ATPL Navigation (Sri Lanka CAA Pilot Licence Examinations) key terms

The line of intersection between Earth's surface and a plane passing through Earth's centre. It gives the shortest surface distance between two points and appears concave toward the nearer pole on a Mercator chart.

A track crossing every meridian at the same angle, so it plots as a straight line on a Mercator chart. It is longer than the great circle except along a meridian or the equator.

The angle at which two adjacent meridians meet, equal to the change of longitude between them multiplied by the sine of the latitude. It explains why a great circle track must be continuously altered.

Half the convergency between two meridians, giving the angular difference between the great circle track and the rhumb line track joining the same two points. Used to convert between the two track types.

The east-west distance between two meridians measured along a parallel of latitude, expressed in nautical miles. It equals change of longitude in minutes of arc multiplied by the cosine of the latitude.

A cylindrical projection on which meridians are parallel straight lines and the latitude scale expands with latitude so that rhumb lines plot straight. Distance must be measured at the latitude in use.

A conic projection on which meridians are straight lines converging at the pole and parallels are concentric arcs. Great circles plot nearly straight, making it popular for aeronautical charts.

The direction of the great circle at the point of departure, measured from true north. It differs from the final track by the total convergency along the route.

The direction of the aircraft's path over the ground measured clockwise from true north. It is the sum of true heading and drift, or true course plus wind effect.

The direction in which the aircraft's longitudinal axis points, measured clockwise from true north. It equals true track minus drift, or true course plus wind correction angle.

The angular difference between the aircraft's heading and the track it actually makes over the ground. It arises from wind and is said to be left or right of heading.

A vector diagram relating true airspeed and heading, wind velocity, and resulting ground speed and track. It is the basis of the navigation computer solution for drift and ground speed.

The speed at which the aircraft travels over the Earth's surface, obtained by applying the wind component along track to the true airspeed. Used to calculate times and fuel.

The process of calculating the aircraft's present position from a known position using heading, airspeed, wind and elapsed time. It must be corrected by fixes because errors accumulate.

A confirmed position established from external references such as radio navigation aids, GNSS, or visual ground features. It resets dead reckoning and confirms the flight's progress.

A line on the chart along which the aircraft must lie, derived from a single navigation measurement such as a bearing, range, or pressure altitude. Two or more intersecting position lines give a fix.

A magnetic bearing from a VOR ground station, identified by the number of degrees measured clockwise from the station's magnetic north. An aircraft flying outbound on a radial has that radial as its track from the station.

Slant distance in nautical miles from the aircraft to a DME transponder, measured by timing the round trip of pulse pairs. It reads higher than true ground range and includes a small fixed error.

The straight-line distance from the aircraft to a ground station, as opposed to the horizontal range along the ground. At high altitude or close range the difference becomes significant.

The lateral guidance component of an instrument landing system, transmitting on VHF between 108 and 112 MHz. It defines the extended runway centreline and is most sensitive at the threshold.

The vertical guidance component of an instrument landing system, normally set at a 3-degree angle and transmitted on UHF. It provides a descent profile to a defined reference point near the runway threshold.

The lowest altitude at which a missed approach must be initiated if the required visual reference has not been established on a precision approach. It is referenced to the altimeter.

The lowest altitude to which descent is permitted on a non-precision approach before the required visual reference is established. Unlike decision altitude it is not a decision point for an immediate go-around.

The direction of an aircraft from a non-directional beacon, indicated by a relative bearing added to the aircraft heading. It is subject to errors such as coastal refraction and night effect.

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

Airborne equipment that indicates the relative bearing of a received radio signal, typically from an NDB. It displays a needle pointing toward the station and requires heading to convert to a true bearing.

A satellite navigation system that provides position, velocity and time by ranging to multiple satellites. Accuracy depends on satellite geometry and can be degraded by ionospheric and multipath errors.

Receiver autonomous integrity monitoring, a function that checks whether the GNSS signals are consistent enough to be trusted for navigation. It warns of a satellite fault or poor geometry.

A measure of how satellite geometry weakens the accuracy of a GNSS position fix. A high value means poor geometry and greater position uncertainty even with strong signals.

The pressure value set on the subscale so the altimeter indicates the intended reference, such as QNH for altitude or QNE for flight level. An incorrect setting causes a systematic height error.

The pressure setting that makes an altimeter read elevation above mean sea level at the aerodrome when on the ground. It is used below the transition altitude.

A surface of constant pressure expressed in hundreds of feet, referenced to the standard pressure setting of 1013.25 hPa. It is used above the transition altitude so all aircraft share a common datum.

The altitude at or below which the altimeter is set to QNH and above which flight levels are used. It varies by location and is published on approach and en-route charts.

The ratio of true airspeed to the local speed of sound. It determines compressibility effects and is the limiting speed for high-altitude operations and structural certification.

The narrow band of altitude and speed where the low-speed stall margin and the high-speed Mach limit converge. It restricts the usable speed range at high altitude.

The point through which the total weight of the aircraft acts. Its position must remain within specified forward and aft limits for control and performance to be assured.

The chord of an equivalent rectangular wing that would produce the same aerodynamic characteristics as the actual wing. Centre of gravity limits are often expressed as a percentage of it.

The mass of the aircraft with all operating equipment, unusable fuel and full hydraulic and oil systems, but without crew, passengers, payload or usable fuel. It is the starting point for mass and balance calculations.

The mass of the aircraft including crew, passengers and cargo but excluding usable fuel. It must not exceed the maximum zero fuel mass limit.

The horizontal distance from the reference datum to the point where a mass acts. Multiplying mass by moment arm gives the moment used to locate the centre of gravity.

The product of a mass and its moment arm, expressing the turning effect of that mass about the datum. Summing moments and dividing by total mass gives the centre of gravity position.

The total mass of passengers, baggage, cargo and mail carried for revenue or operational purposes. It is limited by structural mass limits and by the space available.

The total mass of the aircraft at the start of the take-off run, comprising zero fuel mass plus usable fuel. It must not exceed the maximum certified take-off mass for the conditions.

The total mass of the aircraft at touchdown, equal to take-off mass minus fuel burned during the flight. It must not exceed the maximum certified landing mass.

The process of calculating usable fuel for taxi, trip, contingency, alternate, final reserve and any additional requirements. It ensures legal and safe reserves remain at destination.

The minimum fuel that must remain on arrival at the destination alternate, calculated to allow holding for a specified period. It is not planned for use in normal operations.

The fuel required to fly from the destination to the chosen alternate aerodrome, including approach and landing. It is carried when an alternate is required by the flight plan.

Fuel carried to cover unforeseen factors such as weather, routing or traffic delays. It is usually a percentage of trip fuel or a time-based allowance.

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 alternate availability.

The position from which it takes the same time to fly to either of two aerodromes. It is used to decide the diversion point when one destination may become unavailable.

The distance an aircraft can fly per unit of fuel, usually expressed in nautical miles per kilogram or per pound. It varies with altitude, mass and speed and is used to optimise cruise.

A flight management setting that trades fuel cost against time cost to select cruise speed. A higher index favours speed, a lower index favours fuel economy.