Remote Indicating Compass Systems

Principles of Operation

A remote indicating compass system consists of a flux valve (or flux gate detector), a gyro unit, signal transducers, precession amplifiers, annunciators, and cockpit display units. The flux valve, typically installed in a wingtip or tail, detects the Earth's magnetic field with minimal interference from aircraft magnetism. Its output is sent electrically to a gyro-stabilized indicator in the cockpit, often displayed as a rotating compass card.

The gyro provides short-term stability, resisting rapid changes and oscillations, while the flux valve ensures long-term alignment with magnetic north. The system continuously 'slaves' the gyro to the magnetic heading, correcting for drift at a controlled rate. This combination is often called a slaved compass or flux valve compass.

Advantages and Disadvantages

Advantages:

• Reduced deviation errors due to remote placement of the flux valve.
• Less susceptible to turning and acceleration errors compared to direct-reading compasses.
• Stable and accurate heading display, even during manoeuvres.
• Heading information can be distributed to multiple systems (e.g., RMI, HSI, autopilot).
• Easier to read and interpret, especially in turbulent conditions.

Disadvantages:

• Requires electrical power and is heavier/more complex than a simple magnetic compass.
• Still subject to residual deviation and errors if not properly compensated or calibrated.
• Turning and acceleration errors can occur if the flux valve tilts, though mitigated by system design.

System Components Explained

• Flux Detection Unit: Senses the Earth's magnetic field, mounted remotely to avoid interference.
• Gyro Unit: Provides heading stability and resists rapid changes.
• Transducers & Precession Amplifiers: Convert magnetic signals to electrical signals and control gyro corrections.
• Annunciator: Indicates the status of the slaving loop; flickering shows correct operation.
• Display Unit: Shows heading on a compass card, often with synchronising and set-heading knobs, and mode switches (DG/compass/slave/free).

Errors and Corrections

• Deviation: Minimized by remote mounting and compensation magnets, but not eliminated.
• Turning/Acceleration Errors: Reduced, but can occur if the flux valve tilts; some systems disconnect slaving during excessive pitch/bank.
• Attitude Errors: System design helps maintain accuracy during normal flight attitudes.

Information Availability

Remote indicating compass systems can feed heading data to multiple displays and avionics, supporting advanced navigation and autopilot functions, unlike direct-reading compasses which are limited to a single cockpit display.