The Direct Indicating Compass

1. The Magnetic Compass and Direct Indicating Compass

A compass indicates direction relative to a known datum. The magnetic compass uses the horizontal component (H) of the earth's magnetic field as its directional datum. Unfortunately, this datum (the magnetic meridian) is generally not aligned with the true meridian — the angular difference between them is magnetic variation.

Additionally, iron/steel components and electric currents in the aircraft distort the earth's field locally, causing the compass magnet to deviate from the magnetic meridian. This is compass deviation.

2. The Vertical Card Compass

The vertical card compass (B-type / E-type) is the standard direct reading compass for general use — the main magnetic heading reference in light aircraft and the standby compass in larger aircraft.

• A circular compass card is attached directly to the magnet assembly.
• The combined unit is suspended in liquid within the compass bowl.
• A vertical lubber line on the glass window enables heading to be read off the rotating card.

3. Compass Requirements – Horizontality, Sensitivity, Aperiodicity

3.1 Horizontality

The magnets must lie as nearly as possible in the horizontal plane. A freely suspended magnet would align with the total field T and dip — only horizontal at the magnetic equator. To achieve horizontality, the assembly is pendulously suspended: the centre of gravity (CG) is below the supporting pivot.

Two couples act: (1) Z pulling the red end downward; (2) the weight W acting down through CG (displaced by tilt) and reaction R upward through pivot. Equilibrium results in a residual tilt of approximately 2° (red/north end down) in mid-latitudes, Northern Hemisphere. (Blue/south end down in Southern Hemisphere.)

3.2 Sensitivity

The compass must seek the horizontal component H in all areas except near the poles. Sensitivity is increased by:

• Using two, four, or six short magnets or a circular magnet (high pole strength alloy)
• Iridium-tipped pivot in a jewelled cup (reduces friction)
• Lubrication from the compass liquid
• Reduced effective weight through liquid buoyancy

3.3 Aperiodicity (Dead Beat)

The assembly must settle quickly after being disturbed, with oscillations rapidly damped. Achieved by:

• Several short magnets (mass near centre → low moment of inertia → easier to damp)
• Light alloy framework to minimise weight
• Compass liquid acts as primary damping medium
• Grid ring compass adds damping wires for faster damping than vertical card type

4. The Compass Liquid

The liquid is essential — it dampens oscillations and lubricates the pivot. Required properties:

• Low coefficient of expansion (temperature changes cause expansion — handled by an expansion chamber or Sylphon tube)
• Low viscosity (to minimise liquid swirl — discussed later)
• Transparency
• Low freezing point
• High boiling point
• Non-corrosive

5. Deviation and Accuracy

Deviation is produced by the iron/steel components in the aircraft distorting the earth's field. It is the angle between the local magnetic meridian and the direction in which the compass magnets are lying.

During the swing, normal flying conditions should be simulated: engines running, electrical/radio services on, aircraft in level flight attitude. Ferromagnetic tools, watches, and objects must not be near the compass during a swing.

6. Acceleration and Turning Errors – Theory

Acceleration and turning errors occur whenever the CG of the magnet assembly moves away from its normal position almost directly below the pivot. When the CG is displaced, the earth's vertical component Z exerts a torque about the pivot, rotating the magnet assembly and producing a false heading indication.

No error occurs when the manoeuvre displaces the CG north or south (in the plane of the magnetic meridian) — only the north-south tilt changes, with no azimuth rotation.

7. Errors Caused by Linear Acceleration

7.1 Acceleration on 270°M (Northern Hemisphere)

The inertia of the pendulously suspended assembly causes it to swing back behind the pivot (which is north of the CG). Z produces an anticlockwise rotation → compass over-reads → indicates an apparent turn towards north. Example: real heading 270°, compass reads ~280°.

7.2 Acceleration on 090°M (Northern Hemisphere)

Inertia causes assembly to swing back → clockwise rotation → compass under-reads → indicates apparent turn towards north. Example: real heading 090°, compass reads ~080°.

7.3 Deceleration on 090°M (Northern Hemisphere)

Inertia causes assembly to swing forward → anticlockwise rotation → compass over-reads → apparent turn towards south.

7.4 Acceleration on 270°M (Southern Hemisphere)

Now the pivot is offset to the south of the CG (blue/south-seeking end dips down). Inertia swings assembly back → clockwise rotation → compass under-reads → apparent turn towards south.

7.5 Acceleration on a Northerly Heading (Northern Hemisphere)

CG lags — but the CG and pivot remain in the plane of the magnetic meridian → only a change in N/S tilt → no error.

8. Summary of Acceleration Errors

flowchart TD
    A["Linear Acceleration"] --> B{"Heading?"}
    B -->|N or S| C["No Error
(CG stays in meridian plane)"]
    B -->|E or W| D["Maximum Error"]
    D --> E{"Hemisphere?"}
    E -->|NH, accel| F["Apparent turn NORTH
(towards nearer pole)"]
    E -->|NH, decel| G["Apparent turn SOUTH
(towards further pole)"]
    E -->|SH, accel| H["Apparent turn SOUTH
(towards nearer pole)"]
    E -->|SH, decel| I["Apparent turn NORTH
(towards further pole)"]

9. Turning Errors

In a turn, centripetal force acts through the pivot towards the centre of the turn; centrifugal (inertial) force acts outward through the CG. This causes the magnet assembly to "swing out" from the turn, rotating it around the pivot.

9.1 Key Terminology

9.2 Turning from 045° to 315° through North (NH — Left Turn)

In NH, CG is south of pivot. In a left turn through north, inertia throws magnet assembly anticlockwise. Aircraft and magnet rotate in same direction → compass is sluggish. Anticlockwise → over-reads. Pilot sees fewer than 90° pass → must undershoot. Stop at ~335° indicated to achieve 315° actual.

9.3 Turning from 315° to 045° through North (NH — Right Turn)

Inertia throws assembly clockwise. Same direction as aircraft → sluggish. Clockwise → under-reads. Stop at ~025° indicated to achieve 045° actual. Must undershoot.

9.4 Turning from 135° to 225° through South (NH — Right Turn)

Aircraft turns right (clockwise) but inertia throws magnet anticlockwise (opposite direction to aircraft) → compass is lively. Anticlockwise → over-reads. Pilot sees more than 90° pass → must overshoot. Stop at ~245° indicated to achieve 225° actual.

9.5 Turning from 135° to 225° through South (SH — Right Turn)

In SH, CG is north of pivot (blue end dips toward south pole). Inertia throws magnet clockwise. Same direction as aircraft → sluggish. Must undershoot. Stop at ~205° to achieve 225°.

9.6 Turning through East or West (No Error)

Magnets are tilted north-south but this tilt is in the vertical plane of the magnetic meridian through the pivot. There is no rotational couple about the pivot → turning error is zero when passing through east or west.

10. Turning Errors – Liquid Swirl

During a turn, the liquid in contact with the bowl tends to be dragged around with the bowl, creating eddies that drift inward and deflect the magnet assembly in the direction of the turn.

11. Summary of Turning Errors

flowchart TD
    T["Turning Error"] --> NH["Northern Hemisphere"]
    T --> SH["Southern Hemisphere"]
    NH --> NHP["Turning through North (nearer pole)"]
    NH --> NHS["Turning through South (further pole)"]
    NHP --> NHP1["Sluggish, Undershoot, Swirl INCREASES error"]
    NHS --> NHS1["Lively, Overshoot, Swirl REDUCES error"]
    SH --> SHP["Turning through South (nearer pole)"]
    SH --> SHN["Turning through North (further pole)"]
    SHP --> SHP1["Sluggish, Undershoot, Swirl INCREASES error"]
    SHN --> SHN1["Lively, Overshoot, Swirl REDUCES error"]

Master Reference Tables

Answer Key