DGCA ATPL Air Navigation key terms

The direction the aircraft's longitudinal axis points, measured in degrees clockwise from True North. It differs from the track actually flown when wind pushes the aircraft sideways.

The path the aircraft actually travels over the ground, measured in degrees clockwise from True North. It equals heading corrected for wind drift.

The angular difference at a given place between True North and Magnetic North, east or west. It is applied to convert between true and magnetic directions and changes slowly with location and time.

The error of a magnetic compass caused by the aircraft's own magnetic fields and electrical currents, varying with heading. It is read from the compass deviation card and applied to magnetic heading.

A line on a chart joining points of equal magnetic variation. Pilots use it to interpolate variation between the chart's printed values.

The angular difference between the heading the aircraft points and the track it actually makes good over the ground, caused by wind. It is named left or right of heading.

The vector relationship among true airspeed and heading, wind velocity, and resulting ground speed and track. It is the basis for calculating drift and the heading needed to fly a desired track.

The speed of the aircraft relative to the ground, obtained by combining true airspeed with the wind component along track. It determines time en route and fuel needed.

Navigation by calculating position from a known starting point using heading, speed and elapsed time, corrected for wind. It requires no external fixes and accumulates error over time.

A chart projection on which meridians are straight converging lines and parallels are concentric arcs, with scale nearly constant. It is widely used for aeronautical charts of mid-latitude areas.

A cylindrical projection on which meridians and parallels are straight lines and a rhumb line plots as a straight line. Scale expands toward the poles, so it suits equatorial charts better than polar ones.

The shortest path between two points on the Earth's surface, formed by a plane through the Earth's centre. Its direction changes continuously along the route unless the path is the equator or a meridian.

A line crossing all meridians at the same angle, appearing straight on a Mercator chart. It is longer than the great circle between the same points but has a constant track.

The rate at which meridians approach each other toward the pole, equal to the change of longitude multiplied by the sine of latitude. It explains why a constant-heading flight does not follow a straight great circle.

The primary time standard, referenced to the prime meridian and kept within a second of mean solar time at Greenwich. Flight plans and meteorological data are normally expressed in it.

Time based on the mean sun's position at a specific longitude, differing from UTC by one hour for every 15 degrees of longitude. It is used to relate local daylight to flight times.

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 radials but not distance by itself.

A magnetic bearing extending outward from a VOR station, numbered from 000 to 359 degrees. An aircraft flying along a given radial has that magnetic bearing from the station.

Distance measuring equipment that measures slant range from the aircraft to a ground transponder by timing signal round trips. The reading exceeds true horizontal distance and is most in error at high altitude close to the station.

A non-directional beacon transmitting in the low or medium frequency band, used with an automatic direction finder for homing and tracking. It is subject to static interference and night-time skywave errors.

Automatic direction finder equipment that indicates the relative bearing of an NDB or broadcast station. Combined with heading, it gives the magnetic bearing to the station.

The angle measured clockwise from the aircraft's nose to the direction of a radio station, as shown on an ADF. Adding it to heading gives the magnetic bearing to the station.

The magnetic bearing from an aircraft to a station, that is, the heading that would take the aircraft directly to it with no wind. It equals the station's QDR plus 180 degrees.

The magnetic bearing from a station to an aircraft, equivalent to the VOR radial being received. It is the reciprocal of the QDM.

Instrument landing system providing precision horizontal and vertical guidance to a runway using localizer and glidepath signals, with marker beacons or DME for range. It enables approaches in low visibility.

The lateral guidance component of an ILS, transmitting on a frequency in the 108 to 112 MHz band to define the extended runway centreline. Its course is the most sensitive near the runway threshold.

The vertical guidance component of an ILS, normally set at about a three-degree descent angle to a reference point near the touchdown zone. Following it keeps the aircraft on a safe approach slope.

Global navigation satellite systems such as GPS, GLONASS and Galileo that provide position, velocity and time worldwide. Accuracy depends on satellite geometry and can be degraded by ionospheric and multipath effects.

Receiver autonomous integrity monitoring, a function that checks whether enough satellites are in view to detect a faulty signal and warn the pilot. It is required for certain GNSS approaches.

The altitude indicated when the altimeter subscale is set to standard pressure, 1013.25 hectopascals. It is used for flight levels and performance calculations, not for terrain clearance below transition altitude.

The altimeter setting that makes the instrument read elevation above mean sea level when on the ground at that location. It is used below the transition altitude for terrain clearance.

The altimeter setting that makes the instrument read zero on the ground at the reference point, giving height above that point. It is used at some aerodromes for circuit and approach work.

The altitude at and below which the altimeter is set to QNH and altitude is used, above which flight levels on standard pressure apply. It is published for each aerodrome.

A surface of constant pressure altitude referenced to 1013.25 hectopascals, expressed in hundreds of feet. Flight levels provide a common vertical reference for all aircraft above the transition altitude.

The chart limits of total mass and centre of gravity position within which an aircraft may be operated safely. Exceeding them can degrade performance, control and structural loading.

The point through which the total weight of the aircraft acts, expressed as a position along the fuselage datum. Its position affects stability, trim, and take-off and landing performance.

The mass of the aircraft with all traffic load but without usable fuel. It is a limiting structural value that must not be exceeded before fuel is added.

The take-off decision speed at which a failure of the critical engine still allows the aircraft to continue or to stop within the runway. It balances accelerate-stop and accelerate-go distances.

The take-off safety speed to be attained by the time the aircraft reaches 35 feet above the runway with one engine failed. It ensures adequate climb gradient and control.

The runway length at which the accelerate-stop distance equals the accelerate-go distance, giving the shortest field required. It is determined by the chosen V1 for the conditions.

The ratio of height gained to horizontal distance flown, often expressed as a percentage. Required gradients depend on the number of engines and the take-off or approach phase.

Wind at flight altitudes, usually given in forecasts as direction in degrees true and speed in knots for specified levels. Its strength and direction strongly affect ground speed and fuel planning.

The difference between the actual temperature at a level and the International Standard Atmosphere value for that level. Positive values mean warmer air, which increases density altitude and reduces performance.

The pressure altitude corrected for temperature deviation from standard, representing the altitude at which the air density would occur in the standard atmosphere. High values reduce engine power and lift.

The settable pressure reference on an altimeter, adjusted to QNH, QFE or standard pressure. Correct setting is essential to avoid height errors and loss of separation.

A confirmed aircraft position determined from external references such as radio aids, GNSS or visual landmarks. Fixes update dead reckoning and reveal accumulated navigation error.

A line on which the aircraft is known to lie, derived from a bearing, distance or other measurement. Intersecting two or more position lines establishes a fix.

Area navigation that allows an aircraft to fly any desired path using onboard computation of position from various sensors. It removes the need to overfly ground stations.

A published ATC departure procedure of tracks and altitudes linking the runway to the en-route structure. It simplifies clearance delivery and ensures terrain and traffic separation.