Attitude Indicator (Artificial Horizon): Erection Systems and Errors

022-04-03Pablo Asensio Martínez2026-03-271 min

The Attitude Indicator (AI) or Artificial Horizon provides pitch and roll information relative to the Earth's horizon. It is the only instrument that gives an immediate, direct picture of the aircraft's attitude.

Principles

  • Gyro: Earth Gyro (Spin axis is vertical).
  • Degrees of Freedom: 2 (Pitch and Roll).
  • Erection System: The gyro must remain vertical relative to the Earth.
    • Air-Driven: Uses Pendulous Vanes. Gravity acts on the vanes to open/close air ports, creating a precession force to right the gyro.
    • Electrically-Driven: Uses torque motors and mercury/bubble switches.

Indication Errors (Air-Driven)

Air-driven AIs suffer from specific errors caused by the movement of the pendulous vanes during maneuvers.

  1. Acceleration Error:
    • During acceleration, the vanes lag, causing a false precession.
    • Indication: False Climb (Pitch Up) + False Turn to the Right.
    • Correction during Deceleration: False Descent + Turn Left.
  2. Turning Error:
    • During a turn, centrifugal force acts on the vanes.
    • Errors in both pitch and bank indications, but usually small in modern instruments.

The Attitude Director Indicator (ADI)

An advanced version of the AI found in modern cockpits.

  • Inputs: Pitch/Roll Gyro + Flight Computer.
  • Displays: Flight Director (Command Bars), ILS deviations (Localizer/Glideslope), and sometimes Speed/Altitude tapes (EFIS).
  • Color: Blue (Sky) / Brown (Ground).