PPL Navigation & Communication Theory key terms

The largest circle that can be drawn on a sphere's surface, formed by a plane passing through the sphere's centre. It marks the shortest surface distance between two points and is the basis of long-range route plotting.

A line crossing every meridian at the same constant angle, appearing straight on a Mercator chart. It is easier to fly but longer than the great circle except along the equator or a meridian.

The direction of the great-circle path at the point of departure, measured from true north. It differs from the final track because meridians converge toward the poles along the route.

Half the difference between the initial and final great-circle tracks, also equal to half the change of longitude between the two points multiplied by the sine of the mean latitude.

The angular difference at a given place between true north and magnetic north, caused by the Earth's magnetic field. It is easterly or westerly and changes slowly over time and with position.

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

A line on a chart joining places of equal magnetic variation. Pilots use it to interpolate the local variation when converting between true and magnetic directions.

A cylindrical chart projection on which meridians are parallel straight lines and the scale expands with latitude. Rhumb lines plot as straight lines, making it suited to plotting but not to measuring great-circle distances directly.

A chart projection using a cone intersecting the Earth, giving straight-line great circles approximately and a nearly constant scale along a standard parallel. It is widely used for aeronautical charts of mid-latitude areas.

The ratio between a distance measured on a chart and the corresponding distance on the Earth. It may be expressed as a representative fraction, such as 1:500 000, or graphically as a scale bar.

A map produced for navigation that shows airspace, routes, radio aids, obstacles and terrain at a stated scale. The 1:500 000 ICAO chart is the standard for visual navigation in many states.

The Aeronautical Information Publication, a state's official document containing regulations, procedures, aerodrome data and airspace information needed for operations in that country. It is amended regularly and is a primary pre-flight reference.

A notice distributed by aeronautical information services giving timely warning of temporary or permanent changes to facilities, services or procedures that affect flight safety. It must be checked during pre-flight preparation.

A method of estimating position by advancing a known position using heading, true airspeed, elapsed time and wind. It requires no external fixes but its accuracy degrades as errors accumulate.

A vector diagram relating true heading and true airspeed to wind direction and speed, producing true track and groundspeed. It is the core calculation for dead-reckoning navigation and flight planning.

The direction in which the aircraft's longitudinal axis points, measured from a reference such as true or magnetic north. It differs from track whenever the aircraft drifts in wind.

The actual path of the aircraft over the ground, expressed as a direction from a reference north. The track made good may differ from the planned track because of wind or heading errors.

The angular difference between the aircraft's heading and the track it actually makes good over the ground, caused by wind. It is applied to heading to obtain track, or subtracted to find the heading needed.

The speed of the aircraft relative to the ground, obtained by combining true airspeed with the wind component along the track. It determines the time required to cover a given distance.

A mechanical or electronic calculating device used to solve the wind triangle, convert units and compute time, distance and fuel. The circular slide-rule type also provides temperature and altitude conversions.

A very high frequency omnidirectional radio range that transmits a signal allowing an aircraft to determine its magnetic bearing from the station. It provides course guidance along airways and radial position fixing.

The magnetic bearing of a line extending outward from a VOR station, measured clockwise from the station's magnetic north. An aircraft on a given radial is positioned along that bearing from the station.

Distance measuring equipment that measures the slant range from the aircraft to a ground transponder, usually paired with a VOR or ILS. Indicated distance is slightly greater than ground distance and is affected by altitude.

The difference between the straight-line distance measured by DME and the horizontal distance over the ground. It becomes significant at high altitude and short range, and is negligible at long range.

A non-directional radio beacon transmitting in the low and medium frequency bands, used with an automatic direction finder to give a bearing to the station. It is simple but subject to static and night-time interference.

Automatic direction finder, airborne equipment that continuously displays the relative bearing of a received radio signal, most commonly from an NDB. The pilot converts relative bearing to magnetic bearing using heading.

The angle measured clockwise from the aircraft's nose to the direction of a radio station or other bearing source. Adding it to the aircraft's magnetic heading gives the magnetic bearing to the station.

Global navigation satellite system, a constellation of satellites providing position, velocity and time worldwide. Aircraft receivers use ranging signals from several satellites to compute position and support area navigation.

Receiver autonomous integrity monitoring, a technique in which the GNSS receiver checks the consistency of satellite signals to detect a faulty one. It requires sufficient satellites in view and is needed for certain navigation approvals.

A defined geographical position used as a reference in area navigation, identified by name or coordinates. Routes are built by connecting waypoints, and the navigation system sequences from one to the next.

A circuit that mutes the radio receiver's audio output when no usable signal is present, suppressing background noise. It must be set correctly so weak transmissions are not lost.

A communication method in which transmission and reception occur on the same frequency but not simultaneously, so only one station transmits at a time. Most VHF aeronautical voice channels operate this way.

The procedure in which a pilot repeats back safety-related ATC instructions and clearances to confirm they were received correctly. Items such as route clearances, altitudes and runway assignments require it.

An authorisation from air traffic control for an aircraft to proceed under specified conditions. It does not imply protection from all traffic or terrain, and it must be read back when it affects safety.

The altimeter sub-scale setting that makes the altimeter indicate elevation above mean sea level when on the ground at the reporting station. It is used for en-route and approach altitude references.

The altimeter sub-scale setting that makes the altimeter read height above a specified reference, normally the aerodrome. It is used in some countries for traffic-circuit and approach height references.

The altitude at or below which the altimeter is set to QNH and above which the standard pressure setting is used. Its value varies by state and aerodrome and is published in the AIP.

A surface of constant pressure shown on the altimeter when the standard pressure setting is selected, expressed in hundreds of feet. Flight levels provide a common vertical reference above the transition altitude.

Airspace of defined dimensions within which air traffic control service is provided and in which all flights require a clearance. It includes control areas and control zones around busy aerodromes.

Airspace in which no air traffic control clearance is required, though advisory services may be available. Traffic separation is not provided, so pilots rely on see-and-avoid and position reporting.

A radio message giving the aircraft's identification, position, time, altitude and next reporting point, used where procedural separation applies. Accurate reporting supports separation in areas without radar coverage.