Compass systems

022 InstrumentationChapter 3

Earth’s magnetic field

Key fact

Magnetic meridian — horizontal component of earth’s field. Freely suspended magnet aligns north-seeking pole with magnetic meridian. Flux flows north to south — imagine magnet upside down inside earth. Poles are not antipodal. Magnetic north moves ~10 nm/year; poles swap ~every 10 000 years.

Key fact

Variation: westerly = negative (−), easterly = positive (+). Isogonals — equal variation. Agonic — zero variation. Dip: angle between needle and horizontal; greatest at poles. Isoclinals — equal dip. Aclinic — zero dip (magnetic equator).

Exam trap

When horizontal component drops below 6 microtesla, compass too unreliable — occurs above 70° N/S. ANDS — Accelerate North, Decelerate South (NH apparent turn). UNOS — Undershoot North, Overshoot South.

Direct reading compass requirements

Key fact

1. Horizontality — pendulous suspension; CoG below pivot opposes dip moment → residual dip less than actual dip. 2. Sensitivity — more magnets, low pivot friction; within ±10°. 3. Aperiodicity — fluid damping. Compass liquid: low expansion, low viscosity, transparent, non-corrosive.

Compass errors

Key fact

Acceleration error (NH): due to dip, CoG displaced toward equatorial side — acceleration → apparent turn toward north; deceleration → toward south. No error on north/south headings. Turning error: centripetal on pivot, centrifugal on CoG — max turning through N/S, min through E/W; increases with latitude. Liquid swirl increases error turning toward nearer pole.

Aircraft magnetism — deviation

Key fact

Deviation = difference between compass north and magnetic north. Easterly +, westerly −. Hard iron — permanent, not affected by heading/latitude (electrical circuits). Soft iron — temporary, induced by earth’s field (quadrantal deviation — corrected by pairs of permanent magnets).

Formula — Deviation coefficients

Deviation = A + Bmax × sin(HDG) + Cmax × cos(HDG)

A — mechanical misalignment (same all headings). B — longitudinal (max E/W, zero N/S). C — lateral (max N/S, zero E/W).

Key fact

Coefficient A positive if longitudinal component toward nose. B positive toward right. Compensation corrects A, B, C; calibration determines residual deviation (compass swing at 30° intervals, deviation card).

Remote indicating compass (RIC)

Key fact

Combines DIC long-term accuracy (magnetic north) with DG short-term accuracy (unaffected by manoeuvres). Flux valve on wing/tail — three legs, AC-excited; earth’s field causes resultant flux → EMF in secondary (Faraday). Accuracy better than 0.5%. Max deviation ±10°.

Key fact

Slaved mode: torque motor precesses gyro at 2–4°/min. DG mode: reversionary; slew switch align every 15 min at 60–120°/min. Annunciator deviates when HSI not aligned with selsyn. Pendulous compensation: Hooke’s joint, 25° pitch/roll freedom, liquid damping.

Exam trap

Compensation = correct deviation (CoCo). Calibration = determine residual (CaRe). FEAT/Gyromagnetic same as DH2 — slaved uses FEAT; conventional uses DRMC.

Heading change in turns

Formula — Compass turning error approximation

Heading change ≈ (Bank angle + Latitude) ÷ 2