A compass needs a reference and a condition

Which correction belongs between true and compass?

Give east and west a sign, then keep the reference chain explicit.

The Earth and the aircraft add different effects

The horizontal component of the Earth’s magnetic field gives a magnetic compass its useful directional reference. Variation is the angle between true north and magnetic north at a location and date. Magnetic dip is the field’s inclination below or above the local horizontal. Near the magnetic poles the horizontal component becomes weak and a direct-reading magnetic compass becomes an unsuitable directional reference.

Deviation is caused by the aircraft’s own magnetic environment, including permanent magnetism and electrical currents. It can vary with heading, equipment state and configuration. A compass swing measures errors; compensation reduces them; a calibration card records the residual deviation under the stated conditions. A chart’s variation and an aircraft’s deviation card address different causes and cannot simply replace one another.

Use one signed convention throughout

With east positive, write true = magnetic + variation and magnetic = compass + deviation. Therefore magnetic = true − variation, and compass = magnetic − deviation. Normalise the result to 000–359 degrees as appropriate. Writing the reference equation first is more reliable than applying a rhyme without knowing the direction of conversion. A west value is negative in this stated convention.

For true heading 100°, variation 8°E gives magnetic 092°. If the applicable deviation is 3°W, compass is 092 − (−3) = 095°. The deviation must come from the relevant heading and equipment condition; some cards directly supply headings to steer rather than signed deviations. Read the table heading before applying an equation. The laboratory lets each correction change independently so the two reference transitions remain visible.

When should the direct-reading compass be trusted?

Its physical response includes acceleration and dip, not direction alone.

The magnet seeks a three-dimensional field

A suspended magnetic assembly tends to align with the complete magnetic field, including its vertical component. The instrument is designed to read direction in approximately straight, level and unaccelerated flight. Turning, accelerating or changing pitch disturbs the balance and allows the vertical component to influence the indication. The resulting error depends on hemisphere, heading, manoeuvre and compass design.

Common northern-hemisphere acceleration mnemonics apply to a defined heading and conventional compass; they are not universal laws for every display. Near east or west headings, acceleration and deceleration can produce apparent direction changes. During a turn through north or south, the compass can lag or lead. A remote-reading magnetic system has different dynamics, so avoid transferring a direct-reading compass rule to every electronic heading indication.

Cross-check and timed turns

Before flight, inspect liquid, bubbles, freedom of movement, markings and the correction card as applicable. Compare against a known direction while allowing for true versus magnetic reference and local disturbances. A runway designation is rounded and may not equal its precise magnetic bearing. Significant disagreement needs the approved serviceability response rather than an improvised correction.

A timed rate-one turn uses 3° per second, reducing dependence on a compass indication while the compass is disturbed. A 90° heading change takes 30 seconds at that rate, with entry and rollout accounted for by the flying technique. This does not guarantee an exact heading if the turn rate is wrong. After levelling and stabilising, compare the steady compass indication. The model shows reference conversion without pretending to calculate every dynamic dip error of an installed compass.

Can you explain and apply the mechanism?

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