The Magnetic Compass, Turning Errors, and the ANDS and UNOS Rules That Trip Up Students at the Worst Possible Moment

The magnetic compass is the oldest navigation instrument in the cockpit, required for VFR day flight under FAR 91.205, and the one instrument that still works when the entire electrical system fails. It also behaves in ways that confuse nearly every student pilot the first time they see it in a turn. Understanding the physics behind compass errors makes the memory rules stick permanently - and makes partial-panel flying manageable instead of disorienting.

Why Does the Compass Do Strange Things?

The root cause of nearly every compass error is magnetic dip. Earth’s magnetic field doesn’t run horizontally. It angles downward toward the ground as you move away from the magnetic equator. In the continental United States, that dip angle is significant enough to affect compass behavior every time the airplane banks or changes speed.

To compensate, compass manufacturers install a pendulous vane system - a small weight under the compass card that keeps it roughly level during straight, unaccelerated flight. The moment you bank the airplane or change airspeed, that balance is disrupted, and the errors begin.

What Are the Two Categories of Compass Error?

There are two distinct categories: turning errors and acceleration errors. Each has its own memory rule.

Turning errors are most pronounced near north and south headings and nearly disappear near east and west. When the airplane is oriented east or west, the compass card sits relatively level in the bowl during a bank, and magnetic dip has minimal lateral effect. Near north or south, banking causes the card to tilt significantly, which makes it appear to move in the wrong direction or lag far behind the actual turn.

Acceleration errors appear during straight-and-level flight whenever airspeed changes. They’re most pronounced on east or west headings and essentially zero on north or south headings.

How Do I Use the UNOS Rule When Turning to North or South?

UNOS: Undershoot North, Overshoot South.

When turning to a northerly heading, the compass card lags badly behind the actual turn. It may still show south or south-southeast even as you’re rolling out on a northerly heading. If you wait for the card to reach north before rolling out, you’ll overshoot by a wide margin.

The practical rule: start your rollout when the compass reads approximately your latitude. Flying over the Carolinas near 35° north latitude? Roll out when the compass shows roughly 35°. The card still has momentum and will swing forward to north as the wings level.

For southerly headings, the opposite applies - the compass rushes ahead of the actual turn and reads closer to south before you’ve actually reached it. You have to let it go past. Roll out when the compass shows approximately 180° plus your latitude. At 30° north latitude, that means rolling out at about 210° on the compass, then watching the card settle back to south.

These rules assume a standard rate turn of three degrees per second. A steeper bank means shorter timing; a shallower bank gives more time. Adjust accordingly.

What About East and West Turns?

East and west turns require minimal correction. The card stays relatively level in the bank, and turning errors are small. For practical purposes, you can roll out when the compass shows the target heading with only a few degrees of adjustment. Most pilots find east and west turns feel intuitive once they’ve experienced how disorienting north and south turns can be.

How Does the ANDS Rule Work for Acceleration Errors?

ANDS: Accelerate North, Decelerate South.

In straight and level flight on an east or west heading, push the throttle forward and the compass swings north. Pull power back and it swings south. Same root cause - the pendulous vane system tilts with the change in momentum, and the dip angle turns that tilt into a false heading indication.

The practical rule is straightforward: never change airspeed and read the compass at the same time. Let the airplane stabilize with speed steady and power set before trusting the indication.

What’s the Difference Between Deviation and Variation?

These are two separate corrections that pilots often conflate.

Variation is the angular difference between true north and magnetic north. It’s shown on sectional charts as isogonic lines. In the eastern United States, variation is east, so you subtract; in the west, it’s west, so you add. The memory phrase: east is least, west is best.

Deviation is the smaller remaining error caused by the magnetic fields inside the airplane itself - the engine, avionics, and steel structure all nudge the compass off by a few degrees depending on heading. The deviation card, the small white card mounted next to the compass, shows the specific correction for each heading in that aircraft. These corrections are unique to each airplane and are updated when the airplane receives a compass swing during an inspection.

On a long cross-country using compass alone, a few degrees of uncorrected deviation adds up over an hour.

How Often Should I Reset the Heading Indicator?

The heading indicator is the primary heading reference in normal operations, but gyroscopic precession causes it to drift over time. The standard practice is to reset it to the compass every 15 minutes in cruise.

To do this correctly: choose a calm, stable period in cruise flight, wait for the compass card to settle, then align the heading indicator to match. In moderate turbulence, the compass card oscillates and becomes unreliable - use the heading indicator to hold heading through the rough air, then reset during smooth air.

Neglecting this for 90 minutes of flight can result in 10 to 15 degrees of heading error by the time you reach your destination. On a long cross-country, that compounds into a significant navigational error.

What Does the Airman Certification Standards Expect?

The ACS requires that applicants understand compass errors, explain them, and demonstrate turns to headings using the magnetic compass. Examiners aren’t testing obscure trivia - they want confirmation that a pilot can navigate if the glass panel goes dark.

The magnetic compass needs no electricity, no databases, and no software. As long as the airplane is flying, the card is pointing north. That’s a capability worth knowing how to use.

Most of the physics behind these errors is covered in Chapter 8 of the Pilot’s Handbook of Aeronautical Knowledge (PHAK), available free at faa.gov.

Key Takeaways

• Magnetic dip is the root cause of all compass errors - it makes the card behave differently in banks and during acceleration because the Earth’s field pulls it downward, not horizontally.
• UNOS: Undershoot North (roll out early, when compass shows ~your latitude), Overshoot South (roll out late, when compass shows ~180° + your latitude).
• ANDS: Accelerate North, Decelerate South - let the airplane stabilize before reading the compass after any power change.
• Turning errors are worst near north and south headings; east and west turns require minimal correction.
• Reset the heading indicator to the compass every 15 minutes in cruise, and know the difference between deviation (deviation card, aircraft-specific) and variation (sectional charts, region-specific).