A320 Auto Flight: Approach, Landing and Go-Around

On approach the A320's auto flight system narrows its task to a single runway. The APPR pushbutton on the FCU arms the approach modes; the autopilots and flight directors then capture the final approach path and, on an ILS with the right capability, fly the flare and hold the centreline after touchdown. The FMA shows what is armed, what is engaged and what landing capability the aircraft has.

This article follows the approach from arming to touchdown, then covers the go-around. The general modes, FCU and FMA are described in A320 auto flight: FCU and AP/FD modes; the principles in autoland and low-visibility operations.

Approach modes and the APPR pushbutton

The APPR mode depends on the approach selected in the flight plan. For an ILS, pressing APPR arms LOC and G/S; for a non-precision approach it arms APP NAV and FINAL. The crew press it once cleared for the approach, on an intercept trajectory for the final approach course and with localiser deviation available; LOC and G/S capture no sooner than 3 s after arming.

With APPR selected both autopilots should be engaged; the second AP can engage only once LOC and G/S are armed, captured or tracked. Above 5,000 ft AGL the FMA shows CAT 1 (APPR1 for an approach with SLS); below it, the capability available for the intended approach. Pressing APPR or LOC again disarms the approach modes, whether the guidance is ILS, MLS, GLS, FLS, SLS or FINAL APP.

ILS approach: LOC and G/S

LOC can arm only with HDG, TRK or NAV engaged, the vertical mode not SRS, and the aircraft above 400 ft RA. LOC* (capture) engages in the capture zone: a deviation below 2.3 dots when the geometry allows a single-turn capture, with the track 25° to 115° from the localiser course, or below 1.3 dot in any case. Enhanced LOC capture adds a pre-capture zone beyond 2.3 dots, used when the FMS is in GPS PRIMARY, the intercept angle is 25° to 115° and a single-turn capture is possible. LOC (track) follows once the aircraft is established. Above 700 ft RA, a DIR TO inserted during LOC* returns the aircraft to NAV.

The LOC convergence function helps NAV hand over to the localiser: it applies with NAV engaged and LOC armed, within 20 NM of the destination runway, when the track differs from the runway course (QFU) by less than 20°.

G/S* engages when the aircraft reaches the glideslope capture zone with G/S armed and LOC* or LOC engaged; from ALT the change is automatic. When the aircraft is established on the localiser but above the glideslope, Airbus recommends V/S 1,500 ft/min initially, with the FCU altitude set above the aircraft; above 2,000 ft/min the speed rises towards VFE, which the AP then holds by reducing the vertical speed. On final the PM calls a deviation of half a dot on either beam. If the radio altimeters fail, LOC, G/S and both AP/FDs disengage and the FDs return in the basic modes. Above 100 ft RA the pitch bar flashes if the vertical deviation cannot be computed; above 15 ft RA the roll bar does so for the lateral deviation.

RNAV approach: FINAL APP

FINAL APP combines APP NAV, the managed lateral approach mode, with FINAL, the vertical one, to fly a non-precision approach along the FMS final descent profile. It engages with NAV or APP NAV engaged, FINAL armed, predictions available and the approach phase active, in the capture zone of the profile: ±1.5 NM from the final descent point (FDP) and up to 250 ft above it.

FINAL APP disengages at the missed approach point (MAP), on a go-around, when the AP/FD TCAS mode engages, or when the crew push or pull the V/S knob. If NAV is lost, the modes revert to HDG or TRK and V/S or FPA. The AP disconnects at the MAP, at MDA (or MDH) minus 50 ft, or at 400 ft AGL if no minimum was entered. The minimum height for AP/FD use with FINAL APP, V/S or FPA is 250 ft AGL, and DES or OP DES may be used on approach only with the FCU altitude at or above the higher of the MDA/MDH and 500 ft AGL.

Managed FMGS guidance is certified for instrument approaches other than the final segments of ILS, LOC, LOC B/C, LDA, SDF, GLS, MLS and FLS approaches. An RNAV (GNSS) approach requires GPS PRIMARY, and RNP AR to 0.3 NM is demonstrated with the AP on, the final approach in FINAL APP (see RNP approaches).

FLS approach: F-LOC and F-G/S

The FLS function guides with two modes: F-LOC, lateral, and F-G/S, vertical, captured through F-G/S*. It is certified, with the FMS giving lateral and vertical guidance, for RNAV and RNAV (GNSS) approaches with LNAV/VNAV or LNAV minima and for GPS, VOR, VOR/DME, NDB and NDB/DME approaches; and, with the localiser for lateral and the FMS for vertical guidance, for LOC, ILS with the glideslope out and LOC B/C approaches. The minimum height for AP/FD use in F-G/S is 200 ft AGL. The separate SLS function is certified for RNAV (GNSS) approaches with LPV or LP minima, showing APPR1 on the FMA, with a minimum of 160 ft AGL.

Decelerated approach and stabilisation

The decelerated approach, flown with FD or AP/FD guidance, is the standard technique for ILS, MLS and GLS approaches and approaches with FLS, SLS or FINAL APP guidance. The aircraft reaches 1,000 ft in the landing configuration at VAPP; in most cases it is in CONF 1 at S speed at the FDP. Flaps 1 are selected at green dot more than 3 NM before the FDP, and the aircraft must be established on the final descent with flaps 1 and S speed at or above 2,000 ft AGL. Flaps 2 follow at 2,000 ft AGL at the latest; on an ILS, MLS, GLS, SLS or FLS approach intercepted below 2,000 ft AGL, they are selected one dot below the path.

The approach must be stabilised at the stabilisation height: 1,000 ft above aerodrome elevation in IMC, 500 ft in VMC, or another height set by the operator. The aircraft must be on the correct path, in the landing configuration, at the target speed with thrust stabilised, without excessive deviations; in IMC speed and thrust may stabilise later if the operator allows, but not below 500 ft. Otherwise the crew go around, unless only small corrections are needed (see stabilised approach). A PF who intends to land with manual thrust disconnects the autothrust at 1,000 ft above aerodrome level at the latest.

Landing capability and AUTOLAND warning

The FMA shows the approach and landing capability, which reflects the systems available:

Below 200 ft RA, CAT 3 SINGLE or DUAL is memorised as long as an AP is engaged in LAND, FLARE or ROLL OUT.

Automatic landing is approved in CONF 3 and FULL and demonstrated with glideslopes from −2.5° to −3.15°, airport elevations up to 2,500 ft and weights below the maximum landing weight. The wind limits are 30 kt headwind, 10 kt tailwind and 20 kt crosswind, as reported by the tower. With an engine out, CAT II and CAT III fail-passive autoland are approved only in CONF FULL, the engine-out procedures being completed before 1,000 ft. VAPP for an autoland is at least VLS + 5 kt.

The AUTOLAND warning light flashes below 200 ft RA, with LAND or FLARE on the FMA, when one of its trigger conditions is met; among them is a localiser deviation of more than a quarter of a dot while the aircraft is above 15 ft RA.

View full schematic

LAND, FLARE and ROLL OUT

LAND engages below 400 ft RA with LOC and G/S engaged and a CAT 1, 2 or 3 capability, and tracks both beams down to about 40 ft. Once it is engaged, V/S, FPA and DES can no longer engage. At 350 ft the PF checks LAND on the FMA: without it, no autoland. If the ILS course pointer and the runway track differ by more than 5°, the crew go around or land manually with sufficient visual references.

FLARE engages at about 40 ft RA. It is a common mode: in yaw it reduces the sideslip to align the aircraft with the runway, in pitch it reduces the rate of descent. Below 30 ft RA the PFD replaces the roll bar with a green yaw bar. In an automatic landing the autothrust's RETARD mode engages at about 30 ft and sets idle thrust (THR IDLE on the FMA). The RETARD callout comes at 10 ft in an automatic landing, 20 ft in a manual one, and repeats until the levers are at idle or reverse; the PF then sets the levers to idle and the autothrust disconnects.

ROLL OUT engages at touchdown: it tracks the centreline and lowers the nosewheel, and the pitch bar disappears. It disengages, with the AP/FDs, if the aircraft's track differs from the runway's by more than 20°. With a nosewheel steering or anti-skid failure the AP is set off at touchdown; otherwise it is disengaged at the end of the roll-out.

Go-around and discontinued approach

At or above the FCU altitude, the crew may either go around or use the discontinued approach technique: announce "CANCEL APPROACH", press APPR or LOC to disarm the approach modes, then select the lateral and vertical modes and speed. Below the FCU altitude, the go-around procedure is mandatory.

SRS GA engages when at least one thrust lever is set to TOGA with the slats or flaps extended, in flight or within 30 s of touchdown. With the flaps lever at 0, the AP/FD stay in the approach and landing modes and the FMS does not activate the go-around phase.

GA TRK engages with SRS GA unless NAV, HDG or TRK is engaged, and cannot be disengaged below 100 ft RA; depending on the approach modes, NAV is armed or engaged. The FDs engage if they were off, extended speedbrakes retract and the FMS activates the go-around phase. With both APs engaged, leaving the go-around mode on either axis disconnects AP2. V/S and FPA cannot engage with SRS GA below 100 ft RA.

The PF sets TOGA, rotates towards 15° of pitch (about 12.5° with one engine out) and announces the go-around; the PM retracts the flaps one step. The FMA reads MAN TOGA, SRS, GA TRK or NAV, and A/THR. The PM calls BANK beyond 7°, PITCH above 20° or below 10° nose up, and SINK RATE without a climb rate. SRS GA ends at the go-around acceleration altitude, or when ALT* or ALT CST* engages above 400 ft RA, but not at the engine-out acceleration altitude; selecting a speed with the FCU altitude above the aircraft reverts it to OP CLB. At the go-around thrust reduction altitude, LVR CLB flashes and the levers go to CL (see go-around and missed approach).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.