CPL Navigation key terms

The line of intersection between Earth's surface and a plane passing through Earth's centre. It is the shortest surface route between two points and is the basis of all long-range track calculations.

A track that crosses every meridian at the same true angle, appearing straight on a Mercator chart. It is longer than a great circle except along the equator or a meridian.

Half the difference between the initial great circle track and the constant rhumb line track between the same two points. It is used when converting a plotted rhumb line to a great circle course.

The true direction, measured at the departure point, of the great circle joining departure to destination. It differs from the final track because meridians converge as latitude increases.

The angle at which two adjacent meridians meet, equal to the difference in longitude between them. It explains why a great circle track changes direction continuously along the route.

A semi-great circle running from the North Pole to the South Pole along which longitude is constant. Meridians are true north-south reference lines for measuring latitude and track.

A small circle on Earth's surface parallel to the equator, joining points of equal latitude. Only the equator is a great circle; other parallels get shorter towards the poles.

The meridian through Greenwich, defined as zero degrees longitude. Longitude is measured east or west from it up to 180 degrees towards the antimeridian.

The east-west distance in nautical miles between two meridians along a given parallel of latitude. It equals change of longitude times the cosine of the latitude.

A distance unit of 1852 metres, approximating one minute of latitude. It is the standard unit for air navigation distances, speeds in knots and chart scale measurement.

A speed of one nautical mile per hour. Wind velocity, true airspeed and ground speed in aviation are normally expressed in knots.

A cylindrical chart projection on which meridians are parallel and rhumb lines plot as straight lines. Scale increases with latitude, so great circle tracks appear curved.

A chart projection using a cone intersecting the globe, giving a nearly straight great circle and minimal scale distortion in mid-latitudes. It is common for aeronautical charts.

The ratio between a distance measured on a chart and the corresponding distance on Earth. It may be stated as a representative fraction or as a graduated bar.

A direction measured clockwise from true north, expressed in degrees from 000 to 359. It is the reference from which magnetic and compass directions are derived.

The angular difference at a location between true north and magnetic north, shown on charts as east or west. It must be applied to convert between true and magnetic directions.

A line on a chart joining places of equal magnetic variation. Isogonals allow the navigator to interpolate variation along a route.

The particular isogonal along which magnetic variation is zero, so true and magnetic directions coincide. It is marked on charts and shifts slowly with time.

The error of a magnetic compass caused by magnetic fields within the aircraft. It varies with heading and is recorded on a compass correction card.

The direction indicated by the aircraft's magnetic compass after deviation is applied to magnetic heading. It is the heading actually steered to achieve a desired track.

The actual path of the aircraft over the ground, expressed as a direction. Track made good differs from heading whenever there is drift.

The direction in which the aircraft's longitudinal axis points, measured clockwise from a chosen north. It is steered by the pilot and differs from track when drifting.

The angular difference between heading and track caused by wind. It is the angle between the aircraft's longitudinal axis and its actual path over the ground.

A vector diagram combining true airspeed and heading with wind velocity to give ground speed and track. It is the fundamental tool for dead reckoning flight planning.

The speed of the aircraft relative to the ground, obtained by vectorially adding wind velocity to true airspeed. It determines time en route and fuel required.

The speed of the aircraft through the air mass, corrected for altitude and temperature from indicated airspeed. It is the airspeed value used in the wind triangle.

Navigation by calculating position from a known starting point using heading, airspeed, elapsed time and wind. It requires no external fixing aid but accumulates error over time.

A confirmed aircraft position obtained from an external source such as radio aids, GNSS or visual ground reference. It resets the dead reckoning plot.

A defined geographical point, identified by coordinates or a named code, used to build a route. Routes are flown from waypoint to waypoint with legs between them.

The horizontal direction of one point from another, measured clockwise from a stated north reference. It is used to fix position by intersecting two or more bearings.

A magnetic bearing from a VOR station, broadcast as one of 360 radials. An aircraft on a given radial lies along that bearing from the station.

A document filed with air traffic services stating the aircraft, route, cruising level, speed and estimated times. It supports search and rescue and traffic coordination.

A worksheet recording legs, headings, wind, ground speeds, times, fuel figures and position reports for a flight. It is the working record of the flight plan in the air.

The calculation of fuel required for taxi, trip, contingency, alternate and final reserve. It ensures legal reserves remain on arrival at the destination.

A map produced for navigation showing airspace, routes, navaids, terrain and obstacles at a stated scale. The type chosen must suit the phase and level of flight.

A notice distributed to flight crews about temporary changes to facilities, services or hazards. Relevant NOTAMs must be reviewed during pre-flight preparation.

The worldwide time standard, abbreviated UTC, used for all flight plan and position reporting times. It avoids confusion between local time zones along a route.

The heading referenced to magnetic north, obtained by applying variation to true heading. It is converted to compass heading using the deviation card.