Lateral Guidance and Leg Transitions

Article 21 laid the road; this article is how the flight guidance rounds the corners: who designs the transition between two legs, why the ND sometimes draws the bend and sometimes doesn't, what XTK actually measures, and why NAV capture is sometimes instant and sometimes patient. It is the most AMM-dense article of the series — the pilot value is being able to read the curve on the ND. (The AMM section quoted carries the Thales-configuration text — FSN 105-150 and 352-400 group; the transition geometry is generic FMS2 mechanics, with the parallel clauses in the other configuration's chapter.)

1. One road, two geometries

Per AMM 22-72-00:

The lateral guidance is responsible for the construction of a lateral path of the ACTIVE primary flight plan. This lateral path consists of the active leg path geometry to be used to control the aircraft and the multiple leg geometry which is used for generating EIS displays. The lateral path is made of geometric legs and transitions between the legs.

Two builds of the same road: the active-leg path geometry (active leg + next leg + the transition between) flies the aircraft; the multiple-leg geometry draws the ND. The ND's recursion rule explains why you sometimes see further ahead: the display normally carries active + next + their transition — but if the next-to-third transition is a Type III or V, the third leg and its transition are drawn too, and third-to-fourth recurses once more. How far the bends are drawn is itself a signboard of the transition types ahead. Four leg families are guidable: great-circle courses, planar magnetic courses, magnetic headings, and circular arcs (AF/HX) — the geometric face of article 21's leg registry.

2. The five transitions — a taxonomy of bends

Per AMM 22-72-00:

(a) Type I is a curved path transition between two fix referenced legs (FX or XF) that terminate and begin at the same point. ... It is performed with a constant turn radius. The nominal bank angle is then corrected to take ground speed evolution into account.

Type I is the standard fly-by inside cut: constant radius, bank corrected for evolving ground speed — the reason high-altitude turns anticipate so far on the ND (fast ground speed, big radius, early lead). Its understudy:

(b) Type II is a next course capture between two fix referenced legs (which are not connected) or from a heading/course leg (VX or CX legs) to a fix referenced leg. ... Substitution of a type II transition for a type I transition can also occur under the following conditions: - an overfly has been specified for the active leg - a turn direction is specified for the next leg which causes course change of greater than 180 deg. (except for PX, AF, HX) - course change between active and next leg is less than or equal to 3 deg. or more than or equal to 175 deg. - distance between the termination of the active leg and the path of the next leg is greater than 1 NM.

Type II is a next-course capture — fly through, then cut back. It substitutes for Type I in four cases: an overfly on the active leg (there is the geometric truth of the Δ key — article 21); a specified turn direction forcing a course change beyond 180°; a course change of ≤ 3° or ≥ 175° (too straight to need a bend, too sharp to cut one); or the legs more than 1 NM apart (nothing to connect). Type II also owns the establishment and cancellation of parallel offsets, and the three holding entries. So: overfly is not "fly a bit further" — it is a different transition: the smooth inside cut becomes an over-the-fix swing-out and recapture, visible in the ND's changed shape. Likewise, no bend drawn between two near-collinear legs is not a display omission — ≤ 3° earns no transition at all.

The holding entries' inner geometry (article 21 said "system-chosen"; here is the drawing):

2 Teardrop entry The aircraft captures a 30 deg. intercept course from the holding pattern fix and holds this course until the distance from the holding fix is equal to 2.56 times the radius of the circular arc of the holding pattern (fixed path control law). Then, the aircraft performs a circular arc (turn control law) (constant turn radius, course change = 180 deg.) before capturing the inbound leg using a type II transition.

A 30° cut-out from the fix, held to 2.56 hold-radii, then a 180° constant-radius arc, then a Type II capture of the inbound — that 2.56 R is the teardrop's length. The parallel entry is the same construction along the reciprocal; the direct entry is a straight Type II capture of the first turn or outbound leg; and the entry type is decided by the sector you cross the fix from.

The remaining three:

(c) Type III is a transition onto a heading leg (VX legs)... (d) Type IV is a transition onto a direct type leg (DF). A circular arc path is used to capture the great circle course of a DF leg... (e) Type V is a transition onto course legs that are not fix referenced (CA, CD, CR legs).

III onto heading legs, IV onto direct-to-fix legs (an arc capturing the great circle), V onto non-fix-referenced course legs. Four boundary notes from the table: an IF fix belongs to the next leg and takes no transition; inside a discontinuity there is no transition — NAV disengages (article 21's gap, in geometry); adjoining arc legs must share a radius; and a next leg with a conditional altitude turns into a DF leg once the condition is met, else transitions as unconditional.

3. XTK and TKE — the two rulers

Per AMM 22-72-00:

- The TRACK ANGLE ERROR is always the difference between the DESIRED TRACK and the TRUE TRACK. - The CROSSTRACK ERROR to a course leg is the perpendicular distance to the leg. ... - The CROSSTRACK ERROR to a circular arc is the difference between the distance from the aircraft position to the turn center and the turn radius of the arc.

TKE is the angular ruler (desired minus true track, one definition everywhere). XTK is the distance ruler with two readings: to a straight leg, the perpendicular distance; to an arc (holds, AF legs), the distance to the turn centre minus the radius — XTK = 0 in a turn means "standing exactly on the circle". The two rulers feed three mouths — the lateral control laws, path-capture decisions, and sequencing — and only XTK is displayed (TKE is internal).

Reference parameters, four per leg: bearing/direct distance/time to the leg's termination (TTG = DTG ÷ current ground speed) plus the desired track; a leg without a fixed endpoint (altitude- or radial-terminated) references its predicted endpoint; a MANUAL leg computes only the desired track (article 21's endless line, quantified). Three destination distances coexist: DIRECT DIST (great circle via the LOC-capture point to the runway), ALONG TRACK DIST TO GO (down the active leg — the one performance eats), and DIST TO DEST (sum of remaining legs + along-track; F-PLN page line 6). And the transitions swap reference frames: in a Type I, DTG/TTG/BTG reference the active leg while XTK/desired track reference the transition curve; in a Type II everything references the captured path (holding entries reference the fix); in a Type IV the XTK references the arc until it is sequenced — the decoder ring for "the ND numbers look odd in the bend".

4. NAV capture — and the control-law family

Per AMM 22-72-00:

Managed control engagement depends upon whether the aircraft is within the capture zone of the active leg. This capture zone is unlimited for heading/track legs (VX or CX legs except CF leg). This means that if LAT AUTO control is armed while one of these legs is active, it is immediately engaged. For fixed path legs, the capture zone is a 1 NM wide zone around the active leg, or at the capture point which allows a comfortable F-PLN capture without overshoot.

The capture zone depends on the leg: heading/course legs — unlimited (arm NAV on one and it engages instantly); fixed-path legs — a 1 NM band around the leg, or the no-overshoot capture point (the AMM original behind article 09's ~1 NM shepherd move and INTCPT conditions). In the zone: engaged, FCU window to dashes; outside: the HDG/TRK value stays live and adjustable until capture. DIR TO does more than point:

Note that performing a DIR TO revision engages NAV if it was not active. In this case, if the track corresponding to the selected heading/track intercepts the F-PLN before the TO waypoint, the F-PLN becomes as shown in the following figure.

If your present track would cut the plan before the TO fix anyway, the FMS rebuilds the plan around an intercept geometry rather than commanding a hard turn. Managed lateral is also re-checked at three moments — a fresh managed push on the FCU, a plan revision touching the active leg, an offset inserted or cancelled — and a lost managed state falls to HDG/TRK synchronised on current values (article 02's window synchronisation, guidance face).

Inside NAV, four control laws share the work:

1 If the active leg is a course leg (CX except CF leg), the desired track is generated and the system holds this track (track control law). 2 If the active leg is a heading leg (VX legs), the desired heading is generated and the system holds this heading (heading control law). 3 In lateral path-mode, the bank angle command is generated by comparing the current A/C state vector (position, ground speed, track) to the desired lateral path. - For circular arc (AF), the turn control law maintains a constant radius using the nominal bank angle. ... - For straight line segments, the fixed path control law holds the desired path using crosstrack error and track angle error. The nominal bank angle is set to zero.

Track law for course legs, heading law for heading legs, turn law for arcs (nominal bank holds the radius, XTK/TKE trim it), fixed-path law for straight segments (double feedback, zero nominal bank). One holding circuit changes law four times — and the ride stays smooth because the reference frames swap cleanly (section 3). The Type II bank discipline:

- Type 2: The fixed path control law is used. Crosstrack error and track angle error are used to capture the desired track. If a turn direction is specified for the next leg, the bank angle command is set to 25° until the track angle error is less than 60° and then the fixed path control law is used.

Coarse first, fine second: 25° of bank until the TKE closes inside 60°, then the fixed-path law polishes. And the capture-law selection at NAV engagement: heading leg → heading law; course leg → track law; DF leg → fixed-path law; anything else judged twice — TKE > 90° (facing away) → fixed-path law hauls you around; otherwise compare XTK with the crosstrack limit: beyond it, hold the current heading (heading law) until the limit, then capture with the fixed-path law; within it, capture immediately. A plan revision mid-capture yields to the new active leg's normal law.

5. Dual-FM path consistency

The other configuration's chapter dedicates a paragraph to dual behaviour (attributed summary, per AMM 22-70-00): each FM builds its own lateral path, and in dual mode the crosstalk keeps the two paths consistent. This is the mirror image of article 19's INDEPENDENT warning (two different active legs, an AP switch turns the aircraft): in healthy dual operation the two computers' geometry is reconciled — whichever FMGEC is master, the FMA and FD draw the same bend.

6. Reading the bend — operations

On the ND (each rule quoted above): a big turn anticipation = high ground speed (Type I constant radius); swing-out past the fix and recapture = overfly or one of Type II's four conditions; no bend between near-collinear legs = the ≤ 3° exemption; bends drawn further downstream = Type III/V recursion; the line breaking at a discontinuity = NAV really will drop there — not a rendering bug.

XTK monitoring calibration (the geometry behind the callouts of article 30 and article 34): on straight segments XTK is the intuitive perpendicular distance; in a bend XTK references the arc or transition curve — so the reading stays valid inside an RF leg or a holding turn (radial distance off the circle), and the RNP AR 0.1 NM callout works mid-turn. Interfaces: the FCOM's NAV arming/engagement conditions (article 09) and this article's AMM capture zones are two faces of one mechanism; sequencing rules live in article 21, whose DIR TO procedures get their geometric floor here.

[!warning]- Four misconceptions this article corrects (1) Overfly does not add distance — it swaps the transition type: the inside cut (Type I) becomes an over-the-fix recapture (Type II), and the ND's shape change is the proof. (2) No bend drawn between nearly-aligned legs is by design — course changes of 3° or less take no transition at all. (3) XTK does not go blind in turns — on an arc it reads the radial distance off the circle, so deviation callouts remain valid inside RF legs and holds. (4) NAV arming does not always wait — on heading/course legs the capture zone is unlimited and engagement is immediate; only fixed-path legs demand the 1 NM band.

Self-test

[!note]- Q1. The two geometries' names and consumers — and why does the ND sometimes draw two extra legs of bends?

Active-leg path geometry controls the aircraft; multiple-leg geometry generates the EIS display. When the next-to-third transition is Type III or V, the third leg and its transition are drawn, recursing once more to the fourth.

[!note]- Q2. Type I's two geometric signatures — and why is the bank corrected by ground speed?

A curved path between fix-referenced legs meeting at one point, flown at constant turn radius. Ground speed evolves through the turn, so the nominal bank is corrected to hold that radius — hence bigger anticipation at altitude.

[!note]- Q3. The four conditions that substitute Type II for Type I — and what does overfly change geometrically?

Overfly specified on the active leg; a specified turn direction forcing more than 180° of course change; course change ≤ 3° or ≥ 175°; leg-end to next-leg distance over 1 NM. Overfly replaces the inside cut with fly-through-then-capture.

[!note]- Q4. In the teardrop entry, what are the 30° and the 2.56 R — and what decides the entry type?

A 30° intercept course away from the fix, held until 2.56 holding-arc radii from it, then a 180° constant-radius arc and a Type II capture of the inbound. The sector from which the fix is crossed decides direct/teardrop/parallel.

[!note]- Q5. How is XTK defined for an arc — and is TKE shown on the ND?

Distance from the aircraft to the turn centre minus the turn radius — zero means standing on the circle. TKE (desired minus true track) is internal; only XTK is displayed.

[!note]- Q6. Which reference parameters exist on a MANUAL leg?

Only the desired track — no termination point means no BTG/DTG/TTG.

[!note]- Q7. The NAV capture-zone widths for heading/course legs versus fixed-path legs?

Unlimited for heading/course legs (VX/CX except CF) — arm and it engages at once. A 1 NM band (or the no-overshoot capture point) for fixed-path legs.

[!note]- Q8. The Type II two-stage bank discipline when a turn direction is specified?

Bank commanded at 25° until the track angle error closes below 60°, then the fixed-path law (XTK + TKE feedback) takes over for the fine capture.

[!note]- Q9. Which law captures when TKE exceeds 90°? And when XTK exceeds the crosstrack limit?

Facing away (TKE > 90°): the fixed-path law hauls the aircraft around directly. XTK beyond the limit: heading law holds the current heading until reaching the limit, then the fixed-path law captures; within the limit, capture is immediate.

[!note]- Q10. Why do both FMs draw the same bend in dual mode?

Each builds its own lateral path, but dual-mode crosstalk keeps the paths consistent — the reconciled geometry is why an AP change in healthy dual operation does not turn the aircraft (unlike INDEPENDENT operation).

Key takeaways

References

Lateral-path construction, the two geometries and display recursion per AMM 22-72-00 (lateral guidance, description and operation); the five transition types, substitution conditions, holding-entry geometry and boundary notes per its transition paragraphs; reference parameters, XTK/TKE definitions, destination distances and frame swaps per its guidance-parameter paragraphs; capture zones, DIR TO reconstruction, managed re-checks, the four control laws, the 25°/60° discipline and capture-law selection per its control paragraphs (Thales-configuration text; generic FMS2 mechanics, parallel clauses in the other configuration's chapter). Dual-path consistency per AMM 22-70-00 (attributed summary). The ND-reading guide is an integrative synthesis of the quoted rules.

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.