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Overview

The Autopilot System is the automation layer that relieves pilots of the continuous workload of manually flying the aircraft, allowing them to manage the flight at a higher level — monitoring, planning, and communicating — while the autopilot maintains precise attitude, heading, altitude, and track. Modern airline autopilots integrate tightly with the Flight Management System (FMS) and can fly a complete route from shortly after takeoff to a fully-automatic landing (ILS Category IIIc autoland) with near-zero visibility.

How It Works

At its core, the autopilot is a set of feedback control loops. Position and rate errors — the difference between the desired and actual state — are computed and converted into surface or thrust commands. A pitch outer loop commands elevator (or stabiliser trim) to hold altitude; a roll outer loop commands ailerons and roll spoilers to hold heading or track a lateral navigation path. Inner rate loops damp oscillations and stabilise the loops.

The autopilot couples to the Auto-Throttle via a shared flight guidance computer so that, for example, a commanded descent can reduce thrust while the autopilot simultaneously commands a nose-down pitch. During autoland sequences, the autopilot tracks localiser and glideslope beams from the ILS ground station all the way to the flare, while a separate flare law gently rotates the aircraft for touchdown.

Key Components

• Flight Guidance Computer (FGC) / Autopilot Flight Director Computer (AFDC): Computes guidance commands from mode selections and FMS inputs.
• Mode Control Panel (MCP) / Flight Control Unit (FCU): Cockpit interface where crews select autopilot modes, target values, and engage/disengage.
• Flight Director: Visual cue on the primary flight display showing where to fly; autopilot follows the same commands when engaged.
• Inertial Reference System / Air Data: Provides position, attitude, airspeed, and altitude inputs to the autopilot loops.
• Radio Altimeter: Low-level altitude input used during approach and autoland flare.

Aircraft Applications

All modern commercial jets carry at least dual (and often triple) autopilot systems for redundancy. Autoland capability requires redundant autopilots operating in parallel, with automatic monitoring and disconnect on disagreement.

• Boeing 737-800: Dual autopilots with Autoland capability on suitably equipped aircraft; coupled to a glass cockpit via the FCC (Flight Control Computer).
• Airbus A320: Two Flight Augmentation Computers (FAC) and two Flight Management and Guidance Computers (FMGC) provide autopilot and flight director functions; supports Cat III autoland with dual AP engaged.
• Boeing 777: Triple autopilot for Cat IIIb autoland; autopilot commands are sent directly to the FBW Flight Control Computers.
• Boeing 787: Integrated autopilot within the Common Core System; uses synthetic vision and enhanced navigation for RNP approaches.
• Airbus A350: Latest-generation Honeywell-based autopilot with improved turbulence handling and enhanced Cat III capability.

Advantages and Limitations

Autopilots dramatically reduce crew fatigue on long-haul flights and enable precision approaches in very low visibility. However, over-reliance on automation can degrade manual flying skills — a concern regulators have addressed with requirements for periodic manual flight training. Autopilot-related mode confusion has featured in multiple incidents where crews did not understand what mode the autopilot was in or what it would do next. Operators counter this through robust training on automation philosophy and mode awareness.