Direct Reading Magnetic Compass (DRC): Turning and Acceleration Errors

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

The Direct Reading Compass (DRC), often called the "whiskey compass", is a standalone instrument that aligns itself with the Earth's magnetic field.

Principle and Limitations

The magnet assembly tries to align with the magnetic field lines. However, due to the Earth's Dip ($Z$ component), the assembly tends to tilt downwards towards the pole. To counter this, the center of gravity (CG) of the magnet assembly is deliberately offset (lowered) relative to the pivot point to keep it horizontal. This pendulous suspension creates dynamic errors during flight. The compass is only accurate in straight, level, unaccelerated flight.

Turning Errors (UNOS)

Occur when turning from North or South headings (worst on N/S, zero on E/W).

  • Turning through North: The compass lags (Under-turns) or moves initially in the opposite direction.
  • Turning through South: The compass leads (Over-turns).
  • Mnemonic (Northern Hemisphere): UNOS
    • Undershoot North
    • Overshoot South

Acceleration Errors (ANDS)

Occur on East or West headings (worst on E/W, zero on N/S).

  • Acceleration: Center of Gravity inertia causes a rotation indicating a turn to the North.
  • Deceleration: Inertia causes a rotation indicating a turn to the South.
  • Mnemonic (Northern Hemisphere): ANDS
    • Accelerate North
    • Decelerate South

Serviceability Checks

Before flight, check:

  1. Fluid: No bubbles or discoloration (dampens oscillation).
  2. Card/Casing: Intact and readable.
  3. Indication: Must match a known heading (Runway or Taxiway) within limits (typically $\pm 10^\circ$).
  4. Deviation Card: Must be present and legible.