3.magnetic-compass-errors. Magnetic Compass Errors
The magnetic compass is the only direction-seeking instrument in most aircraft that does not require electrical power. It consists of a float with two small magnets aligned with magnetic north, suspended in a bowl of liquid (kerosene or a similar low-friction fluid) that dampens oscillation and reduces wear on the pivot. Although simple and reliable, the compass is subject to several inherent errors that every instrument pilot must understand and compensate for.
Variation
Variation is the angular difference between true north (the geographic pole) and magnetic north (where the compass actually points). It varies by geographic location and changes slowly over time. Lines of equal variation are called isogonic lines, depicted as dashed magenta lines on sectional charts. The line of zero variation is the agonic line.
- East variation: subtract from true course to get magnetic course.
- West variation: add to true course to get magnetic course.
- Memory aid: "East is least, West is best."
Deviation
Deviation is the error caused by magnetic fields generated within the aircraft itself — radios, wiring, ferrous structure, and electrical equipment. Deviation varies by heading and is determined by "swinging the compass" on a compass rose. Residual error after compensation is recorded on the compass correction card mounted near the instrument, listing the compass headings to fly for each desired magnetic heading (typically every 30°).
The full conversion sequence is: True ± Variation = Magnetic ± Deviation = Compass (TVMDC, often remembered as "True Virgins Make Dull Companions").
Magnetic Dip
The Earth's magnetic field runs roughly parallel to the surface near the equator but tilts increasingly downward as latitude increases, becoming vertical at the magnetic poles. This downward pull on the compass magnets is called magnetic dip, and it is the root cause of the dynamic compass errors below. The compass float is designed with a small offset weight to partially counteract dip in straight-and-level flight, but this introduces errors during acceleration and turning.
Acceleration/Deceleration Error (ANDS)
When flying on an east or west heading in the Northern Hemisphere:
- Accelerating causes the compass to indicate a turn to the north.
- Decelerating causes the compass to indicate a turn to the south.
Memory aid: ANDS — Accelerate North, Decelerate South. The error is greatest on east/west headings and zero on north/south headings. It is caused by the offset center of gravity of the compass card responding to acceleration forces.
Northerly/Southerly Turning Error (UNOS)
Turning error is most pronounced when turning to or from a northerly or southerly heading:
- When turning from a northerly heading, the compass initially shows a turn in the opposite direction, then catches up and lags behind the actual heading. To roll out on north, lead the rollout — at approximately 30° latitude, roll out about 30° + half the bank angle early. A common rule: lead by latitude plus half the bank angle.
- When turning from a southerly heading, the compass leads the actual heading. Roll out late by approximately latitude minus half the bank angle.
Memory aid: UNOS — Undershoot North, Overshoot South (when rolling out, undershoot the target heading on northerly turns and overshoot on southerly turns). The magnitude of the error roughly equals the latitude of the aircraft.
On east or west headings, no turning error exists at the moment the turn begins, though acceleration error may appear during the bank.
Oscillation Error
Oscillation is erratic swinging of the compass card caused by turbulence or rough control inputs. The fluid damping helps but cannot eliminate it. When oscillation is present, average the swings rather than reading instantaneous values.
Practical Use Under Instrument Conditions
Under IFR, the heading indicator is the primary directional reference, but it must be reset to the magnetic compass periodically (every 15 minutes is a common practice) because the heading indicator precesses. Always perform compass alignment in straight, level, unaccelerated flight to avoid capturing dip-induced errors. If the heading indicator fails, the pilot must navigate using compass headings directly, applying lead/lag corrections in turns and avoiding alignment checks during acceleration or climbs/descents.
Required Compass Indications (Preflight)
During preflight, verify the compass:
- Is full of fluid (no bubbles).
- Swings freely and indicates known headings while taxiing turns are made.
- Has a current and legible compass correction card installed (required by 14 CFR 91.205(b)(3) for VFR day operations and applicable to IFR by extension).