Aircraft compass system

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An instrument that indicates the bearing, or angle of the direction in which an aircraft is pointing in the horizontal plane. A compass may indicate magnetic heading or bearing, bearing referenced to a radio signal source, or bearing with respect to an inertially maintained line of position.

In modern commercial jet transport or military fighter aircraft, the primary sensor of the aircraft compass system is the inertial reference system (IRS). This system provides a gyroscopically derived reference to an inertial reference axis by sensing the linear and angular accelerations of the aircraft and continuously integrating these values to provide angular and linear velocities. The inertial reference system derives its position and attitude in an inertial frame referenced to true north. This approach represents an increase in precision over the original compass systems, which developed their lines of position referenced to magnetic north. See also Gyroscope; Inertial guidance system.

The magnetic and true bearings of the aircraft are presented on computer-generated cathode-ray tubes or flat-panel displays. These displays present aircraft heading in the form of a compass rose on both the primary flight display and the navigation display.

The magnetic compass remains a simple, inexpensive, and reliable instrument for indicating the aircraft bearing. Limitations in the accuracy of this compass are due to the accelerations and vibrations of the aircraft, the local induced magnetic field of the aircraft, and lack of knowledge of local magnetic variation (the difference between true north and magnetic north). The magnetic compass is a secondary sensor of bearing on jet transports and most military aircraft, although it remains the primary instrument for many small, general aviation aircraft. This compass is typically a stand-alone instrument consisting of a card indicating the bearing installed in a liquid-filled case. The liquid serves to dampen rapid aircraft movements or oscillations. See also Magnetic compass.

In some aircraft, the magnetic compass has been coupled with a gyroscopic element to provide the gyroslaved, or gyrosynchronized, magnetic compass. The gyrosynchronized compass uses a directional gyro to determine the local horizon and to sense accelerations and thus correct for gravity effects on the compass output. See also Aircraft instrumentation.