True, magnetic and compass

How do true, magnetic and compass directions connect?

Use one algebraic convention and test it with the geometry.

Define the east-positive chain

Variation is the angle from true north to magnetic north; deviation is the angle from magnetic north to the compass indication’s north reference caused by the aircraft’s magnetic environment. With east positive, T=M+V and M=C+D; therefore C=T−V−D, normalised into 000–359°.

For true heading 010°, variation 8°E and deviation 3°W, use V=+8, D=−3: magnetic heading 002°, compass heading 005°. Do not apply a track’s wind correction and then call it variation: first obtain true heading from the wind triangle, then change the north reference.

Use the correct local data

Variation changes with place and time. Isogonals join equal variation; the agonic line has zero variation. A chart’s magnetic information has an epoch and may include annual change. Deviation depends on the aircraft and often on heading; use the appropriate deviation card and installed system procedures.

A compass swing measures and compensates aircraft effects; it does not abolish the Earth’s changing magnetic field. Keep electrical and magnetic objects away from the compass and investigate a disagreement instead of averaging incompatible references.

Work the chain backward as a check

Suppose a compass heading of 275° is flown with deviation 2°E and variation 7°W. With the stated east-positive convention, magnetic heading is 275 + 2 = 277°; true heading is 277 − 7 = 270°. Reversing the conversion gives 270 − (−7) − 2 = 275°, recovering the starting value.

If the recovered answer does not match, inspect the signed values before blaming rounding. A mnemonic can obscure whether a correction applies to a heading or track and whether it belongs to the Earth or aircraft. Write T, M and C next to the intermediate results. The chain is a reference conversion; it contains no correction for wind drift and no instruction to alter a cleared track.

Why does the direct-reading compass lag or lead?

Magnetic dip couples aircraft motion into an instrument intended to show horizontal direction.

Horizontal direction competes with dip

The magnetic field has a horizontal component and a vertical component. Dip is small near the magnetic equator and large near magnetic poles, where horizontal directional force becomes weak. A freely suspended magnet tends to align with the full field, so the compass construction must restrain dip.

Acceleration and turning displace the compass assembly; the dipped field can then produce a heading indication error. The familiar northern-hemisphere acceleration error on east/west headings is a transient indication toward north when accelerating and toward south when decelerating. The sense reverses in the southern hemisphere and reduces near the magnetic equator.

Treat the mnemonic as conditional

Turning errors depend on hemisphere, heading, bank and compass design. In the northern hemisphere, the conventional direct-reading compass tends to lag when turning through north and lead through south; the southern hemisphere reverses this. These are compass indications, not changes in the actual aircraft track.

Use stabilised straight flight and the applicable instrument procedures for heading comparison. A slaved system may reduce some direct-reading errors but still depends on valid magnetic sensing. At high latitude a true or grid reference may be required under the operator’s procedures.

Diagnose a transient disagreement

An aircraft is established eastbound in the northern hemisphere. During acceleration, its direct-reading compass moves toward north while a reliable stabilised attitude/heading reference and the ground track remain steady. The observed pattern is consistent with the stated acceleration error. Turning the aircraft merely to chase that transient indication would introduce a real track error.

A persistent disagreement in stabilised straight flight needs a different explanation: incorrect directional-gyro setting, magnetic interference, slaving faults, a reference selection mismatch or actual heading change are possibilities. The animation resolves only horizontal and vertical magnetic field components, so it supports the reason for dip-related susceptibility without pretending to reproduce instrument dynamics. Identify what a model actually demonstrates before using it to diagnose a specific installation.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.