7.magnetic-compass. Magnetic Compass
The magnetic compass is the oldest and simplest instrument in the cockpit, and under 14 CFR 91.205(b) it is required equipment for VFR flight (day and night). It is the only direction-seeking instrument in most light aircraft — the heading indicator is a slaved or free gyro that must be periodically reset to the compass.
Construction
The compass consists of a sealed bowl filled with acid-free white kerosene. Inside the bowl, two small bar magnets are mounted on a brass float, which is suspended on a hardened steel pivot point. A graduated compass card marked every 5° (with letters N, E, S, W replacing 0°, 90°, 180°, 270°) is attached to the float. The pilot reads heading against a fixed lubber line on the back of the bowl. The fluid damps oscillations and supports the weight of the float, reducing pivot wear.
Because you read the card from behind, the numbers appear reversed: a turn to the right (east) shows numbers decreasing under the lubber line, even though you are turning toward higher headings.
Compass Errors
The magnetic compass aligns with Earth's magnetic field, but several errors corrupt its reading:
- Variation — The angular difference between true north (geographic pole) and magnetic north. Variation is depicted on sectional charts by dashed magenta isogonic lines. The line of zero variation is the agonic line. Apply variation by the rule "East is least, West is best": subtract easterly variation from true course to get magnetic course; add westerly variation.
- Deviation — Error caused by magnetic fields generated within the airplane itself (radios, wiring, ferrous metal). Deviation is minimized by the A&P during a compass swing and the residual error is recorded on a compass correction card mounted near the instrument, listing the steer-this heading for each cardinal/intercardinal heading.
- Magnetic dip — Caused by the vertical component of Earth's magnetic field pulling the float down toward the nearer pole. Dip is essentially zero at the magnetic equator and maximum near the poles. Dip is the source of the two dynamic errors below.
- Oscillation error — A combination of all errors that causes the card to swing during turbulence; average the indications.
Dynamic Errors (Northern Hemisphere)
Magnetic dip produces two errors that show up only when the aircraft is accelerating, decelerating, or turning. Both can be remembered with the mnemonics ANDS and UNOS.
Acceleration/Deceleration Error (ANDS) — Most pronounced on east or west headings.
- Accelerate → North (compass swings toward north)
- Decelerate → South (compass swings toward south)
When flying east or west and you accelerate (e.g., add power, lower the nose), the compass momentarily indicates a turn to the north. When you decelerate, it indicates a turn to the south. The actual heading has not changed.
Northerly Turning Error (UNOS) — Most pronounced when passing through north or south headings.
- Undershoot North
- Overshoot South
When rolling out of a turn onto a northerly heading, the compass lags behind the actual heading; you must roll out before the compass indicates north — a common rule of thumb is to lead the rollout by the latitude (e.g., at 30° N latitude, roll out about 30° early). When rolling out on a southerly heading, the compass leads, so you must continue the turn past the indicated heading by approximately the latitude before rolling out. On east or west headings there is no turning error — the compass reads correctly.
In the Southern Hemisphere these effects reverse.
Pre-Flight Check
During preflight, verify:
- The compass is full of fluid (no bubbles).
- The card is readable and rotates freely.
- The compass correction card is present, legible, and dated within the operator's required interval.
- The compass agrees with known runway heading and with the heading indicator after start-up and during taxi turns.
Practical Use
In cruise, the heading indicator is referenced because it is stable. However, the heading indicator drifts due to mechanical precession and Earth rate, so the pilot must reset it to the magnetic compass approximately every 15 minutes — but only in straight-and-level, unaccelerated flight, when the compass is most accurate. In aircraft without a heading indicator (or after a vacuum failure), the magnetic compass becomes the primary directional reference, and the pilot must apply the dynamic-error rules consciously when turning.
Example
You are flying westbound at 30° N latitude in level flight on a magnetic heading of 270°. You lower the nose to descend and accelerate. The compass momentarily swings north, perhaps reading 285°. Recognizing acceleration error, you ignore the indication, hold wings level on the attitude indicator, and the compass settles back to 270° once airspeed stabilizes.