Autoland

1. Introduction — The Landing Challenge

The approach and landing is the most difficult manoeuvre demanded of a pilot because it requires simultaneous control of the aircraft in all three axes AND airspeed. The pilot must:

• Align the aircraft with the runway centreline
• Achieve a sink rate of about 2 ft/sec before touchdown
• Reduce airspeed from 1.3 VS to about 1.15 VS by progressive reduction of engine power
• Level wings before landing
• Yaw aircraft to remove drift angle (drift "kick-off" or de-crabbing)

An automatic landing system must provide guidance and control better than that required of the pilot.

2. Autoland System Requirements

Objectives of Autoland System Operation

• Not disturb the flight path as a result of an active malfunction
• Have adequate authority for sufficiently accurate flight path control
• Warn of a passive failure
• Allow the intended flight manoeuvre to be completed following an active or passive failure

Minimum Equipment Required

3. Autoland Status — Fail-Passive vs Fail-Operational

4. The Automatic Landing Sequence

During cruise and initial approach, the control system operates as a single channel system. At a certain stage of approach, the remaining channels are armed by pressing the APPR switch on the flight control panel — this also arms LOC and GS modes.

Radio Altimeter Role

Altitude information for vertical guidance to touchdown is always provided by radio altimeter, which becomes effective as soon as altitude is within its operating range (typically 2500 feet).

Approach Mode (APP) — Dual A/P

The APP mode arms the AFDS to capture and track LOC and GS. One VHF Nav receiver must be tuned to an ILS frequency before APP can be selected. For dual A/P approach, the second VHF Nav receiver must also be tuned and the corresponding A/P engaged prior to 800 ft RA.

5. Height-by-Height Autoland Detail

flowchart TD
  A["LOC + GS captured\nbelow 1500 ft RA"] --> B["1500 ft RA\n2nd A/P couples\nFLARE mode armed\nGA mode armed\nROLL OUT armed (if avail)\nLAND 2 or LAND 3 annunciated"]
  B --> C["800 ft RA\n2nd A/P must be engaged\nby this point\n(inhibited if not)"]
  C --> D["400–330 ft RA\nStabilizer trimmed nose-up\nElevators neutralize\nIf A/Ps disengage → fwd column force needed"]
  D --> E["~50 ft RA (45 ft GA)\nFLARE mode engaged\nFD command bars retract\nStabilizer again nose-up\nA/T begins retarding thrust"]
  E --> F["~5 ft GA\nFLARE disengages → touchdown\nLOC disengages\nROLL OUT engages"]
  F --> G["~1 ft GA\nPitch attitude decreased to 2°"]
  G --> H["Touchdown\nElevators lower nose\nNose gear contacts runway\nA/T auto-disengages ~2 sec after TD\nAFCS remains on until manually disengaged"]

Key Height Values

6. Other Autoland Features

Runway Alignment (De-crabbing)

Any autoland-capable system must remove drift prior to touchdown. This is the runway alignment mode:

• Typically armed at the same time as flare mode (at 1500 ft descent)
• Engages at less than 100 ft
• From 1500 ft the yaw channel computes the difference between heading and track
• When align mode engages, rudder deflects to align the aircraft with the runway centreline before touchdown
• This manoeuvre is called de-crabbing or drift kick-off

Roll-out

A Cat 3 autoland function. Gives steering commands on the ground proportional to localizer deviation to keep the aircraft on the runway centreline throughout the ground roll.

• Commands displayed via a rotating 'barber's pole' indicator called a Para-Visual Display (PVD), or
• Automatic steering by applying deviation signals to the rudder channel and nosewheel steering

A/T After Landing

• When reverse thrust is applied, the autothrottle system is automatically disengaged
• Irrespective of reverse thrust, the A/T automatically disengages approximately 2 seconds after touchdown
• The automatic flight control system remains on until manually disengaged by the flight crew — this is when the autoland sequence is considered complete

7. A/P Go-around Mode

The A/P GA mode requires dual A/P operation. It becomes armed when FLARE ARMED is annunciated. Cannot be engaged before flare arm is annunciated or after the A/P senses touchdown.

A/P GA Sequence

• Either TOGA switch pressed → GA mode engaged → GA annunciated for AFDS
• MCP IAS/MACH display blanks; airspeed cursors display flap manoeuvring speed
• Thrust levers advance toward reduced GA N1
• A/P commands 15° nose-up pitch initially
• When programmed rate of climb established, A/P controls pitch to hold normal flap manoeuvring speed
• Roll: A/Ps maintain the aeroplane ground track existing at GA engagement

Leaving A/P GA Mode

• Below 400 ft RA: A/Ps must be disengaged to change pitch or roll modes from GA
• Above 400 ft RA: other modes can be selected
• If roll mode changed first: selected mode engages in single A/P roll; pitch remains in dual GA
• If pitch mode changed first: pitch engages in single A/P; 2nd A/P disengages; roll changes to CWS R
• ALT ACQ engages when approaching selected altitude; ALT HOLD engages at selected altitude if stabilizer trim is satisfactory for single A/P

8. ILS — The Ground Guidance System

Ground-based Elements

• Localizer transmitter — runway azimuth (lateral) approach guidance
• Glide slope transmitter — vertical approach guidance
• Marker beacons — distance to runway threshold (now largely replaced by DME)

Airborne Elements

• Localizer receiving antenna (usually same as VOR antenna)
• Glide slope receiving antenna
• Dual ILS receiver installation
• LOC and GS deviation indicator
• Marker beacon antenna and receiver (or DME)
• Marker lights on main instrument panel

9. Weather Minima — Decision Height & RVR

Weather minima = minimum values of DH and RVR, specified by national licensing authorities for various aircraft and airport types.

10. ICAO Categorization of Landing Capabilities

11. Alert Height

The Alert Height is a specified radio height, based on the aircraft's characteristics and its fail-operational landing system. It is the height above which any failure in a required redundant system leads to a go-around (unless reversion to a higher DH is possible).

Master Reference Tables

Autoland Height Sequence

ILS Category Values