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clockwise to a line representing the course of the aircraft. The aircraft course measured with
reference to magnetic north.
True Course (TC)—The angle measured clockwise from true north to the line representing the
intended path of the aircraft.
Course Line—A line of position which is parallel or approximately parallel to the track of the aircraft.
A line of position used to check aircraft position relative to intended course.
Crab Angle—The angle between the aircraft longitudinal axis and ground track used to correct for wind
drift and maintain an intended track.
Cruise Control—The operation of an aircraft to obtain the maximum efficiency on a particular mission
(most miles per amount of fuel).
D Reading—The difference between pressure altitude and true altitude at a different time in flight (true
altitude minus pressure altitude).
Datum—Any numerical or geometrical quantity or set of such quantities which may serve as reference
or base for other quantities.
Day:
Civil Day—The interval of time between two successive lower transits of a meridian by the mean
(or civil) sun.
Sidereal Day—The interval of time between two successive upper transits of a meridian by the first
point of Aries (23 hours, 56 minutes).
Solar Day—The interval of time between two successive lower transits of a meridian by the true
(apparent) sun.
Dead Reckoning (DR)—Finding ones position by means of a compass and calculations based on speed,
time elapsed, effect of wind, and direction from a known position.
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Declination (Dec)—The angular distance to a body on the celestial sphere measured north or south
through 90° from the celestial equator along the hour circle of the body. Comparable to latitude on the
terrestrial sphere.
Deviation (dev)—The angular difference between magnetic and compass headings.
Deviation Correction—The correction applied to a compass reading to correct for deviation error. The
numerical equivalent of deviation with the algebraic sign added to magnetic heading to obtain compass
heading.
Diurnal Circle—See Circles.
Dog Leg—A temporary divergence from the desired track to adjust your course timing (Preplanned or
Spontaneous).
Drift—The rate of lateral displacement of the aircraft by wind, generally expressed in degrees.
Drift Angle (DA)—The angle between true heading and true course, expressed in degrees right or left
according to the way the aircraft has drifted.
Drift Correction (DC)—Correction for drift, expressed in degrees (plus or minus), and applied to true
course to obtain true heading.
Ecliptic—The great circle on the celestial sphere along which the sun, by reason of the earth's annual
revolution about the sun, appears to move. The plane of the ecliptic is tilted to the plane of the equator at
an angle of 23° 27'.
Effective Air Distance (EAD)—The distance measured along the effective air path.
Effective Air Path (EAP)—A straight line on a navigation chart connecting two air positions or a fix
position and an air position. Commonly used between the last known position (fix) and the next air
position associated with a fix position.
Effective True Airspeed (ETAS)—The effective air distance divided by the elapsed time between two
pressure readings.
Elevated Poles—That celestial pole which is on the same side of the equinoctial as the position of the
observer.
Equal Altitude—See Circles.
Equation of Time—The amount of time by which the mean sun leads or lags behind the true sun at any
instant. The difference between mean and apparent times expressed in units of solar time with the
algebraic sign so that, when added to mean time, it gives apparent time.
Equator—The great circle on the earth's surface equidistant from the poles. Latitude is measured north
and south from the equator.
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Equinoctial—See Celestial Equator.
Equinox—The Autumnal Equinox is the point on the equinoctial when the sun, moving along the
ecliptic, passes from north to south declination. The Vernal Equinox is the point of intersection of the
ecliptic and the celestial equator (equinoctial) when the sun is moving from south to north declination.
Also called First Point of Aries.
Field-Elevation Pressure—The existing atmospheric pressure in inches of mercury at the elevation of
the field. Also known as station pressure.
Fix—A geographic position of an aircraft determined from terrestrial, electronic, or astronomical data
for a specific time.
Flight Levels—Surfaces of constant atmospheric pressure that are related to a specific pressure datum,
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