CFI Notebook, All rights reserved. As the aircraft accelerates from this state, to increase its airspeed, the following happens: Similar would be the effect on the magnetic compass if an aircraft decelerates in flight. When the aircraft is in a coordinated turn, the card also banks because of centrifugal force. var today = new Date() // Inertia of the float, along with the bar magnet, forces the rotating float to lag behind in its attempt to catch-up with the speeding airplane. Understand how the magnetic compass works, the associated errors, how to correct for these errors, and rate of turns. On a northerly or southerly heading the compass will usually not turn but dip. This line is referred to as the lubber line. Some other definitions: lines of equal variation are called isogonic and were the variation is zero it is called an agonic line. The magnetic compass of today is modified to decrease an effect categorized as “magnetic dip.” This modification incorporates a pivot assembly that is not connected to the float of the bar magnet at its center of gravity, but is in fact connected to the float at a point well-displaced of the center of gravity. For an aircraft in flight, in the northern hemisphere: Tom, an instructor from the Caesar Creek Soaring Club, explains acceleration errors in a magnetic compass on his glider: This causes acceleration errors in an aircraft’s magnetic compass. | Privacy Policy | Terms of Service | Sitemap | Glossary | Patreon | Contact, Advisory Circular (43.13-1B CHG 1) Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair, Federal Aviation Administration - Pilot/Controller Glossary, Federal Aviation Regulation (91.205) Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements, Instrument Flying Handbook (3-10) Magnetism, The magnetic compass was one of the first flight instruments developed, A compass is a simple magnetic bar suspended in fluid, It floats in a hardened steel pivot in its center that rides inside a special, spring-loaded, hard glass jewel cup, The magnetic compass is a reliable, self-contained unit requiring no external power source, It is extremely useful as a standby or emergency instrument, 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 30° between these letters, The final "0" is omitted from these directions; for example, 3 = 30°, 6 = 60°, and 33 = 330°, There are long and short graduation marks between the letters and numbers, with each long mark representing 10° and each short mark representing 5°, The Earth is a huge magnet with lines of flux which make its magnetic field, These lines extend from the poles around the Earth, Any magnet that is free to rotate (such as an aircraft's magnetic compass) will align with them, An electrical current is induced into any conductor that cuts across them, A magnet, typically made of iron, attracts and holds lines of flux, Opposite poles attract, while similar poles repel, The float and card assembly has a hardened steel pivot in its center that rides inside a special, spring-loaded, hard-glass jewel cup, An aircraft magnetic compass has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid, The compass housing is entirely full of compass fluid, similar to kerosene, to avoid freezing at lower temperatures/higher altitudes, The buoyancy of the float takes most of the weight off the pivot, and the fluid damps the oscillation of the float and card, This jewel-and-pivot type mounting allows the float freedom to rotate and tilt up to approximately 18° angle of bank, At steeper bank angles, the compass indications are erratic and unpredictable, To prevent damage or leakage when the fluid expands and contracts with temperature changes, the rear of the compass case is sealed with a flexible diaphragm or with a metal bellows in some compasses, The magnets align with the Earth's magnetic field and the pilot reads the direction on the scale opposite the lubber line, When the pilot is flying north as the compass shows, east is to the pilot's right, but on the card "33", which represents 330° (west of north), is to the right of north, The reason for this apparent backward graduation is that the card remains stationary, and the compass housing and the pilot turn around it, always viewing the card from its backside, A compensator assembly mounted on the top or bottom of the compass allows an aviation maintenance technician (AMT) to create a magnetic field inside the compass housing that cancels the influence of local outside magnetic fields, This is done to correct for deviation error, The compensator assembly has two shafts whose ends have screwdriver slots accessible from the front of the compass, Each shaft rotates one or two small compensating magnets, The end of one shaft is marked E-W and its magnets affect the compass when the aircraft is pointed east or west, The other shaft is marked N-S and its magnets affect the compass when the aircraft is pointed north or south, The magnetic compass is the simplest instrument in the panel, but it is subject to a number of errors that must be considered, These errors can be remembered with the acronym "VD-MONA", Latitude and longitude are based on "true" directions meaning they provide a constant horizontal and vertical plane with which to reference on maps and charts, The magnetic pole to which the magnetic compass points is not collocated with the geographic "true" 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, The amount of variation depends on your location in relation to the poles, Points of equal variation can be connected by an "isogonic" line on a map, The line that passes near Chicago (0°) is called the agonic line, East of this line, the magnetic pole is to the west of the geographic pole and a correction must be applied to a compass indication to get a true direction, Flying in the Washington, D.C. area, for example, the variation is 10° west, If the pilot wants to fly a true course of south (180°), the variation must be added to this resulting in a magnetic course to fly of 190°, West of this line, the magnetic pole is to the east of the geographic pole and a correction must be applied to a compass indication to get a true direction, Flying in the Los Angeles, CA area, the variation is 14° east, To fly a true course of 180° there, the pilot would have to subtract the variation and fly a magnetic course of 166°, The variation error does not change with the heading of the aircraft; it is the same anywhere along the isogonic line, Isogonic lines can be found on sectional charts with a dashed magenta line and the number associated, Used to convert true course to magnetic course, Variation east, magnetic track least (-) while variation west, magnetic track best (+), Local magnetic fields in an aircraft caused by electrical current flowing in the structure, in nearby wiring or any magnetized part of the structure, cause a compass error called deviation, Deviation manifests itself differently between aircraft and depending on heading however, but it is not affected by the geographic location, Deviation error can be minimized when a pilot or AMT performs the maintenance task known as ", Most airports have a compass rose, which is a series of lines marked out on a taxiway or ramp at some location where there is no magnetic interference, Lines, oriented to magnetic north, are painted every 30°, The pilot or AMT aligns the aircraft on each magnetic heading and adjusts the compensating magnets to minimize the difference between the compass indication and the actual magnetic heading of the aircraft, Any error that cannot be removed is recorded on a compass correction card and placed in a card-holder near the compass, If the pilot wants to fly a magnetic heading of 120° and the aircraft is operating with the radios on, the pilot should fly a compass heading of 123°, The corrections for variation and deviation must be applied in the correct sequence and is shown below starting from the true course desired, Error due to magnetic interference with metal components in the aircraft as well as magnetic fields from the aircraft's electrical equipment, Compensating magnets inside the compass casing can help reduce this error, but not completely eliminate it, True Course (180°) ± Variation (+10°) = Magnetic Course (190°), The Magnetic Course (190°) is steered if there is no deviation error to be applied, The compass card must now be considered for the compass course of 190°, Magnetic Course (190°, from step 1) ± Deviation (-2°, from correction card) = Compass Course (188°).

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