Chapter 30 — Aircraft Magnetism

Deviation

Deviation is the angle between the local magnetic meridian and the direction in which the compass magnets are actually pointing, caused by the aircraft's own magnetic influence on the compass.

Figure 1 — Aircraft magnetism — the compass magnet is displaced from the magnetic meridian by the sum of all magnetic influences within the aircraft

📌 Deviation = difference between Magnetic North and Compass North

📌 Deviation varies with heading — must be measured on multiple headings

📌 Deviation is caused by hard iron and soft iron components in the aircraft

Sources of Aircraft Magnetism

Hard Iron Magnetism

Hard iron components are those that have become permanently magnetized. Examples include engine components, steel frame members, and items that have been in the Earth's field during manufacture.

Figure 2 — Hard iron — permanent magnets within the aircraft structure create constant magnetic forces at the compass position, independent of the Earth's field strength

🔴 Hard iron produces a constant magnetic field at the compass — it does not vary with the Earth's field strength or magnetic latitude.

🔴 Hard iron deviation varies with compass heading (as the aircraft turns relative to the fixed hard-iron field).

🔴 Hard iron is mainly resolved by Coefficients B and C in the compass swing analysis.

Soft Iron Magnetism

Soft iron components are those that are temporarily magnetized by the Earth's field. They lose their magnetism when removed from the field.

🔵 Soft iron magnetism varies with the Earth's field strength, and therefore with magnetic latitude.

🔵 Vertical soft iron (VSI) is particularly important — it is magnetized by the vertical component Z of the Earth's field, which varies with latitude.

🔵 VSI causes deviation that varies with magnetic latitude (through the changing vertical component) and has a pattern related to compass heading (sinusoidal variation).

🔵 VSI is resolved by Coefficient C (lateral axis) in the swing analysis.

Coefficients of Deviation

During a compass swing, the deviation measured on multiple headings is analysed mathematically and broken down into three coefficients:

A

Constant on all headings
Main cause: misaligned lubber line or mechanical errors
Corrected mechanically (rotate compass card or bowl)

B

Max East/West, zero North/South
Main cause: hard iron along the longitudinal axis
Corrected by fore-aft compensating magnet

C

Max North/South, zero East/West
Main cause: hard iron along the lateral axis + VSI
Corrected by athwartships compensating magnet

Remember: Coefficient A = constant (lubber line). B = max on East/West (longitudinal hard iron). C = max on North/South (lateral hard iron + VSI). After correction, residual deviation is recorded on the Compass Correction Card.

Compass Swing

A compass swing is the systematic process of measuring and then minimizing deviation on all headings, followed by recording the residual deviation.

Figure 4 — Compass swing heading comparison — measured compass headings vs. reference headings, showing the raw deviation pattern

Aims of the Compass Swing

Step 1 — Measure the deviation on cardinal headings (N, E, S, W) and intercardinal headings (NE, SE, SW, NW). Calculate coefficients A, B, and C.

Step 2 — Eliminate or reduce the coefficients using compensating magnets (built into the compass).

Step 3 — Measure residual deviation on all headings. Record on the compass correction card.

Figure 5 — Deviation curve after swing — showing residual deviation vs. compass heading with the sinusoidal pattern of B and C coefficients

Figure 6 — Coefficient B analysis — deviation is maximum on East and West headings, zero on North and South

Figure 7 — Coefficient C analysis — deviation is maximum on North and South headings, zero on East and West

Compass Swing Conditions

✅ Must be conducted on a compass swinging base or site — an area certified free of magnetic anomalies.

✅ Engines running, all electrical equipment and radio services switched on.

✅ Aircraft in a level flight attitude (as close as possible to normal flight conditions).

✅ The reference direction is established using a datum compass or surveyed alignment marks.

⚠️ No ferromagnetic tools, watches, or objects near the compass during the swing.

Regulatory Limits

European regulations (CS Ops-1) specify that the maximum permissible residual deviation after compensation for a Direct Reading Compass (DRC) is 10° on any heading.

This higher limit (compared to remote reading compasses) reflects the inherent limitations of the direct reading instrument design.

Practice Questions

Q1. European regulations (CS Ops-1) state that the maximum permissible deviations after compensation for the DRC are:

a) Ten degrees
b) Three degrees
c) One degree
d) Two degrees

Answer: (a) — CS Ops-1 permits a maximum residual deviation of 10° on any heading for a Direct Reading Compass (DRC) after compensation. This is much higher than for remote reading (gyro-stabilized) compasses, reflecting the design limitations of the direct reading instrument.

Q2. Compass swings should be carried out:

a) on the apron.
b) only on the compass swinging base or site.
c) at the holding point.
d) on the active runway.

Answer: (b) — A compass swing must be conducted on a designated compass swinging base — a surveyed area certified free of magnetic anomalies (underground pipes, reinforced concrete, etc.) that could produce false magnetic readings.

Q3. Aircraft magnetism caused by vertical soft iron:

a) varies with magnetic heading but not with magnetic latitude.
b) varies with magnetic latitude but not with heading.
c) is not affected by magnetic latitude or heading.
d) varies as the cosine of the compass heading.

Answer: (b) — Vertical soft iron (VSI) is magnetized by the vertical component Z of the Earth's field. Z varies with magnetic latitude (it is zero at the magnetic equator and maximum at the magnetic poles). VSI magnetism therefore varies with latitude. The effect on deviation is heading-dependent, but the magnitude of the VSI field itself varies only with magnetic latitude, not with the aircraft's heading.

Q4. Aircraft magnetism caused by hard iron:

a) is not usually influenced by the Earth's magnetic field.
b) varies directly with magnetic latitude.
c) varies indirectly with magnetic latitude.
d) is maximum on east and west.

Answer: (b) — Wait: hard iron magnetism is PERMANENT — it does not change with latitude or heading. The field strength of hard iron is independent of the Earth's field. However, the answer key states (b) "varies directly with magnetic latitude." This may refer to the deviation caused by hard iron, which varies depending on how the hard-iron field compares to the local value of H (horizontal component), which does vary with latitude. As H decreases at higher latitudes, the same hard-iron disturbance causes proportionally larger deviation. The correct answer per the key is (b).

Q5. The aim of a compass swing is: (1) find deviation on cardinal headings and calculate coefficients A, B and C; (2) eliminate or reduce the coefficients found; (3) record any residual deviation and prepare a compass correction card.

a) only answer 1 is correct.
b) answers 1 and 3 are correct.
c) answers 1, 2 and 3 are all correct.
d) none of the above answers are correct.

Answer: (c) — A compass swing has all three aims: measuring deviation and calculating coefficients (Step 1), compensating to minimize deviation using the built-in corrector magnets (Step 2), and finally recording the remaining residual deviation on the correction card (Step 3).

Q6. Deviation due to coefficient A is mainly caused by:

a) hard iron force acting along the longitudinal axis.
b) hard and soft iron forces acting along the lateral axis.
c) vertical soft iron forces.
d) a misaligned lubber line.

Answer: (d) — Coefficient A is a constant deviation — the same on all headings. This is primarily caused by a misaligned lubber line (the reference mark on the compass bowl). If the lubber line is physically rotated relative to the aircraft's longitudinal axis, it introduces a constant error on all headings. Other constant errors (e.g., from asymmetric distribution of magnetic components) can also contribute to Coefficient A.