Introduction

The gyromagnetic compass is a horizontal gyro slaved to magnetic north by a magnetic field detector (flux valve) usually located in the wing tips of the aircraft away from metal structure. The short term accuracy of the gyro combined with the long term accuracy of the magnetic compass provides a stable and accurate heading reference.

Gyromagnetic Compass System

A Gyromagnetic compass system comprises:

• A magnetic detector (flux valve) which electronically senses the horizontal component of the earth’s magnetic field;
• A turn/accelerator cut out switch which cuts out the feed from the flux valve to the gyro during turns and acceleration;
• A gyroscope which maintains its position reference and continues to point in the same direction regardless of what the aircraft does;
• Compass repeaters to transmit heading information to the associated instruments;

Operating Modes

The GMC has two operational modes (selectable by the pilot through the GMC control panel):

• Magnetic slaved information (also revered to as “SLAVE” mode) – this mode is used when magnetic heading references are reliable. In this mode, the directional gyro is slaved to the magnetic flux detector, which supplies long-term magnetic reference to correct the apparent drift of the gyro.In order to ensure correct indication of the directional gyro indicator it is necessary to align it with the magnetic compass at regular intervals, approximately every 10 to 15 minutes. On some modern aircraft this alignment is performed automatically by the control system.

• Directional gyro (also referred to as “FREE” mode) – this mode is used when magnetic heading references are not reliable (e.g. in polar regions). In this mode, the system furnishes an inertial heading reference, with corrections introduced manually to offset earth rate and other errors.

Further Reading

• Gyroscopic Flight Instruments
• Standby Flight Instruments
• Pilot Equipment Interface
• Inertial Reference System (IRS)