement, HDG/TRACK will engage, when the flight crew pulls the HDG/TRK knob. When the HDG or TRACK engages, the heading/track target is the HDG/TRK preset.

When overflying the MAP, HDG/TRK will synchronize with the current aircraft heading / track. The HDG/TRK preset function is no longer available.

The flight crew can cancel a HDG/TRK preset with one of the following action: ‐ The HDG/TRK knob is pushed, or ‐ A DIR TO is selected in the FMS, or ‐ The AP and FDs are disengaged.

So: a preset survives indefinitely (the 45-second evaporation rule does not apply to it), it disarms NAV when made before takeoff, it is redeemable after a go-around by pulling the knob — and it dies at the missed-approach point, where the window synchronises to the current heading/track.

Put this side by side with article 08's V/S pre-selection trap and you have a deliberately contrasting pair: the lateral preset is an official feature (persistently displayed, honoured after go-around), while the vertical pre-selection is a scratch value (45 s to live, and an ambush target for reversions in the meantime). Don't let students transplant one axis's rules onto the other.

3. NAV — managed, and proud of zero crosstrack

Per FCOM DSC-22_30-40-30:

The NAV mode is a managed lateral mode, that guides the aircraft laterally along the FMS flight plan. It is designed to have a zero crosstrack error. NAV usually arms before engaging.

Arming — five doors, one veto list:

NAV arms, when one of the following occurs: ‐ The aircraft is on ground, and the flight crew does not preset a HDG/TRK for takeoff, or ‐ The flight crew pushes the HDG/TRK knob on the FCU, or ‐ The flight crew activates a DIRECT TO RADIAL IN or DIRECT TO RADIAL OUT when in HDG/TRK mode, or ‐ The flight crew presses the APPR pb and the conditions to arm the FINAL APP mode are applicable, or ‐ The flight crew initiates a go-around.

(On-ground arming is automatic provided you haven't preset; the go-around door reflects the "NAV armed at go-around" option, active on the baseline airframe of this series — article 01.)

However, NAV cannot arm, if one of the following modes is engaged: LOC*, LOC, F-LOC* , F-LOC , LOC B/C*, LOC B/C, LAND, FLARE, ROLL OUT.

Engagement — four ways:

NAV engages, if one of the following occurs: ‐ NAV is armed, and the aircraft reaches 30 ft RA after takeoff, or ‐ NAV is armed, and the aircraft reaches the capture zone of the flight plan active leg, or ‐ The flight crew activates a DIR TO on the MCDU, or

‐L1 The flight crew selects the HDG/TRK when the aircraft is close to (within ~1 NM of) the active flight plan leg, or

DIR TO is the universal "give me NAV back" key (ineffective below 700 ft RA with the LOC family or landing script engaged), and the fourth door is the shepherd move: lead the aircraft to within about a mile of the active leg with HDG/TRK and NAV snaps on. For the armed-capture case, three things must all be true:

When NAV is armed, it automatically engages, if: ‐ The aircraft track line intercepts the flight plan between the FROM and the TO waypoints, and ‐ The intercept waypoint (INTCPT) is displayed on the ND, and ‐ The aircraft reaches the capture zone of the flight plan active leg.

No INTCPT symbol on the ND — don't expect the cyan NAV to turn green. And once it does:

When NAV is engaged, the HDG/TRK window on the FCU displays dashes.

4. NAV loss — and the vertical chain reaction

Per FCOM DSC-22_30-40-30:

NAV disengages, and reverts to the HDG/TRACK mode when: ‐ The aircraft reaches a flight plan discontinuity ‐ The flight plan becomes invalid.

Both land on HDG/TRACK at the current value, with the full reversion annunciation set. But the real cost is vertical:

When NAV is engaged with the managed vertical modes CLB or DES or FINAL APP, the guidance takes into account the altitude and speed constraints, linked to waypoints on the lateral flight plan. When NAV disengages, the managed vertical modes are not available and the altitude constraints are disregarded.

Constraints live on lateral waypoints — kill NAV and the whole managed vertical family loses its footing. The precise mapping (one source of article 10's master table):

Note: If a managed vertical mode was previously engaged, a selected vertical mode engages: ‐ If CLB was engaged, OP CLB engages ‐ If ALT CST*, ALT CRZ*, or DES was engaged, V/S / FPA engages ‐ If ALT CST was engaged, ALT engages.

Anything already level stays level (ALT CST → ALT); anything in motion gets a steady-state substitute (V/S). The rule to hang on the wall: the lateral plan is the vertical plan's landlord — NAV moves out, CLB/DES and every constraint move out with it. One uncleaned F-PLN discontinuity costs you three things at once: lateral to HDG, vertical to selected, constraints void (article 21's insistence on clearing discontinuities is guidance-layer, not housekeeping).

5. RWY and RWY TRK — the takeoff stewards

Per FCOM DSC-22_30-40-20:

The RWY guidance law aims to maintain the aircraft on the runway centerline during the takeoff run. When the aircraft is airborne, the RWY guidance law aims to maintain the sideslip indicator centered.

(The centre-line command is the green yaw bar below 30 ft — article 05.) RWY is choosy about engaging:

RWY engages, if all of the following conditions are applicable: ‐L2 V2 is inserted in the PERF TAKEOFF page, and ‐ The aircraft is on ground, and ‐ The slats are extended, and ‐ The LOC is valid, and ‐ The LOC deviation is below 1/2 dot, and ‐ The aircraft heading is within 20 ° of the ILS (GLS ) (SLS ) course, and

(plus: the LS course within 2° of the F-PLN runway heading, no lateral mode already engaged, thrust levers in the detent). No localizer available or a condition unmet — RWY simply doesn't engage, and a blank lateral FMA on the takeoff roll is a normal presentation: the lateral column fills at 30 ft. Which brings us to the junction:

RWY disengages, when one of the following occurs: ‐ The LOC is lost below 30 ft RA ‐ NAV is armed, and the aircraft reaches 30 ft RA: NAV engages ‐L2 NAV is not armed, and the aircraft reaches 30 ft RA: RWY TRK engages

30 ft RA is the lateral fork: NAV armed → NAV; NAV not armed → RWY TRK; LOC lost before it → RWY steps down early. (RWY also drops if the heading diverges more than 20° from the LS course.) The steward that takes over:

The RWY TRK mode guides the aircraft along a constant track target. The track target is the aircraft track at RWY TRK engagement.

Read carefully: the track at engagement — not the runway direction. Lift off with a 5° drift correction in a crosswind and RWY TRK faithfully keeps that 5°; it will not steer you back over the extended centre line. Its exits:

RWY TRK disengages, when one of the following occurs: ‐ The flight crew pulls the HDG/TRK knob: HDG/TRACK engages. ‐ NAV is armed, and the aircraft reaches the capture zone of the active leg: NAV engages. ‐ The flight crew inserts a DIRECT TO waypoint: NAV engages.

And the trap with a database explanation. Per FCOM standard procedures:

Note: If a radar-vector standard instrument departure (SID) has been selected, a F-PLN discontinuity immediately follows the departure runway, and NAV mode is not armed. After takeoff, RWY TRK mode engages until the crew selects a HDG or performs a DIR TO.

A radar-vector SID encodes a discontinuity right after the runway — so NAV cannot arm, and RWY TRK after takeoff is the inevitable, correct result. Not a malfunction.

6. Flying the lateral axis

Three takeoff plans (synthesis, developed in article 30):

Two keys back into NAV whenever you've left it: DIR TO (any time, except below 700 ft on approach), and the ~1 NM shepherd move. Dispatch interlocks (article 32, some operators' MEL practice): a failed ND mode/range selector affecting the PF side bars the whole PBN family (managed lateral needs its display); both FMAs' lateral indications failed bars RNAV/RNP operations (article 06).

[!warning]- Four misconceptions this article corrects (1) The lateral preset and the vertical V/S pre-selection follow opposite rules — the preset persists and is honoured later; the V/S value dies in 45 s and can ambush you meanwhile. (2) NAV loss is never just lateral — managed vertical modes become unavailable and all constraints are disregarded (CLB→OP CLB, moving captures→V/S, ALT CST→ALT). (3) RWY TRK holds the track at engagement, drift correction included — it does not fly you back to the runway centre line. (4) A blank lateral FMA during the takeoff roll (no LOC available) and RWY TRK after a radar-vector SID are both normal, database-explained presentations — call what you see, fix with HDG or DIR TO.

Self-test

[!note]- Q1. Five ways NAV arms — and what single crew action disarms it before takeoff?

On ground automatically (no preset made); pushing the HDG/TRK knob; DIR TO RADIAL IN/OUT from HDG/TRK; pressing APPR when FINAL APP can arm; initiating a go-around (option-dependent, active on the baseline airframe). Presetting a HDG/TRK before takeoff (up to 30 ft RA) disarms it.

[!note]- Q2. The 30 ft RA fork — what are the three roads and their criteria?

NAV armed → NAV engages. NAV not armed → RWY TRK engages. LOC lost below 30 ft → RWY disengages early (yaw-bar guidance gone).

[!note]- Q3. For RWY to engage, what must the LOC deviation and aircraft heading satisfy — and the LS-course-to-runway agreement?

LOC valid with deviation below ½ dot; heading within 20° of the LS course; and the LS course within 2° of the F-PLN runway heading (plus V2 inserted, on ground, slats out, no lateral mode engaged, levers in the detent).

[!note]- Q4. What track does RWY TRK hold? In a strong crosswind, will it regain the extended centre line?

The aircraft's track at the moment of engagement — drift and all. No: it keeps that track; regaining the centre line takes a preset/HDG or NAV.

[!note]- Q5. Why is RWY TRK the expected mode after a radar-vector SID — and the two ways back to NAV?

The database encodes a discontinuity immediately after the runway, so NAV never arms. Back to NAV: DIR TO, or lead the aircraft to within ~1 NM of the active leg with HDG/TRK (or arm and let the capture zone do it).

[!note]- Q6. When NAV disengages, where do CLB, DES and ALT CST go — and do constraints still apply?

CLB → OP CLB; DES (and ALT CST*/ALT CRZ* captures) → V/S–FPA; ALT CST → ALT. Constraints are disregarded — they live on lateral waypoints.

[!note]- Q7. Three ways to cancel a preset — and does it survive past the missed-approach point?

Push the knob, select a DIR TO, or disengage AP and FDs. No: overflying the MAP synchronises the window to the current heading/track and the preset function lapses.

[!note]- Q8. In TRK-FPA presentation, can HDG mode engage? What extra symbol appears on the PFDs?

No — HDG and V/S cannot engage; TRACK and FPA can. Both PFDs display the green FPV (the bird), which shows path, not guidance orders.

Key takeaways

References

HDG/TRK, presentation switching, preset behaviour, NAV arming/engagement/disengagement and the vertical-mode mapping per FCOM DSC-22_30-40-30; RWY and RWY TRK clauses per FCOM DSC-22_30-40-20; the radar-vector SID note per FCOM standard procedures (article 30); lateral mode definitions per FCOM DSC-22_30-40-10 (article 06). Dispatch interlocks reflect some operators' MEL practice. The takeoff-plans table and the landlord framing are integrative syntheses. GLS/SLS and the F-LOC/LOC B/C mode names appear in quoted condition lists for completeness; 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.