Overview
The standby compass system supplies the flight crew with the magnetic heading data. The standby compass system is in constant operation and is used as a backup display for the magnetic heading.
05/02/16
Standby Compass
The standby compass is installed below the overhead panel and is aligned with the aircraft longitudinal axis.
The standby magnetic compass is a self-contained unit consisting of a free-turning compass card mounted within a transparent, molded bowl. The bowl is filled with silicon fluid which provides the dampening for the compass card. Metal bellows allow the fluid to expand and contract due to changes in temperature and altitude.
The card is pivoted on a jewel bearing on a stem mounted from the bottom of the bowl. The magnet system consists of a circular magnet fitted to the dome of the compass card. The compass card is marked every 10" with a vertical white line. Each 30 degrees line (except the cardinal points) are identified with a longer line and the appropriate numerals representing degrees with the last numeral omitted (i.e. "3" denotes 30 degrees, "24" denotes 240 degrees). The cardinal points are marked "N", "S", "E", and "W".
Heading of the aircraft is read against a vertical lubber line engraved and colored white on the surface of the bowl.
A special miniature lamp bulb, of non-magnetic construction, is housed below the bowl to provide internal illumination of the compass card markings and the lubber line. The lamp is powered by five VDC which is supplied to the compass through a twin-conductor supply lead.
The index error adjustment is used when performing a standby compass swing calibration. During the calibration, the differences in readings between the heading indicator and the compass rose are recorded and an index error is obtained.The center major index mark on the index error adjustment scale is used as a reference point when performing any adjustments to the compass.
Correction to overcome errors affecting East-West and North-South headings are made during the production compass swing by inserting a special corrector key into the "B" correction hole and "C" correction hole, respectively. Rotation of the key applies correction to the compass by turning corrector magnets located within the compass.
A compass correction card is mounted just below the standby compass in the cockpit. The purpose of the card is to provide corrected headings for the compass due to the magnetic interference caused by the electrical and other avionic systems onboard the aircraft. The deflection of the compass caused by applying power to all systems is measured during compass calibration in the aircraft.
These deflections are recorded in the compass correction card. For each of the cardinal headings plus the 45 degrees increments between them, the compass correction card will provide a corrected heading to steer towards (i.e. for 135 degrees, steer 133 degrees).
05/02/16
System Interface
The standby compass system interfaces to the integral lighting system.
The integral lighting system supplies 5 VAC to the light for the standby compass. The light bulb for the compass is a special non-magnetic bulb that provides light for the compass card markings and lubber line.