Remote Magnetic Compasses

Remote indicating compasses were developed to compensate for the errors and limitations of the older type of heading indicators. The two panel-mounted components of a typical system are the pictorial navigation indicator and the slaving control and compensator unit.

The remote-reading compass automatically and continuously compares the output of a magnetic sensing element with the indicated heading of the gyro indicator. It then resets the gyro whenever a discrepancy exists. The gyro output is therefore slaved to magnetic north.

Hence also known as the Slaved Gyro Compass. Other names are Gyromagnetic Compass or Heading Reference Unit. The pilot is no longer required to reset the gyro indicator periodically.

The slaving control and compensator unit has a push button that provides a means of selecting either the "slaved gyro" or "free gyro" mode. This unit also has a slaving meter and two manual heading-drive buttons. The slaving meter indicates the difference between the displayed heading and the magnetic heading. A right deflection indicates a clockwise error of the compass card; a left deflection indicates a counterclockwise error.

Whenever the aircraft is in a turn and the card rotates, the slaving meter shows a full deflection to one side or the other. When the system is in "free gyro" mode, the compass card may be adjusted by depressing the appropriate heading-drive button.

A separate unit, the magnetic slaving transmitter is mounted remotely, usually in a wingtip to eliminate the possibility of magnetic interference. It contains the flux valve, which is the direction-sensing device of the system. A concentration of lines of magnetic force, after being amplified, becomes a signal relayed to the heading indicator unit, which is also remotely mounted.

This signal operates a torque motor in the heading indicator unit that processes the gyro unit until it is aligned with the transmitter signal. The magnetic slaving transmitter is connected electrically to the HSI.

System Errors

The remote indicating gyro compass suffers from errors caused by variations in aircraft magnetism and by changes in the earth's magnetic field. However as compared to a direct reading compasses the errors are to a lesser degree. They must be swung periodically to establish compass deviation. The detector is pendulous so as to be free to remain horizontal and detect only the H component of the earth's field. But its freedom is limited to 25° off the aircraft vertical.

Longitudinal and lateral accelerations will tilt the detector, introducing readings of the Z component. Bank and pitch beyond 25° will also pull the detector out of the horizontal. Because of these potential errors there are various sensors incorporated to detect pitch, bank and acceleration.

When the set limits are exceeded, the magnetic monitoring system is switched off. During manoeuvres and accelerations the gyro is still available as a heading reference but subject to the usual gyro errors of earth rate and transport wander. These errors (as compared to a direct reading compass) are relatively small and with the stability of the gyro element and the relatively slow rate of synchronisation used.

The gyros, like the DI are subject to gimballing error in turns and can be ignored.