Auto-Flight System (AFS)

What Is an Auto-Flight System?

The Auto-Flight System (AFS) is the integrated collection of automation functions that guide and control an aircraft along a desired flight path with minimal direct pilot input. It encompasses the autopilot, auto-throttle (or autothrust), flight director, and their integration with the Flight Management System and ILS or GNSS guidance. The AFS is the primary tool through which pilots manage aircraft energy state and trajectory during normal operations.

How It Works

The autopilot receives roll, pitch, and yaw command signals from the Flight Management Computer (FMC) or from mode selections on the Mode Control Panel (MCP/FCU). It computes the required control surface deflections and sends commands to electrohydraulic or electromechanical actuators — on fly-by-wire aircraft, through the flight control computers. The auto-throttle independently manages engine thrust to maintain selected speed or vertical speed targets.

The flight director generates steering commands displayed on the Primary Flight Display (PFD) as command bars or a crosshair, indicating the attitude the pilot should fly to meet the selected flight path — even when the autopilot is disengaged. This allows pilots to fly manually while receiving precise guidance, maintaining situational awareness and manual skills.

Approach and landing automation has advanced significantly. CAT IIIB autoland systems — certified on aircraft like the Boeing 737 and Airbus A320 — can execute precision ILS approaches to a runway visual range (RVR) of 75 m (246 ft), with decision heights of zero feet. Triple-channel autopilot redundancy enables continued operation through any single computer failure during autoland.

Key Components

• Flight Control Computer (FCC): Central autopilot processor interpreting guidance commands and generating actuator inputs.
• Mode Control Panel (MCP) / Flight Control Unit (FCU): Pilot interface for selecting autopilot modes, targets, and engagement.
• Auto-Throttle / Autothrust: Manages engine thrust to maintain speed or climb/descent rate targets.
• Go-Around Mode: Single-button activation applying go-around thrust and pitch-up attitude for missed approach.
• Approach Modes: LOC (localizer), GS (glideslope), VNAV, LNAV enabling precision and area navigation approaches.

Aircraft Examples

• Airbus A320 family: Flight Management Guidance Computer (FMGC) integrating FMS, autopilot, and autothrust; first fly-by-wire narrowbody with full AFS integration, certified 1988.
• Boeing 777: Dual-channel autopilot with autoland capability to CAT IIIC (RVR 50 m / 164 ft) on selected runways.
• Boeing 787-9: Common display system integrating AFS status with primary flight, navigation, and engine displays.
• Airbus A350: New generation Flight Management and Guidance Computer enabling curved RNP approaches to 0.1 nautical mile accuracy.