The Magnetic Compass, the Acceleration Error That Lies to You on Climbout, and the Northerly Turn Lead That the Checkride Will Expose If You Haven't Already Found It

The magnetic compass is the only navigation instrument in your cockpit that keeps working after a complete electrical failure. It needs no power, no GPS signal, and no avionics stack - the earth’s magnetic field runs it. But it produces consistent, predictable errors during acceleration and turns, and if you only memorized the acronyms without understanding why, those errors will expose you on a partial panel checkride.

How Does the Magnetic Compass Actually Work?

Inside the compass is a magnet - or a pair of magnets - suspended in compass fluid. The fluid dampens movement so the card doesn’t oscillate on every bump. The magnet aligns itself with the earth’s magnetic field lines, and as the airplane turns, the magnet holds its alignment with magnetic north while the airplane rotates around it. You read your heading where the card aligns with the lubber line.

The complication is that the earth’s magnetic field isn’t horizontal. In the northern hemisphere, field lines tilt downward toward the north magnetic pole. In the continental United States, the dip angle is roughly 60 to 70 degrees from horizontal, depending on latitude. So the magnet is being pulled both toward north and downward.

To keep the compass card level, the manufacturer mounts the magnet on a pivot point that sits above the magnet’s center of gravity - the magnet hangs slightly below its pivot. This is called pendulous mounting, and it’s the root cause of every error discussed below.

What Causes Acceleration Error?

When you accelerate, a forward force acts on everything inside the airplane, including the pendulously-mounted compass magnet. Because the magnet hangs below its pivot, that forward acceleration causes the north-seeking end to swing toward the low side. In the northern hemisphere, where the field dips toward north, the north end swings down and the compass card deflects.

The result: accelerating on an east or west heading makes the compass briefly show a turn toward north. Decelerating - reducing power, deploying flaps, adding drag - makes the compass briefly show a turn toward south.

The memory device is ANDS: Accelerate North, Decelerate South.

Acceleration error is most significant on easterly and westerly headings. On a true north or south heading the error is minimal. The maximum effect occurs 90 degrees away from the poles, which is why east and west are the headings where it hurts you.

How Does Acceleration Error Sneak Into Your Flying?

The insidious version of this error doesn’t look like an error - it looks like a heading you need to fix. You’re on a westerly crosswind leg and reduce power to descend. The compass momentarily swings south. You interpret this as a heading deviation and correct slightly toward north. By the time the compass settles, the phantom error is gone - but you’ve built a real heading error chasing it.

The correction is to give the compass a moment to settle after any power change before trusting it. During climbs, descents, and approach, power changes will temporarily displace the compass reading. Do not chase those displacements. If you have a directional gyro, use it for heading during power changes and cross-check the compass only when stabilized.

What Causes Turning Error, and Why Does North Lag?

The same pendulous mounting that causes acceleration errors also deflects the compass during turns. When the airplane banks, centrifugal force acts on the north-seeking end of the magnet. Near the equator, where field lines are nearly horizontal, this effect nearly disappears. Near the poles, where field lines are nearly vertical, it’s severe. In the continental United States, the effect is significant enough to require consistent correction.

When turning through north, the compass lags behind your actual heading. When turning through south, the compass runs ahead. The memory device is UNOS: Undershoot North, Overshoot South.

On a turn toward north, lead your rollout - roll out before the compass reaches north. On a turn toward south, overshoot - let the compass pass south before rolling out.

How Much Lead or Lag Do You Apply?

The general rule from the FAA’s Pilot’s Handbook of Aeronautical Knowledge (Chapter 8): your lead or lag angle is approximately equal to your latitude in degrees.

Flying at 30° north latitude, apply about a 30° correction. Flying at 45° north, apply about 45°. This is a workable approximation for practical flying.

Here’s what that looks like in a specific scenario. Flying in Virginia, approximately 38 degrees north latitude, on partial panel with a turn to north from an easterly heading:

• You roll into a standard rate left turn.
• As you approach north, you need to start your rollout at roughly 30 to 40 degrees of lead.
• On a left turn from east, north is at 270° on your turn indicator. You need to begin rolling out when the compass reads approximately 315 to 325 degrees - well before it hits 360.

It feels wrong. It looks like you’re about to roll out heading northwest. But if you roll out there, the compass will swing through its arc as the bank reduces and settle on north. If you wait until the compass reads 360, you’ll end up heading somewhere around 015 to 020 degrees because compass lag kept pace with you while you waited.

How Do You Execute a Turn to South on Partial Panel?

South works the opposite way, and students often get the direction of error backwards.

Same starting position: heading east, turning right toward south. The compass is running ahead of you. If you roll out when the compass reads 180, you’ll actually be heading somewhere east of south - around 165 degrees - because the compass showed south before you reached it.

The fix: overshoot. Let the compass blow past 180 by about 35 to 40 degrees. Roll out when it reads approximately 215 to 220 degrees. The compass will swing back as the bank comes off and settle on south.

This is deeply counterintuitive - you’re rolling out when the compass says you’re already 35 degrees past your target. The physics are working correctly. Trust them.

What Is the Compass Deviation Card?

Every compass has a deviation card mounted nearby - typically a small laminated card in the compass housing or posted on the panel. It lists corrections for every 30 degrees of heading to account for magnetic interference from the airplane itself: the engine, electrical components, and steel in the airframe. Every installation is different.

The deviation card gets verified during preflight. When holding short of the runway, point the nose straight down the centerline and cross-check the compass against the directional gyro. The runway heading is known. The compass should read within a few degrees of it. If there’s a significant discrepancy, either the DI wasn’t set correctly, the compass is off, or something in the cockpit is creating interference.

What Magnetic Interference Should You Watch For?

Magnetic interference in the cockpit is more common than most pilots realize. Any of these can deflect the compass enough to produce a misleading heading:

• A headset resting on the glare shield too close to the compass
• A metal clip on a clipboard
• A forgotten tool in a flight bag stowed near the panel
• A tablet mount with a magnetic attachment point

Keep the area around the compass clear. If your compass and DI disagree by more than a few degrees in straight and level unaccelerated flight, look around the cockpit for metallic items near the compass before assuming the instrument has failed.

What Does the ACS Require for Partial Panel Compass Turns?

The Airman Certification Standards require rolling out within 10 degrees of the assigned heading on a compass turn. Not 20 degrees. Not “close.” Ten.

If you’ve been sloppy about leading northerly turns and consistently overshoot by 15 degrees, that is a failed task on the practical exam. The examiner is watching the heading you actually roll out on, not whether you said “UNOS” out loud.

The only path to consistent 10-degree accuracy is practicing the lead and lag corrections in your actual airplane, at your actual latitude, until the timing is calibrated to your hands.

How Do You Build the Muscle Memory for Compass Turns?

This drill works. On a clear VFR day with no hood:

1. Get into the practice area and stabilize in level flight.
2. Cover the directional gyro with a sticky note.
3. Turn to north. When you think you’re there, uncover the DI and check yourself.
4. Note how far you missed. Adjust your lead and try again.
5. Turn to south. Repeat.
6. Come back to north from the other direction.

Thirty minutes of this will calibrate your hands to your airplane at your latitude. That calibration will stay with you. There is no classroom substitute for it.

Key Takeaways

• The magnetic compass is a last-resort instrument that works during complete electrical failure - understanding it is survival knowledge, not just checkride content.
• ANDS (Accelerate North, Decelerate South) describes acceleration error, which is most pronounced on east and west headings.
• UNOS (Undershoot North, Overshoot South) describes turning error - lead rollout on northerly turns, overshoot on southerly turns.
• Your lead/lag correction angle approximately equals your latitude in degrees.
• The ACS standard is 10 degrees - sloppy lead/lag timing is a failed task, not a close call.
• Magnetic interference from headsets, clipboards, tools, and tablet mounts can compromise compass accuracy - keep the area around the compass clear.