Chapter 29 — Direct Indicating Compass

The Magnetic Compass

A compass is an instrument designed to indicate direction relative to a known datum. The magnetic compass uses the horizontal component of the Earth's magnetic field as its datum. Since the Earth's field is not aligned with the true meridian, the angular difference between true and magnetic meridians must be accounted for — this is magnetic variation.

Magnetic influences from iron/steel components and electrical currents in the aircraft further distort the Earth's field, causing the compass to deviate from the magnetic meridian. This is compass deviation.

🧭 True Heading = Compass Heading ± Deviation ± Variation

📍 The rules: Compass to True: CADET (Compass + Deviation + Error [variation] = True)
Or: TVDM — True → Variation → Deviation → Magnetic/Compass (to go the other way)

Types of Direct Reading Compass

There are two basic types of direct indicating (direct reading) magnetic compasses used in aircraft:

1. The Vertical Card Compass (B-type / E-type)

This is the most common direct reading compass. It is the main magnetic heading reference in light aircraft and the standby compass in larger aircraft.

Figure 1 — Vertical card compass — the circular compass card is attached directly to the magnet assembly and suspended in damping liquid

The vertical card compass consists of a circular compass card attached directly to the magnet assembly, suspended in liquid within a compass bowl. A vertical lubber line on the glass window allows the heading to be read from the card.

✅ Simple, compact, lightweight
✅ Can be read during manoeuvres
❌ Less stable than the grid ring compass

2. The Grid Ring Compass (P-type)

The grid ring compass is found on older aircraft. It is more accurate and stable than the vertical card compass but has significant disadvantages.

Figure 2 — Grid ring compass — the grid ring must be unclamped and aligned before reading, making it usable only in straight and level flight

✅ More accurate and stable
✅ Greater periodicity due to damping wires
❌ Heavier, bulkier, more expensive
❌ Can only be read in straight and level flight (grid ring must be unclamped and aligned)

Feature	Vertical Card (B/E-type)	Grid Ring (P-type)
Reading condition	Any flight attitude	Straight and level only
Accuracy	Moderate	Higher
Stability	Moderate	Better (damping wires)
Weight/size	Light/compact	Heavy/bulky
Application	Light a/c main; large a/c standby	Older aircraft; rarely used now

Compass Requirements

For the direct reading magnetic compass to function correctly, the pivoted magnet system must satisfy three essential requirements:

⬆️

Horizontality

Magnets must lie in the horizontal plane during normal straight and level flight to measure direction correctly.

🔍

Sensitivity

The magnet system must respond quickly and accurately to the horizontal component H of the Earth's field.

⏹️

Aperiodicity

The assembly must settle rapidly on a steady reading after displacement — it must be 'dead beat' with no oscillation.

Achieving Horizontality — Pendulous Suspension

A freely suspended magnet would align with the Earth's total field (both horizontal H and vertical Z components), causing the magnets to dip. They would only be truly horizontal at the magnetic equator.

To achieve horizontality, the magnet assembly is pendulously suspended — the centre of gravity is placed below the supporting pivot.

Figure 3 — Pendulous suspension — the CG is below the pivot, opposing the dipping effect of the Earth's vertical component Z with a restoring couple from weight W

In the Northern Hemisphere: the vertical component Z pushes the North-seeking (red) end down. The weight couple opposes this, resulting in a very small residual dip of ~2° (red/north-seeking end slightly low) in mid-latitudes.

In the Southern Hemisphere: the South-seeking (blue) end dips slightly.

⚠️ This residual dip causes turning and acceleration errors — a significant limitation of the direct reading compass (covered in the Aircraft Magnetism chapter).

Achieving Sensitivity

Sensitivity requires a high magnetic moment in the magnet assembly and low friction at the pivot. This is achieved by:

🔴 Using 2, 4 or 6 short magnets (or a circular magnet) made of high-retentivity alloy — maximizes pole strength without increasing length.

⚙️ An iridium-tipped pivot in a jewelled cup — minimizes pivot friction.

💧 Compass liquid — lubricates the pivot AND partially buoys the magnet assembly (reducing effective weight on pivot).

Achieving Aperiodicity

Aperiodicity (dead-beat response) is achieved by:

⬇️ Multiple short magnets — keeps mass near the centre, reducing moment of inertia, making oscillations easier to damp.

💧 Damping liquid — the primary purpose of the liquid in the compass bowl is to damp oscillations of the magnet assembly.

🔌 Damping wires (grid ring compass only) — additional wires attached to the magnet assembly, rotating through the liquid for extra damping.

Serviceability Checks

Before flight, the compass should be checked for:

✅ Correct fluid level — liquid must fill the bowl completely (no bubbles); a bubble indicates a leak and makes the compass unserviceable.

✅ Freedom of movement — the card must move freely without sticking.

✅ Correct illumination — if fitted with lighting, it should function.

✅ No excessive deviation — residual deviation should be within regulatory limits and a current compass correction card must be displayed.

Deviation

Deviation is caused by iron/steel components, electrical currents and other magnetic influences within the aircraft distorting the local Earth's field at the compass location. It is the angle between the magnetic meridian and the direction in which the compass magnets are actually pointing.

⬅️ Easterly Deviation (+) — North-seeking (red) end of magnets points East of magnetic North.

➡️ Westerly Deviation (−) — North-seeking (red) end points West of magnetic North.

📊 Deviation varies with heading — it must be measured on multiple headings during a compass swing.

Compass Correction Card: After a compass swing, residual deviation is measured and recorded on a Deviation Card (or Compass Correction Card) mounted in the aircraft. The pilot applies this correction to convert from Compass Heading to Magnetic Heading.

Good Practice with Deviation

⚠️ Never place ferromagnetic objects (tools, watches, metal cases) near the compass — they introduce unknown extra deviation.

📦 Ferromagnetic cargo should be stowed as far from the compass as loading limits permit.

🔄 A new compass swing may be required if a large ferromagnetic load is carried.

✈️ The swing must be conducted with: engines running, all electrical/radio services on, aircraft in level attitude.

The Direct Indicating Compass