Airpath C2400 Cowl-Mount Compass
All compasses in the C–2400 series meet FAA TSO-C7c requirements. Each has been specifically designed for installation either for streamline–type bracket housing or a pedestal–type housing. Each model is provided with mounting hardware, deviation card, deviation card window and either an attached cardholder, deviation plate, or standard unattached cardholder depending on the application.
About the Magnetic Compass
One of the oldest and simplest instruments for indicating direction is the magnetic compass. It is also one of the basic instruments required by Title 14 of the Code of Federal Regulations (14 CFR) part 91 for both VFR and IFR flight.
A magnet is a piece of material, usually a metal containing iron, that attracts and holds lines of magnetic flux. Regardless of size, every magnet has two poles: north and south. When one magnet is placed in the field of another, the unlike poles attract each other, and like poles repel.
An aircraft magnetic compass has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid similar to kerosene. A graduated scale, called a card, is wrapped around the float and viewed through a glass window with a lubber line across it. The card is marked with letters representing the cardinal directions, north, east, south, and west, and a number for each 30º between these letters. The final “0” is omitted from these directions. For example, 3 = 30º, 6 = 60º, and 33 = 330º. There are long and short graduation marks between the letters and numbers, each long mark representing 10º and each short mark representing 5º. – Source: Pilot's Handbook of Aeronautical Knowledge.
Magnetic Compass Deviation
The magnets in a compass align with any magnetic field. Some causes for magnetic fields in aircraft include flowing electrical current, magnetized parts, and conflict with the Earth’s magnetic field. These aircraft magnetic fields create a compass error called deviation.
Deviation, unlike variation, depends on the aircraft heading. Also unlike variation, the aircraft’s geographic location does not affect deviation. While no one can reduce or change variation error, an aviation maintenance technician (AMT) can provide the means to minimize deviation error by performing the maintenance task known as “swinging the compass.” Source: Pilot's Handbook of Aeronautical Knowledge.
Magnetic Compass Variation
The Earth rotates about its geographic axis; maps and charts are drawn using meridians of longitude that pass through the geographic poles. Directions measured from the geographic poles are called true directions. The magnetic North Pole to which the magnetic compass points is not collocated with the geographic North Pole, but is some 1,300 miles away; directions measured from the magnetic poles are called magnetic directions. In aerial navigation, the difference between true and magnetic directions is called variation. This same angular difference in surveying and land navigation is called declination. Source: Pilot's Handbook of Aeronautical Knowledge.
Northerly Turning Errors
The center of gravity of the float assembly is located lower than the pivotal point. As the aircraft turns, the force that results from the magnetic dip causes the float assembly to swing in the same direction that the float turns. The result is a false northerly turn indication. Because of this lead of the compass card, or float assembly, a northerly turn should be stopped prior to arrival at the desired heading. This compass error is amplified with the proximity to either magnetic pole. One rule of thumb to correct for this leading error is to stop the turn 15 degrees plus half of the latitude. Source: Pilot's Handbook of Aeronautical Knowledge.
Southerly Turning Errors
When turning in a southerly direction, the forces are such that the compass float assembly lags rather than leads. The result is a false southerly turn indication. The compass card, or float assembly, should be allowed to pass the desired heading prior to stopping the turn. As with the northerly error, this error is amplified with the proximity to either magnetic pole. To correct this lagging error, the aircraft should be allowed to pass the desired heading prior to stopping the turn. The same rule of 15º plus half of the latitude applies here. Source: Pilot's Handbook of Aeronautical Knowledge.