Magnetic Compass Errors

The magnetic compass is the oldest and simplest instrument in the aircraft and the only heading reference that needs no electrical or vacuum power, which makes it the ultimate backup — but it is accurate only in straight-and-level, unaccelerated flight. Its errors are entirely predictable once a pilot understands that a compass card floats in fluid and aligns itself with Earth's magnetic field, and that this field does not run cleanly parallel to the surface. These behaviors are covered in the FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), Chapter 8, Flight Instruments.

The two static errors are corrected before flight. Variation is the angular difference between true north and magnetic north at a given location; it is depicted on charts by isogonic lines and is applied when converting between true and magnetic headings, using the aid "east is least, west is best" (subtract easterly variation, add westerly). Deviation is the error caused by the aircraft's own magnetic fields — radios, wiring, and metal structure — which pull the compass slightly off. Deviation is measured during a compass swing and recorded on the compass correction card mounted by the instrument, giving the actual heading to fly for a desired compass reading.

The dynamic errors all stem from magnetic dip, the tendency of the compass card to tilt downward toward the magnetic pole because Earth's field has a strong vertical component away from the equator. Dip lets the card respond to accelerations and to the vertical field during turns. Acceleration and deceleration error appears on east and west headings and is remembered by ANDS: Accelerate North, Decelerate South. On an easterly or westerly heading in the Northern Hemisphere, accelerating makes the compass momentarily indicate a turn toward north, and decelerating makes it indicate a turn toward south, even though the aircraft's heading has not changed.

Turning error appears on north and south headings and is remembered by UNOS: Undershoot North, Overshoot South. Rolling out of a turn onto a northerly heading in the Northern Hemisphere, the compass lags and shows a heading short of the target, so the pilot must roll out early — undershoot — relative to the compass; rolling out onto a southerly heading, the compass leads and runs ahead, so the pilot rolls out late — overshoot. A common rule of thumb near south is to lead or lag by the amount of the latitude. Finally, oscillation is the erratic swinging of the card in turbulence or rough handling, which is not a systematic error but simply makes the instrument hard to read until the air smooths out. Because of all this, pilots treat the heading indicator as the primary heading reference in maneuvering flight and use the compass to reset it during steady, level segments — the two instruments cover each other's weaknesses.

For flight schools, the magnetic compass is a favorite of knowledge tests and oral exams precisely because its errors are counterintuitive and easy to confuse. Students memorize ANDS and UNOS, but the deeper lesson is procedural: read the compass only when wings-level and unaccelerated, and use it to correct the heading indicator's drift rather than chasing it in a turn. Instructors reinforce that in a partial-panel scenario where the heading indicator has failed with the gyros, the compass becomes the primary heading reference — and the pilot must then actively compensate for turning and acceleration errors, timing turns rather than watching the compass swing. This links the topic directly to gyroscopic-instrument failures and to dead-reckoning navigation.

Operationally, deviation is a maintenance-touched item: whenever avionics are added or the airframe's magnetic signature changes, the compass should be re-swung and the correction card updated, so a school's records tie the instrument to airworthiness rather than treating it as fit-and-forget. In dead-reckoning and pilotage navigation the compass remains the fundamental heading source, so understanding variation and deviation is essential to plotting and flying an accurate course, especially where charted variation is large.

Aviatize's Training Management module lets instructors track compass-error understanding as graded curriculum items and pair it with partial-panel exercises, so a student's ability to actually fly headings on the compass — timing turns and correcting for ANDS and UNOS — is captured rather than assumed from a memorized acronym. Ground Training & Checking keeps the instrument and navigation syllabus aligned across instructors.

On the airworthiness side, Aviatize's Digital Data & Records module keeps compass correction cards and the maintenance history of avionics changes organized, so when a new radio or antenna alters the aircraft's magnetic signature, the required compass swing and updated card are recorded against the airframe.