ATPL Theory

Autoflight Principles

1. Closed-Loop Control (The Foundation)

A closed-loop system constantly:

  1. Measures what the aircraft is doing
  2. Compares it to what it should be doing
  3. Corrects any error
  4. Monitors the result, and repeats

Core elements

  • Sensors → IRS, gyros, air data
  • Error detector → difference between desired and actual state
  • Signal processor (FCC) → applies control laws
  • Actuators → elevator, ailerons, rudder, spoilers
  • Feedback → confirms the correction is working

👉 Key idea: The correction reduces as the error approaches zero

2. Why Two Control Loops Are Needed

A single closed loop can only correct disturbances — it cannot change targets.

To command something new (altitude, VS, heading), you need two loops:

Inner Loop (Stability Loop)

  • Reacts fast
  • Uses gyros
  • Keeps the aircraft stable
  • Corrects disturbances automatically

Outer Loop (Command Loop)

  • Reacts slower
  • Uses pilot or FMS commands
  • Changes the target (altitude, VS, IAS, heading)
  • “Biases” the inner loop reference

📌 All modern autopilots use this structure

3. Inner Loop Control – What Actually Happens

Example: Uncommanded pitch-up

  1. Gyro senses pitch rate
  2. Signal proportional to rate is generated
  3. Controller compares this to reference attitude
  4. Elevator command sent
  5. Elevator moves → aircraft pitches down
  6. Feedback reduces correction as error shrinks
  7. Elevator returns to neutral

👉 Inner loop:

  • Does not care why the disturbance happened
  • Only cares that it exists

4. Outer Loop Control – How Commands Are Achieved

Example: Vertical Speed selected (1500 fpm climb)

  1. Pilot selects VS on MCP
  2. FCC computes required pitch change
  3. Elevator moves → aircraft pitches up
  4. Rate of climb sensed by IRS/ADIRS
  5. Actual VS compared to commanded VS
  6. Pitch is adjusted until error = 0

💡 Important insight: The outer loop “tricks” the inner loop into correcting a fake disturbance

5. Gain, Stability, and Controllability

Gain = how aggressively the system corrects errors

  • High gain

    • Fast response
    • Risk of oscillation

  • Low gain

    • Stable
    • Sluggish response

AFCS continuously adjusts gain based on:

  • Airspeed
  • Configuration (flaps/slats)
  • Phase of flight
  • Aircraft mass & CG

👉 High speed → lower gain 👉 Low speed → higher gain

6. Oscillation (Why Autopilots Can “Hunt”)

Oscillation occurs when:

  • Corrections are too large
  • System reacts too late (latency)
  • Aircraft overshoots the target

Classic example

Passengers move aft → CG shifts → pitch up → AFCS overcorrects → pitch down → repeat

⚠️ If oscillations:

  • Increase in magnitude → unstable
  • Pilot must disconnect AP/AT, stabilize manually, then re-engage

7. Types of Autopilot Systems

Single-Axis

  • Roll only
  • “Wing leveller”

Two-Axis

  • Pitch + Roll
  • Can hold altitude and heading
  • Has 2 inner loops + 2 outer loops

Three-Axis (CAT aircraft)

  • Pitch, Roll, Yaw

  • Rudder used for coordination & stability

  • Can:

    • Hold altitude, VS, IAS, Mach
    • Track VOR/ILS
    • Perform autoland
    • Follow FMS vertical & lateral profiles

📌 Three-axis system:

  • 3 inner loops (pitch, roll, yaw)
  • 2 outer loops (pitch & roll)

One-Sentence Memory Model (Exam Gold)

The inner loop keeps the aircraft stable; the outer loop changes what “stable” means.