AFCS General Principles: Autopilot Loops, Modes, and Crew Monitoring

022-06-01Pablo Asensio Martínez2026-04-012 min

Automatic Flight Control System (AFCS) Overview

The Automatic Flight Control System (AFCS) reduces pilot workload, enhances flight accuracy, and minimizes stress on the flight controls. It typically consists of:

  • Autopilot Flight Director System (AFDS): Controls pitch and roll to follow a path or hold parameters (e.g., altitude).
  • Autothrust (A/T): Manages engine thrust to control speed or rate of climb/descent.

Pilot Roles

In multi-pilot aircraft, roles are defined to ensure safety:

  • Pilot Flying (PF): Focuses on "flying the aircraft" (controlling attitude, speed, path) and managing the AFCS.
  • Pilot Monitoring (PM): Monitors flight parameters, calls out deviations, handles ATC communications, and manages checklists/systems.

Control Loops

Autopilots use closed-loop control systems to maintain stability and follow paths.

Loop Types

  1. Inner Loop (Stability): Senses displacement (e.g., from a rate gyro) and corrects it to stabilize the aircraft around its Center of Gravity (CG). Fast reaction.
  2. Outer Loop (Guidance): Takes commands from the Mode Control Panel (MCP) or Flight Management Computer (FMC) to guide the aircraft's path (e.g., altitude hold, heading intercept).

Feedback Mechanism

A closed loop compares the actual state (feedback) with the desired state (input).

  • Example: The system commands an elevator movement. A feedback sensor measures the actual position of the elevator and compares it to the commanded position to ensure accuracy.
  • Oscillations: If the system is too sensitive or overcompensates, it can cause self-induced oscillations (instability).

Modes of Operation

Basic (Stabilization) Modes

Operate via the inner loops to maintain aircraft attitude.

  • Attitude Hold: Maintains Pitch and Bank angle.
  • Wings Level: Keeps wings level.

Guidance Modes

Operate via the outer loops to navigate in 3D space.

  • Vertical: Altitude Hold (ALT), Vertical Speed (V/S), Glideslope (G/S), VNAV.
  • Lateral: Heading (HDG), Track (TRK), LNAV, Localizer (VOR/LOC).

Disengagement and Safety

Manual Intervention

The crew must be ready to disconnect the autopilot if it behaves abnormally (e.g., large pitch oscillations near the ground).

  • Disengage Methods: Pressing the instinctive cut-out button on the control column/sidestick, pushing the stick/column beyond a threshold, or using the MCP/FCU pushbutton.
  • Warnings: Disconnection triggers audio/visual warnings.

Envelope Protection

Modern systems (especially Fly-By-Wire) include protections to prevent the aircraft from exceeding safe limits (Stall, Overspeed, Excessive Bank).

  • Command Speed Limiting: Prevents exceeding VMO/MMO or stall speeds.
  • Mode Reversion: If a limit is approached (e.g., stall speed), the AFCS may automatically revert modes (e.g., to level change) to protect the aircraft.