Chapter 17
Remote Indicating Magnetic Compass

1. Limitations of the DRMC and DGI

Direct Reading Magnetic Compass (DRMC) Limitations

• Turning and acceleration errors — cannot be read accurately during a turn.
• Magnetic sensing element is within the instrument, close to the pilot — hence close to deviation sources (lights, motors, ferrous metal in the cockpit).
• Self-contained — cannot feed heading information to other equipment.

Directional Gyro Indicator (DGI) Limitations

• No magnetic element — if the gyro drifts, there is no correction except by the pilot manually synchronizing to the DRMC at regular intervals.
• Turning and acceleration errors are eliminated, and output can be taken to other equipment — but long-term heading accuracy suffers from drift.

2. Requirement for the Gyro-magnetic Compass

The ideal system combines the short-term rigidity of the gyro (overcoming turning and acceleration errors) with the long-term magnetic monitoring of the earth's field (correcting gyro drift via a servo/slaving system). This is the gyro-magnetic compass.

3. Basic System Description and Components

flowchart LR
    FV["Flux Valve\n(Detector Unit)"] --"AC Error Signal"--> AMP["Precession Amplifier\n(Amplify, Phase-detect,\nRectify to DC)"]
    AMP --"DC Signal"--> PM["Precession Motor"]
    PM --"Corrects"--> GY["Horizontal Gyro"]
    GY --"Direct Drive\nShaft (bevel gear)"--> HI["Heading Indicator\n(Compass Card)"]
    HI --"Rotor-Stator\nComparison"--> FV
    HI --"Heading Output\n(Selsyn)"--> OTH["Other Instruments\n(ADI, RMI, FMS...)"]

4. Operation with Steady Heading — Gyro Drift Correction

1. The flux valve senses the earth's magnetic field and reproduces it within the compass unit, where it is compared with the position of the gyro drive shaft.
2. If the two are aligned → no error signal → compass reads correctly. No action needed.
3. If the gyro drifts → drive shaft misaligns with flux valve field → an AC error signal is generated and passed to the precession amplifier.
4. The amplifier amplifies, phase-detects, and rectifies the AC error signal to DC.
5. The DC signal drives the precession motor, which turns the gyro.
6. The gyro output is fed via the direct drive shaft to the heading indicator for comparison with the flux valve signal.
7. When aligned → compass synchronized → no further action.

5. Operation in a Turn — Gyro Drift Small Over Period of Turn

During a turn, the aircraft turns but the gyro (having rigidity) does not. The relative rotation between the horizontal gyro and the instrument case operates the bevel gear on the direct drive shaft, changing the heading indication on the compass card. At the same time, the heading sensed by the flux valve is also changing at the same rate. Therefore, no error signal is generated and the compass remains synchronized during the turn.

If there is some gyro drift during the turn, on completion of the turn a small error signal will appear and be corrected at the normal rate of ≈3°/min.

6. Rapid Synchronization

On initial switch-on, the gyro adopts a random alignment unlikely to be synchronized with the earth's magnetic field. At the normal rate of ≈3°/minute, a 90° misalignment would take 30 minutes to correct — unacceptable.

7. Detector Unit — The Flux Valve

The detector unit is positioned in a part of the aircraft least affected by on-board electrical fields — usually the wing tip or tail fin, where aircraft-generated magnetic disturbances are at a minimum.

Its function is to sense the direction of the earth's magnetic field. It contains a pendulous magnetic detecting element mounted on a Hooke's Joint which enables the detector to swing within limits of 25° about the pitch and roll axes, but allows no rotation in azimuth. The unit is contained in a sealed case partially filled with oil to dampen oscillations.

The primary component is the flux valve — a 3-spoked device, fixed in azimuth but with some freedom in the vertical to align with the plane of the earth's magnetic field. The curved 'rams horns' at the end of each leg improve magnetic flux gathering efficiency.

8. Flux Valve — How It Works (Faraday's Law)

Single Leg Operation

AC is fed to the coil wound around the centre post, producing fields of opposite sign (anti-phase) in the top and bottom legs of the flux valve leg. Without the earth's background field, these two fields cancel → resultant flux = zero → no current in the pick-off coil.

When the earth's magnetic field is present as a background, the positive and negative flux start from a non-zero baseline. The physical characteristics of the flux valve metal are such that it magnetically saturates at a certain level — it will not magnetize further beyond the saturation level.

Faraday's Law of Electromagnetic Induction: "If the number of lines of force threading a circuit is changing, an induced electromotive force (EMF) will be set up in the circuit, the magnitude of the EMF being proportional to the rate of change in the number of lines of force threading the circuit."

The secondary winding picks up the change in magnetic flux density (the dips in the resultant flux) as an EMF, detected as an AC signal.

Why 3 Legs Are Needed

The EMF varies as the cosine of the angle between the flux valve leg and the earth's field — but for any given voltage, there are two possible heading values (cos is not a unique function from 0–360°). A single leg cannot unambiguously determine heading.

The 3-leg system solves this: the output from each leg is fed to one of 3 legs of a stator, recreating the earth's field relative to the flux valve direction around the direct drive shaft from the gyro to the heading indicator compass card. The 3-phase output uniquely determines the field direction.

9. Error Signal Comparison — Rotor-Stator

A wound coil (the rotor) is mounted on the gyro drive shaft. If the coil is in line with the AC field generated by the stators, a secondary AC voltage is induced (maximum voltage). If the rotor is at 90° to the AC field (null position), no secondary voltage is induced. At any other position, some voltage is induced.

This secondary voltage = the error signal. It is passed to the precession amplifier where it is:

10. Heading Indicator

The heading indicator dial (compass card) is directly driven by the shaft from the gyro. The compass card rotates as heading changes and the heading is read against the index line in the 12 o'clock position (the lubber line).

A heading bug (heading select marker) can be set by the pilot to indicate a desired heading. If the magnetic input from the flux valve fails, a heading warning flag appears.

11. Operation as a DGI — FREE Mode vs. SLAVED Mode

12. Annunciator

The annunciator is an indicator positioned in the error signal path between the precession amplifier and the precession motor. During normal flight in SLAVED mode, continuous slight oscillations of heading mean the rotor/stator comparison continuously generates very small error signals. This continuous 'hunting' is normal and by design.

13. Keeping the Gyro Axis Horizontal

Gyro wander takes two forms: drift (azimuth wander — handled by the slaving system) and topple (vertical wander — must be corrected separately).

The gyro must be tied either to the aircraft yaw axis or to gravity to stay horizontal. Both methods use a levelling switch and a torque motor:

• Yaw axis method: Inner and outer gimbals maintained at 90° to each other by a system of commutators, insulating strips, and brushes.
• Gravity method: Mercury gravity switches used.

Either way, correcting signals are passed to a torque motor which applies rotational force to the gyro in the yaw axis. The resulting precession returns the gyro to the horizontal — but at a slow precession rate so it does not react wildly to temporary departures (turns, accelerations, climbs, descents).

14. Transmitting Heading Output — Selsyn Unit

Heading information is picked off from the drive shaft between the gyro and the compass card and transmitted to other instruments using a Selsyn Unit (synchro transmitter-receiver pair).

How it works:

1. The transmitter rotor is attached to the heading drive shaft and rotates with it. It is supplied with a constant primary AC voltage, which induces a field in the stators.
2. The stators are directly connected by 3-strand wire to the 3 stator arms of the repeater, reproducing an identical field there.
3. If the repeater rotor is not perpendicular to the repeater stator field, an AC voltage is induced in the repeater rotor, passed to an amplifier and then a motor to turn the repeater shaft until no further voltage is detected.
4. The repeater shaft therefore follows any heading changes in the main gyro drive shaft — driving displays in other instruments (RMI, EHSI, autopilot, FMS, etc.).

15. Summary

Master Reference Tables

Signal Flow Summary

Answer Key Summary