Aviation Calcs

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Compass Turning Errors Calculator

A magnetic compass has dip, turning, and acceleration errors that every private-pilot oral still asks about. Enter a latitude and this page estimates the dipole dip angle, names the hemisphere memory item — UNOS and ANDS in the north, the reverse in the south — and, if you enter a heading, tells you qualitatively whether to roll out early or late, or whether an acceleration would pull the card toward north or south. It does not invent a number of degrees of lag: that depends on the compass, the bank, and the rate, and is not a single published formula.

Latitude is used as a stand-in for magnetic latitude. The magnetic and geographic equators are not the same, so the dip is an estimate, not a charted value. Headings are magnetic, 0° to 360°.

Result

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Dip, UNOS, and ANDS

In a centered dipole field the magnetic inclination (dip) I and magnetic latitude λ are related by tan I = 2 tan λ. At 45° that is about 63.4° of dip; at the equator it is zero; at the pole it is 90°. The vertical component of that field is what produces the compass errors taught in the Pilot’s Handbook.

Turning error in the northern hemisphere: the compass lags when you turn through north and leads when you turn through south. The memory item is UNOS — Undershoot North, Overshoot South — roll out early when rolling out on a northerly heading, and late when rolling out on a southerly heading. South of the magnetic equator the sense reverses (OSUN). Acceleration error, on an easterly or westerly heading, is ANDS in the north — Accelerate North, Decelerate South — and the reverse in the south. Both errors vanish at the magnetic equator and, for acceleration, on a heading of north or south. This page reports those qualitative rules. It does not compute a lag angle in degrees.

For converting true, magnetic, and compass heading with variation and deviation, use the true/magnetic heading calculator. Neither page replaces a compass correction card or a timed standard-rate turn on the attitude indicator.

Sources

Frequently asked questions

Why doesn’t the page give a number of degrees to lead or lag?

There is no single published lag formula that applies to every wet compass and every bank. The PHAK teaches the direction of the error (UNOS / ANDS), not a universal number of degrees. Inventing one would be less honest than saying “roll out early.”

Can I use geographic latitude for dip?

Only as an estimate. The formula uses magnetic latitude. The magnetic equator is offset from the geographic equator, so a dip chart or a current model is better if you need a precise inclination.

Does this replace a timed turn or a GPS track?

No. In IMC, a standard-rate turn is flown on the attitude indicator and checked against the heading indicator, not chased on the magnetic compass.