Magnetic compass errors

The magnetic compass is the one instrument that needs no power and never quits — and it lies to you in four predictable ways. Knowing when to trust it is the whole skill.

Variation and deviation

Two fixed errors apply even in level flight:

• Variation — the angle between true north and magnetic north at your location. It's drawn on the sectional as isogonic lines; the agonic line is where variation is zero. To convert a true course to magnetic: "East is least (subtract), West is best (add)." A course of 090° true in an area of 10° east variation is 080° magnetic.
• Deviation — error from the airplane's own magnetic fields (radios, wiring, metal). It's specific to each airplane and each heading, and it's corrected with the compass correction card mounted beside the compass — a small table reading "for 0° steer 2°, for 30° steer 28°," and so on. It's why your heading indicator and compass can disagree by a couple of degrees even when both are working.

The dip errors: UNOS and ANDS

The compass card hangs on a pivot and tries to follow Earth's magnetic field, which dips downward toward the poles. That dip produces two errors that appear only while you're maneuvering:

• Northerly turning error — UNOS (Undershoot North, Overshoot South). Turning toward north, the compass lags, so roll out before it reads north. Turning toward south, it leads, so roll out after it passes south. The error is worst near N/S headings, and the lead/lag roughly equals your latitude. Flying near 30° N and rolling out to the north, you'd start stopping the turn about 30° early; turning to south, you'd let it run about 30° past.
• Acceleration error — ANDS (Accelerate North, Decelerate South). On east/west headings, accelerating swings the card toward north and decelerating swings it toward south. Pushing the throttle up while straight-and-level on an east heading briefly shows a turn toward north even though the wings are level; it settles once the speed stabilizes.

A fifth behavior, oscillation, just makes the card swing in turbulence — read the average.

When the compass tells the truth

The compass is accurate only in straight-and-level, unaccelerated flight. In a turn or a speed change, believe the heading indicator and let the compass settle before you reset.

In practice, that's exactly how you use it in dead reckoning (K1): during smooth cruise you set the heading indicator to the compass, then fly the steadier heading indicator and re-sync it to the compass every ten or fifteen minutes — because the heading indicator slowly precesses and drifts off on its own.

Flying the compass when the heading indicator quits

If the heading indicator fails (a vacuum or gyro problem), you fly the compass directly — and now its errors matter on every turn. Roll out onto northerly headings early (UNOS) and southerly headings late, keep the airspeed steady to avoid acceleration error on east/west legs, and use timed turns (standard rate is 3°/second, so a 90° turn takes 30 seconds) when you need a precision the swinging card can't give you mid-turn.

What the DPE asks: "You're rolling out onto a heading of north — what does the compass do?" Answer: it undershoots north (UNOS), so you lead the rollout and stop the turn early.