Unit 5 · Flight Instruments · Lesson 3 of 3

The magnetic compass and its errors

8 min readLesson 23 of 59

By the end of this lesson you'll be able to:

Explain variation and deviation
Describe acceleration and turning errors
Know when the compass is accurate

The magnetic compass needs no power at all, which makes it the ultimate backup. It's also the least convenient instrument, because it has several errors.

Variation

The magnetic north pole isn't at the geographic (true) north pole. The angle between true north and magnetic north at your location is variation. It's shown on charts by isogonic lines. To convert a true course to a magnetic course: "East is least, west is best" (subtract easterly variation, add westerly).

Deviation

Metal and electrical equipment in the airplane also pull the compass. That error is deviation, and it differs on each heading. It's recorded on the compass correction card near the compass.

Magnetic dip errors

The earth's magnetic field lines dip toward the ground near the poles, which causes two errors in the Northern Hemisphere:

Acceleration error (on east or west headings): accelerating makes the compass swing toward north; decelerating makes it swing toward south. Remember ANDS: Accelerate North, Decelerate South.
Turning error (strongest on north or south headings): turning from a northerly heading, the compass initially lags; turning from a southerly heading, it leads. When rolling out using the compass, undershoot north, overshoot south (UNOS).

Oscillation

Turbulence makes the card swing, so it's hard to read in rough air.

The compass is accurate only in straight-and-level, unaccelerated flight. That's exactly when you should read it and set your heading indicator.

Key takeaways

Variation (true vs magnetic north, on charts) and deviation (airplane, on the correction card).
ANDS: accelerate north, decelerate south (east/west headings).
UNOS: undershoot north, overshoot south when turning.

What the examiner may ask

Practice answering out loud before you open each answer.

Why does the compass lag when turning from a northerly heading?

Magnetic dip pulls the compass card, so it initially shows a turn in the opposite direction or lags behind; roll out before the compass shows north (undershoot).

What's the difference between variation and deviation?

Variation is the difference between true and magnetic north, from the chart; deviation is error from the airplane's own magnetic fields, from the correction card.

When should you read the compass?

Only in straight-and-level, unaccelerated flight.

Common mistakes

Rolling out on the compass reading during turns to north or south without correcting for turning error.
Forgetting to apply variation when converting a true course to magnetic.
Reading the compass in turbulence.

Read it in the FAA sources

PHAK · Chapter 8Flight InstrumentsOpens the PHAK at this chapter ↗PHAK · Chapter 16NavigationOpens the PHAK at this chapter ↗

Lesson check

Get every question right to complete the lesson. Take your time; wrong answers explain why.

Check: On an easterly heading in the Northern Hemisphere, you accelerate. The compass will briefly indicate:Answer: A turn toward north. ANDS: Accelerate North, Decelerate South.

Check: Where do you find the deviation for a particular heading?Answer: On the compass correction card in the airplane. Deviation depends on the airplane and is recorded on its compass correction card.

Check: When is the magnetic compass most accurate?Answer: In straight-and-level, unaccelerated flight. Dip, acceleration and turning errors disappear in straight, level, unaccelerated flight.

Check: Turning to a heading of north using only the compass, you should roll out:Answer: Before the compass shows north (undershoot). UNOS: undershoot north, overshoot south.

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