Magnetism

061 General NavigationChapter 2

Magnetic compass theory and direction conversions are heavily examined. Know where errors are greatest and how true, magnetic, grid, and compass headings relate.

Schematic

True ± Variation = Magnetic ± Deviation = Compass (West is best, East is least).

True, magnetic & compass

Definition — True north

Direction of the observer’s meridian toward the North Pole.

Definition — Directive force

Resultant of the earth’s magnetic field in the horizontal plane at the compass location — aligns the compass needle.

Key fact

The compass needle’s red end is the north-seeking pole. It aligns with magnetic lines of force (field direction: magnet’s red pole toward blue pole).

Formula — Direction conversion

True ± Variation = Magnetic ± Deviation = Compass

Mnemonic: True virgins make dull company

Exam trap

West is best, East is least — add west variation/deviation, subtract east. With west variation, magnetic heading is greater than true.

Key fact

Relative bearing + Heading (M/T) = Bearing (M/T) to the object. MH + RB = MB. TH + RB = TB.

Magnetic field behaviour

Key fact

The horizontal component is maximum at the magnetic equator and zero at the magnetic poles. Total field strength is greatest at the poles.

Key fact

The magnetic compass is most effective midway between the magnetic poles. A DRC (direct reading compass) accuracy ≈ ±5° (CS-25 standby compass tolerance to 10°).

Exam trap

At the magnetic poles, a freely suspended needle stands vertical (dip = 90°) — compass unusable for heading.

Key fact

Magnetic north moves about 1° in 5 years. Earth magnetic poles are not antipodal.

Variation & isogonals

Definition — Magnetic variation

Angular difference between true north and magnetic north at a place. Max 180° when directly between TN and MN.

Key fact

West variation = negative (−) in many conventions; east = positive (+). Variation is east when MN is east of TN.

Definition — Isogonals

Lines joining points of equal magnetic variation. Converge toward geographic and magnetic poles.

Definition — Agonic line

Line of zero variation — true and magnetic meridians parallel.

Definition — Isoclinals

Lines of equal magnetic dip. Aclinic — zero dip (magnetic equator).

Compass deviation

Definition — Deviation

Angular difference between magnetic north and compass north caused by aircraft magnetic fields (metal, electrics).

Key fact

Easterly deviation (+), westerly deviation (−). Deviation depends on aircraft heading, magnetic latitude, and electrical equipment. Apply deviation to compass heading to get magnetic heading.

Key fact

A compass swing is required after about one year stationary on the ground, or after a large permanent change in magnetic latitude.

Compass errors

Key fact

Turning errors — greatest on N/S headings and at high latitude. ANDS (Northern Hemisphere): Accelerate = North turn indication; Decelerate = South. Reversed in Southern Hemisphere (UNOS).

Key fact

Acceleration errors — greatest on E/W headings. Undershoot north, overshoot south (Northern Hemisphere) when turning — break turn early/later accordingly. Liquid swirl modifies the effect.

Key fact

Drift (gyro) — apparent movement in the horizontal plane (earth rate / transport wander) — not a magnetic compass error.

Grid navigation & grivation

Key fact

Grid navigation — near the true or magnetic pole, true track changes too rapidly; grid north gives a near-constant reference. Grid lines are parallel to a reference meridian (grid datum) where convergence is zero.

Formula — Grid track

Grid Track = True Track ± Change in Longitude from datum

Convergence west, true best · Convergence east, true least

Formula — Grivation

Grivation = Grid Convergence + Variation

Difference between grid heading and magnetic heading. Isogrivs — lines of equal grivation.

Key fact

True track increases with longitude in the Northern Hemisphere, decreases in the Southern Hemisphere (when flying east).

Quick revision

Topic	Remember
Turning errors	N/S, high latitude
Acceleration errors	E/W headings
Magnetic equator	Strongest horizontal field
DRC swing	Large Δ magnetic latitude
TVMDC	West add, East subtract