Directional Gyro (DG): Heading Stability, Drift, and Re-Synchronization

022-04-04Pablo Asensio Martínez2026-04-011 min

The Directional Gyro (DG) or Direction Indicator (DI) provides a stable heading reference, free from the turning and acceleration errors of a magnetic compass.

Principles

  • Gyro: Horizontal Axis (Tied gyro).
  • Degrees of Freedom: 2 (Pitch and Roll of the aircraft do not affect it).
  • Operation: The gyro stays rigid in space while the aircraft turns around it. The instrument detects this relative motion and drives the compass card.

Drift and Synchronization

The DG has no magnetic sensing capability. It is purely inertial.

  1. Real Drift: Caused by friction.
  2. Apparent Drift: Caused by Earth's rotation ($15^\circ/hr \times \sin Lat$).
  3. Transport Wander: Caused by moving across the globe.

Because of this drift, the pilot must manually synchronize the DG with the Magnetic Compass:

  • When: Every 10-15 minutes.
  • Condition: Only in straight, level, unaccelerated flight (when the magnetic compass is accurate).

Advantages over Magnetic Compass

  • Stable: No oscillation or swinging.
  • No turning errors: Does not overshoot/undershoot on N/S headings.
  • No acceleration errors: Does not show false turns during speed changes.