Brunei DCA CPL(A) General Navigation key terms

The equipotential surface of the Earth's gravity field that coincides with mean sea level, extended under the continents; used as the reference for heights and for defining the Earth's shape.

A mathematically smooth spheroid approximating the Earth, defined by semi-major axis and flattening, on which latitude, longitude and chart projections are computed.

A defined set of parameters fixing an ellipsoid to the Earth, including origin and orientation, so that coordinates of points are unambiguous; changing datum shifts positions by up to hundreds of metres.

The World Geodetic System 1984 datum and ellipsoid used by GNSS and most modern aeronautical charts; positions referenced to other datums must be converted before use.

The intersection of a sphere with a plane through its centre; it gives the shortest surface distance between two points and appears as a curved line on most charts.

A line crossing every meridian at the same true track, appearing straight on a Mercator chart; it is longer than the great circle except along meridians and the equator.

The true track at the starting point of a great circle route; it changes continuously along the route and must be recomputed or waypointed for practical navigation.

The angle between two meridians, equal to the change of longitude between them; it equals the change in great circle track between the two meridians.

A line of constant longitude running north-south between the poles; meridians converge toward the poles and define the reference direction for true north.

A line of constant latitude, parallel to the equator; its length decreases with latitude and it is a small circle except at the equator itself.

The east-west distance in nautical miles between two meridians along a given parallel, equal to change of longitude multiplied by the cosine of the latitude.

A cylindrical conformal projection on which rhumb lines plot straight and angles are preserved, but scale increases with latitude, so great circles appear curved and polar areas are unusable.

A conic conformal projection using a cone intersecting the Earth at one or two standard parallels; it preserves angles, gives near-straight great circles, and suits mid-latitude charts.

The ratio between a distance on the chart and the corresponding distance on the Earth; on some projections it varies with position, so distances must be measured at the correct latitude.

The intended or actual path of the aircraft over the ground, expressed as a true or magnetic direction from north; it differs from heading whenever drift occurs.

The direction in which the aircraft's longitudinal axis points, referenced to true or magnetic north; it is the direction the pilot steers and differs from track by drift.

A vector described by wind direction (the direction the wind blows from) and wind speed; it combines with true airspeed to determine groundspeed and drift.

The angular difference between heading and track caused by wind, positive to the right of heading; it is found from the wind triangle and applied to obtain the heading to fly.

The speed of the aircraft relative to the ground, obtained by combining true airspeed with the wind component along track; used to calculate times and fuel between waypoints.

The calculation of present position by applying known track, groundspeed and elapsed time to a known starting point, without reference to external position-fixing aids.

A vector diagram relating true airspeed and heading to wind velocity and groundspeed, used to solve for drift, heading, groundspeed or wind given the other elements.

The angle between true north and magnetic north at a given place and date, shown on charts as isogonals; it is east or west and changes slowly over time.

The angle between magnetic north and the north indicated by a particular compass, caused by the aircraft's own magnetic fields; it depends on heading and is read from a calibration card.

A line on a chart joining places of equal magnetic variation; isogonals are annotated with their value and annual rate of change for updating.

The process of converting between true, magnetic and compass directions by adding or subtracting variation and deviation using the memory aid east is least, west is best.

The primary time standard, based on atomic clocks and kept within one second of mean solar time at the prime meridian; it is used for all flight planning and ATC communications.

The mean solar time at a specific longitude, equal to UTC plus or minus four minutes for each degree of longitude east or west of the prime meridian.

Standard time within a time zone, usually UTC offset by whole hours according to the zone's central meridian; it differs from local mean time except on that meridian.

The times the upper limb of the Sun crosses the horizon, tabulated in UTC or local time for latitude and date; they depend on latitude, season and refraction assumptions.

The total time an aircraft can remain airborne with the fuel available, calculated from fuel quantity and hourly consumption; it must exceed the planned flight time plus reserves.

The point along a route beyond which the aircraft can no longer return to the departure aerodrome with the fuel remaining; it depends on wind, fuel and consumption.

The position from which the time to continue to destination equals the time to turn back to departure, considering wind in both directions; used for diversion planning.

A magnetic bearing from a VOR station, identified by its phase relationship with a reference signal; an aircraft on a given radial is on that bearing from the station.

A determined aircraft position obtained by intersecting lines of position from radio aids, GNSS or visual references, used to update dead reckoning.

Area navigation allowing the aircraft to fly a defined path between waypoints not necessarily located over ground-based aids, using onboard computation of position and guidance.

A specified geographical location, defined by coordinates or by reference to a navigation aid, used to define an RNAV route or terminal procedure.

A position derived from ranging measurements to satellites of a global navigation satellite system, giving latitude, longitude, altitude and time referenced to WGS-84.

Required Navigation Performance: a navigation specification stating the on-board performance monitoring and alerting and the lateral accuracy, such as RNP 1, to be achieved in a given airspace.

The point through which the total weight of the aircraft acts; its position affects stability, control and fuel consumption, and must remain within specified limits.

The mass of the aircraft with all operating equipment, unusable fuel and full oil, excluding crew, passengers, usable fuel and payload; the starting point for mass and balance calculations.

The difference between maximum certificated take-off mass and basic empty mass, comprising crew, passengers, usable fuel and payload that can be carried.