Since the magnetic compass works on the principle of magnetism, it is well for the pilot to have at least a basic understanding of magnetism. A simple bar magnet has two centers of magnetism which are called poles. Lines of magnetic force flow out from each pole in all directions, eventually bending around and returning to the other pole. The area through which these lines of force flow are called the field of the magnet. For the purpose of this discussion, the poles are designated "north" and "south." If two bar magnets are placed near each other, the North Pole of one will attract the South Pole of the other. There is evidence that there is a magnetic field surrounding the Earth, and this theory is applied in the design of the magnetic compass. It acts very much as though there were a huge bar magnet running along the axis of the Earth which ends several hundred miles below the surface.
The geographic north and south poles form the axis for the Earth's rotation. These positions are also referred to as true north and south. The magnetic north and south poles form another axis. Lines of magnetic force flow out from each pole in all directions, and eventually return to the opposite pole. A compass aligns itself with the magnetic axis formed by the north/south magnetic field of the Earth.
The lines of force have a vertical component (or pull) which is zero at the Equator, but builds to 100 percent of the total force at the magnetic poles. If magnetic needles, such as in the airplane's magnetic compass, are held along these lines of force, the vertical component causes one end of the needle to dip or deflect downward. The amount of dip increases as the needles are moved closer and closer to the poles. It is this deflection, or dip, that causes some of the larger compass errors.
The magnetic compass, which is usually the only direction-seeking instrument in the airplane, is simple in construction. It contains two steel magnetized needles fastened to a float, around which is mounted a compass card. The needles are parallel, with their north-seeking ends pointing in the same direction. The compass card has letters for cardinal headings, and a number, the last zero of which is omitted, represents each 30° interval. For example, 30° appear as a 3 and 300° appears as a 30. Between these numbers, the card is graduated for each 5°. The magnetic compass is required equipment in all airplanes. It is used to set the gyroscopic heading indicator, correct for precession, and as a backup in the event the heading indicator(s) fails.
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