The Magnetic Compass, the Northerly Turning Error, and the Two Memory Tricks That Make Sense Once You Know Why It Lies

The magnetic compass is required equipment for day VFR flight under FAR Part 91 and is one of aviation’s original six instruments. It operates without electricity, has no moving parts except a fluid-suspended card, and will keep pointing north when every other system on the aircraft has failed. Understanding why it produces errors - and how to compensate for them - is the difference between a pilot who can use it and one who is defeated by it.

How Does a Magnetic Compass Actually Work?

Inside the sealed compass housing is a float suspended in compass fluid, typically a mixture of kerosene and white spirits. The fluid damps oscillations that would otherwise make the card unreadable, and it nearly neutralizes the weight of the assembly so the card can rotate with minimal friction. Two small permanent magnets attached to the float align with the Earth’s magnetic field, rotating the card to indicate the aircraft’s heading.

The pivot point is positioned above the center of gravity of the float assembly, causing it to hang like a pendulum - level by gravity. This design is what makes the compass work. It is also what makes it lie.

What Is Magnetic Dip and Why Does It Cause Compass Errors?

The Earth’s magnetic field does not run parallel to the surface. Near the equator, the field is nearly horizontal, but in the northern hemisphere it tilts increasingly downward toward the poles. In the contiguous United States, this dip angle ranges from roughly 55 degrees in the south to over 70 degrees in the northern states.

Because the compass magnets want to chase that tilted field, the float assembly wants to tilt as well - north end dipping toward the ground. Gravity counteracts this, keeping the card roughly level. That constant tension between magnetic pull and gravity is the root cause of both major compass errors: northerly turning error and acceleration error.

What Is Northerly Turning Error and When Does It Occur?

Northerly turning error appears every time the aircraft rolls into a bank. When the aircraft tilts, the compass housing tilts with it. The float, still trying to follow the dipping magnetic field, shifts relative to the card’s pivot point. The result is that the compass card does not accurately track the aircraft’s actual heading during the turn.

The error is worst when turning through north or south and nearly zero when passing through east or west. Whether the card leads or lags depends on which direction you are turning and which heading you are passing through.

How Do I Roll Out on the Correct Heading? (OSUN)

The memory aid is OSUN: Overshoot South, Undershoot North.

Rolling out on a northerly heading: The compass lags during the turn. If you wait for the card to reach your target before leveling the wings, the aircraft will have already passed through it. Begin the rollout early - the standard technique is to start leveling the wings when the compass reads approximately your latitude in degrees before the desired heading. At 40° north latitude, begin the rollout when the compass reads about 40 degrees shy of the target. The card will continue swinging as the wings level and settle on the correct heading.

Rolling out on a southerly heading: The compass leads during the turn. If you wait for the card to show your target heading, you will roll out short of it. Keep turning past the card’s indication; it will swing back to the correct heading as the wings level.

The private pilot Airman Certification Standards include compass turns as a demonstrated skill. Examiners expect pilots to explain the physics, not just recite the mnemonic.

What Is Acceleration Error and When Does It Matter?

Acceleration error is produced by speed changes, not turns. The memory aid is ANDS: Accelerate North, Decelerate South.

When the aircraft accelerates, inertia causes the float assembly to swing rearward. Because of the magnetic dip and the way the pivot hangs the float, that rearward swing tilts the north-seeking end of the magnets downward - and the card appears to swing toward north. A false north indication during acceleration.

When the aircraft decelerates, the float swings forward and the opposite effect occurs. The card appears to swing toward south.

This error is most pronounced on east and west headings and nearly zero on north and south headings. Reduce power for a descent while heading east and the compass will swing noticeably toward south. Chasing that indication with rudder will put the aircraft on the wrong heading once the speed stabilizes.

The fix: recognize any speed change on an east or west heading as a temporary false swing. Wait for airspeed to stabilize before trusting the compass reading.

How Should I Actually Use the Magnetic Compass in Flight?

The magnetic compass is not a real-time instrument. Unlike a heading indicator or HSI, it does not provide reliable feedback during a turn. What it does well is confirm where the aircraft ended up after the wings are level and the speed has stabilized.

The practical discipline: before the turn, confirm your heading. During the turn, use time. After the turn, let the compass settle, then confirm.

A standard rate turn is 3 degrees per second. A 90-degree turn takes 30 seconds; a 180-degree turn takes 60 seconds. Set a timer, count the seconds, roll out on time, and let the compass confirm the result. Timing turns rather than chasing the card is what makes partial panel work manageable.

How Can I Practice Magnetic Compass Skills?

Go to a practice area on a clear day with the heading indicator working normally as a reference. Let the compass settle in straight-and-level cruise and note the heading.

1. Accelerate, then observe the false swing. Decelerate back to cruise and note the swing in the opposite direction.
2. Fly a series of timed turns to cardinal headings - north, east, south, west. Roll out purely by the clock, ignoring the compass during the bank.
3. After each rollout, let the aircraft settle and compare the compass to the heading indicator. Note how close the timing brought the heading.

Every aircraft is slightly different. Fluid quality, local deviation, and geographic latitude all affect how a specific compass behaves. Learning the instrument in a particular aircraft matters.

What Does the Deviation Card Tell Me?

Deviation is not the same as magnetic variation. Variation is the geographic difference between magnetic north and true north - it is the same for every aircraft at a given location. Deviation is error introduced by a specific aircraft: the radios, avionics, airframe metal, and electrical wiring all create local magnetic fields that pull the compass card off.

The deviation card is produced by swinging the compass - taxiing the aircraft onto a compass rose painted on the ramp, usually with a licensed avionics technician, and comparing compass readings to known headings around the full 360 degrees. Small compensating magnets inside the housing can correct some of the error; the remainder is recorded on the placard mounted next to the compass.

Errors greater than 10 degrees in any direction mean the compass needs attention. Cloudy fluid, bubbles, or a sluggish card belong in the squawk book. A degraded compass compounds the errors already built in from magnetic dip - not something to discover over mountains with the GPS offline.

Key Takeaways

• The magnetic compass is required for day VFR under FAR Part 91 and operates with no electrical power.
• Magnetic dip causes two predictable errors: northerly turning error (during banks) and acceleration error (during speed changes).
• OSUN - Overshoot South, Undershoot North - governs rollout technique for timed compass turns.
• ANDS - Accelerate North, Decelerate South - describes the false swing during speed changes, most pronounced on east and west headings.
• Use the compass to confirm where the aircraft ended up; fly timed turns at 3°/second during partial panel and wait for the card to settle before reading it.
• Deviation errors over 10 degrees or degraded fluid require maintenance before that instrument becomes a primary reference.