Direct Reading Compass The purpose of liquid compass is to indicate in which direction the magnetic north pole lies, to enable and ac position to be determine with respect to that pole. They are classified under two headings.
(a) Pilot Type. It is used to indicate the magnetic heading of the aircraft.
(b) Observer Type. It is used with adjustable sights to enable bearing to be taken of distant object (ac)
Pilot Type. The magnetic compass is a simple, self-contained instrument, which operates on the principle of magnetic attraction. It is designed to indicate the direction in which aircraft is headed. The magnetic compass is an independently operating instrument described here as mechanical because it requires no power from any aircraft system
If a bar magnet is mounted on a pivot to be free to rotate in horizontal plane, it will assume a position with one of its ends pointing towards earths north magnetic pole. The magnetic compass consists of a liquid filled bowl containing a pivoted float element to which one or more bar magnets are fastened. The liquid in the bowl dampens oscillation of the float, decreases the friction of pivot and lubricants the pivot bearing. A diaphragm and vent provide for expansion and contraction of the liquid that results from change in temperature and altitude. A circular compass card, usually graduated in increments of 5/10 deg is attached to the float element of the compass. A fixed reference marker, called a lubber line, is attached to the compass bowl. The lubber line and graduation on the card is visible through a glass window. The compass card assembly is mounted on a jeweled pivot bearing on the jewel port assembly.
The compass provides an indication of magnetic direction rather than true direction. The bar magnet in the compass aligns itself with the magnetic lines of force in the earth’s magnetic field.
Magnetic compass is subjected to directional inaccuracy, which is caused by other stray magnetic influence other than earth’s magnetic field. These are also caused by the magnetic influence developed inside the airplane due to the proximity of magnetic material, such as iron and steel and by the current flowing through nearby electrical circuit. The compass is adjustable, so as to correct for the NORTH-SOUTH deviation and EAST-WEST deviation. A compensating device containing small permanent magnet is incorporated in the compass to correct for deviation of the compass. Two screws on the face of instruments are used to move the magnets and thus counter balance the local magnetic influences acting on the main compass magnet. Two set screws are labeled as N-S and E-W.
Vertical Dial Compass. The vertical dial direction indicator is actually a direct reading compass, instead of the direction is being read from swinging compass card, however, the reading is taken from a vertical dial.
The reference index on the vertical dial can be set any desired heading by turning the knob at the bottom of the dial, and it is necessary only to match the indicator needle with the reference pointer to hold a course. The design of the dial provides easy reading of direction and also quick indication of deviation from a selected heading. The compass liquid in the instrument is contained in a separate chamber and the dial, instead of floating in the liquid as in other types of magnetic compasses, is completely dry. This makes the indicator easier to read and eliminates fluid leakage around the dial. The float assembly, which contains the direction magnet, is located in fluid filled bowl.
Occasion of Compass Swinging. The process of “compensating for errors” is known as Compass Swinging. Compass compensation procedure varies, depending on the type of the aircraft. There are following few Occasions, when compass swinging should be carried out.
(a) Any change of compass or corrector box.
(b) After any change of major components of aircraft such as tail plane, oleo leg, engine, radio equipment or any other equipment that could have an effect on the compass.
(c) After the ac has been standing on any one heading for more than four weeks.
(d) Periodical swinging (every three months).
(e) The direction of flight commander.
(f) At any time when its accuracy is doubted.