Aviation & flight tracking glossary
We've put together a glossary of terms which you may encounter either on our site or in aviation in general that we hope enriches your flight tracking experience. The Aviation industry uses many complex acronyms, jargon and terminology which this glossary will help you unravel. The vocabulary we look at centers around flight tracking, characteristics of flight, flight tracking technology/communication and general aviation terminology.
Automatic Dependent Surveillance-Broadcast is the main technology we use to track aircraft. Automatic: data is sent from the aircraft without pilot intervention; Dependent: it is dependent on the aircraft's navigation system for position information. An aircraft that has a transponder that is ADS-B equipped enables extensive flight details to be sent such as altitude, ground speed, call sign to be transmitted. In some cases additional information, like airspeed, wind, and temperature are sent as well. With our global network of ADS-B receivers we are able to use these transmissions to provide a unique flight tracking service.
You can read more about ADS-B here.
The aircraft's age listed on Flightradar24 is from the first flight. Delivery of the aircraft to the operator usually occurs within weeks of the first flight. The age of commercial airliners varies greatly, and largely depends on the number of pressurisation cycles. A Boeing 747-400 for example was designed to have a 20 year lifespan.
Each kind of aircraft is assigned an aircraft type, which is a standardized name for the aircraft. Type codes are assigned by ICAO and consist of two to four letters and numbers. These type codes are often used when filing flight plans. We use these type codes on Flightradar24 for filtering aircraft.
For each flight tracked on Flightradar24 the calibrated altitude reported from the aircraft, which is a pressure-derived value, is displayed. Extended Mode S Data received from some aircraft also includes the GPS-derived altitude of the aircraft.
Air traffic control (ATC)
Air traffic controllers are responsible for the safe coordination of flights from pushback to arrival. They are a ground based service whose primary objective is to safely facilitate the movement of air traffic, prevent aircraft collisions, as well as providing any other information a pilot may require.
Auxiliary power unit (APU)
An aircraft auxiliary power unit (APU) is a device that provides energy in order to power electrical systems. Most APUs are simply small jet engines which take fuel directly from the aircraft's main fuel tanks. In normal operations an APU will be started prior to main engine start, shut down during flight, and then restarted after the main engines are shut down again.
Automatic Terminal Information Service (ATIS)
The Automatic Terminal Information Service (ATIS) is a continuous, recorded broadcast, which provides information such as weather, active runway, runway state, NOTAMs and any other information that may be needed by Pilots. Typically a broadcast is coded with a reference 'Information' letter (Information Alpha) signifying the current version of an ATIS. Air traffic controllers will inform approaching aircraft of the current ATIS ('Information November is current') to ensure pilots have the information they need to carry out a safe landing.
Taken together, the CVR (Cockpit Voice Recorder) and FDR (Flight Data Recorders) are informally known as the black boxes, though in fact, most are bright orange. Both recorders are designed to survive a crash and are equipped with beacons to assist investigators in locating them quickly after an aircraft accident.
Call signs are used by Air Traffic Control to denote a specific flight. These can differ from flight numbers in two respects. Some airline radio call signs are different from the airline's name, either for reasons of history or for easy understanding over the radio. Additionally, in recent years, flights have begun using alphanumeric call signs in order to avoid call sign confusion. For example, flight number British Airways 123 would use the call sign Speedbird 1PW, where "Speedbird" is the radio call sign and 1PW is the alphanumeric call sign. This helps avoid confusion over the radio, where two British Airways flights with similar numbers may be talking to the same air traffic controller.
Calibrated airspeed (CAS)
Calibrated airspeed is the airspeed corrected for position and instrument error and is equal to True Airspeed (TAS) at sea level. 'KCAS' is a further abbreviation of Calibrated Airspeed which denotes CAS measured in knots. In some countries the term 'reflected airspeed' is used instead of calibrated airspeed.
Calibrated altitude values reflect the aircraft's altitude above Mean Sea Level, a constant value used in aviation and other applications. 0 feet/meters above Mean Sea Level does not necessarily reflect an aircraft's altitude above the ground.
A climb is a period of flight where an aircraft's altitude increases. Typically a climb occurs immediately after takeoff. It's common to see commercial airliners perform an additional climb enroute, known as a 'Step climb', and this can often be clearly seen in our flight profile graphs.
A descent is a period of flight where an aircraft's altitude decreases. 'Top of descent' is the point where an aircraft will typically begin descending towards its destination. In an emergency situation such as a rapid decompression an aircraft will perform an emergency descent to below 10,000ft where the oxygen levels in the air are able to ensure passengers and crew may breath unassisted by supplemental oxygen.
For flights that are no longer in our coverage area, we may estimate their position for up to 240 minutes. These flight tracks are shown as black dashed lines. Read more about estimated positions on Flightradar24.
Extended Mode S
Extended Mode S data is received directly from the aircraft and decoded for display on Flightradar24. Extended Mode S data can include additional information about the speed and altitude of the aircraft as well as weather information, such as wind and outside air temperature.
Flight data recorder (FDR)
Flight Data Recorders record a variety of flight parameters on a constant basis and are used as a forensic tool in the event of a crash or other serious incident. The International Civil Aviation Organization (ICAO) have stringent requirements for the survivability of both the Flight Data Recorder (FDR) and a Cockpit Voice Recorder (CVR).
FLARM (a portmanteau of 'flight' and 'alarm') is similar to ADS-B, but used in light aircraft, such as helicopters and gliders. The installation of FLARM equipment is approved by EASA (European Aviation Safety Agency). FLARM compared to traditional transponders has a much lower power consumption making it ideal for the likes of gliders.
Flight Information Region (FIR) / Upper Information Region (UIR)
Flight Information Regions and Upper Information Regions are geographic areas of air traffic control responsibility. The size of a FIRs and UIRs varies and is typically decided by the country, or countries, that the region covers. Oceanic airspace has its own classification as an Oceanic Information Region.
Flight level (FL)
A Flight Level is an aircraft's altitude at standard air pressure, 1013.25 hPa (29.92 inHg), and is expressed in hundreds of feet e.g. 37,000ft is expressed as FL370. Pilots use Flight Levels so that vertical separation can be carried out safely.
A number assigned in the airline's schedule to denote a particular flight or flights. Airlines will regularly use flight number conventions such as designating northbound and eastbound flights with an even number, and southbound and westbound flights with an odd number. Often airlines use larger flight numbers such as BA9171 to indicate a maintenance or positioning flight.
The exact routing a flight will take filed with air traffic control authorities, including specific waypoints the flight will pass over. A flight plan will typically be filed prior a flight by either a pilot or dispatcher. Flight plans are usually filed for IFR flights, with VFR flights only requiring a flight plan if they intend to cross an international border.
When a landing approach cannot be completed safely for any reason, an aircraft will initiate a go-around, which generally includes climbing to a pre-specified altitude and holding point while awaiting further instructions from air traffic control. There are various reasons for why a go-around may occur such as an aircraft ahead slow to vacate the runway or simply an unstable approach. Read more about how go arounds help keep flights safe.
GPS altitude values are based on an ellipsoid of the entire earth, but are also not necessarily indicative of height above the ground.
Ground speed is the speed of the aircraft over the ground; often measured in Knots (nautical miles per hour). If an aircraft is subjected to a headwind this would have a negative impact on the ground speed, whilst a tailwind would have a positive impact.
Indicated Airspeed is the air speed taken directly from the airspeed indicator without any corrections for temperature or pressure. Airspeed is displayed in knots which can be abbreviated further to KIAS (Knots Indicated Airspeed). IAS is sourced from the pitot tube system usually found around the nose of an aircraft.
The International Air Transport Association is a trade organization made up of the world's airlines and has influence over the commercial aspects of flight. For flight tracking purposes, the two most important pieces of information are IATA airline and airport codes, which are two and three letters long, respectively, and differ from ICAO codes.
List of IATA airline codes
List of IATA airport codes
ICAO 24-bit Address (Hex Code)
Mode S transponders are assigned a unique 24-bit address, often called a hex code because of how it is generally displayed. Once registered the HEX code of an aircraft features on its certificate of registration and is rarely changed.
International Civil Aviation Organization is the arm of the United Nations responsible for international air navigation. ICAO performs a number of essential functions, including assigning aircraft type codes, airline codes, and airport codes. ICAO airline codes consist of three letters (Qantas = QFA) ICAO airport codes consist of four letters (Hong Kong = VHHH). These codes differ from IATA codes.
List of ICAO airline codes
List of ICAO airport codes
List of ICAO aircraft type codes
Instrument flight rules (IFR)
Instrument Flight Rules are a set of regulations used for flying aircraft by instruments alone. An IFR flight may occur when the weather is below visual meteorological conditions (VMC) and is therefore deemed to be instrument meteorological conditions (IMC).
Instrument landing system (ILS)
An ILS allows pilots to conduct instrument approaches by providing precision lateral and vertical guidance.
A Mach number is the ratio of True airspeed to the speed of sound at the altitude of a given flight. If an aircraft were to fly a Mach 0.75 it would be flying at 75% of the speed of sound. Mach ranges can be broken down into Subsonic, Transonic, Supersonic, Hypersonic, High-hypersonic, and re-entry speeds. As of now, all commercial flights are subsonic.
Manufacturer's serial number (MSN)
A Manufacturer's serial number, or MSN, is a unique code assigned to an aircraft, often prior to it even being built. The parts that make up an aircraft all reference the individual MSN.
Multilateration is the process of using the time difference of arrival of radio signals from an aircraft to ascertain the aircraft's position and heading. MLAT requires four or more Flightradar24 receivers in a region to receive a signal from the same aircraft. From this we can then measure the Time Difference of Arrival (TDOA).
You can read more about MLAT here.
Mode S is a communication protocol that allows an aircraft to communicate with secondary surveillance radar (SSR) and other systems through its on-board transponder. Aircraft with a Mode S transponder are capable of utilizing TCAS and ACAS II functions.
A METAR is a specialized form of weather reporting used mainly in aviation. METARs are standardized by ICAO and follow a specified format so that they may be understood easily by pilots around the world. METARs differ from a Terminal Aerodrome Forecast (TAF) in that they provide actual weather at that time, rather than a forecast which a TAF provides.
North Atlantic Tracks
North Atlantic Tracks, officially known as North Atlantic Organized Track System (NAT-OTS) are a specified set of routes that stretch Eastbound and Westbound across the North Atlantic. They provide separation for aircraft, as well as taking into account optimum weather conditions. In order to utilise these tracks aircraft require Oceanic clearance.
Notice to Airmen (NOTAM)
A Notice to Airmen NOTAMs are issued by aviation authorities to alert pilots to hazards or other important information they need to navigate the area safely. NOTAMs are filed with a country's aviation authority and then disseminated through a range of channels to provide up to date information to pilots and operators.
When discussing "radar", primary radar is generally what is being referred to. Primary radar outputs a radio signal and attempts to detect any reflection of that signal from an object, such as an airplane.
QNH is an aeronautical 'Q code' which provides a barometric altimeter setting relating to airfield elevation above mean sea level. Standard QNH which is used when flying at Flight Levels is set to 1013.25hPa / 29.92inHg.
QFE is an aeronautical 'Q code' which provides a barometric altimeter setting which causes an altimeter to read zero when at the reference datum point of an airfield.
Registration / Tail Number
Each aircraft is assigned a registration number—often called a tail number—that is unique to the aircraft. Registration numbers are based on the country of registration, with the United Kingdom employing 'G' as their first letter identifier, followed by a hyphen and four further letters e.g. G-STBA. In the United States 'N' is used as the first letter followed by either letters or numbers e.g. N463UA.
Secondary radar, or secondary surveillance radar, sends out a signal to compatible aircraft, which then return a signal of their own with information about a flight, such as speed, altitude, heading, and registration.
Speed, as it relates to aircraft, is more complicated than speed on the ground, as there are different ways to measure speed in flight. The different ways to measure speed can result in differences between the types, whereby the ground speed of an aircraft can vary significantly from its airspeed. The speed shown in the "speed" box on Flightradar24 for all aircraft is ground speed.
Each flight is assigned a four digit code, known as a squawk, by air traffic control. This code is unique to the flight and helps ATC identify each flight. There are a few significant squawk codes, which immediately get the attention of air traffic control. For a longer discussion of squawk codes and emergencies, please see our discussion with a pilot and an air traffic controller.
7600: Radio Failure
7700: General Emergency
Terminal Area Forecasts are similar in format to METARs, but are issued for future periods. For major civil airports TAFs are issued every six hours, usually at 0000, 0600, 1200, 1800 UTC. Military TAFs are normally issued every three hours.
True Airspeed is the airspeed of an aircraft corrected for position and instrument error and taking pressure and temperature into account.
A Traffic Collision Avoidance System allows aircraft to communicate if equipped with a compatible transponder in order to prevent a mid-air collision. The TCAS warns pilots of traffic in the vicinity and can issue directives to help pilots maintain separation if flights come too close to one another.
Track is the aircraft's compass heading, denoted as 1-360. It's the real path, or vector, that the aircraft is flying. In order to work out an aircraft's track, heading +/- WCA (wind correction angle) gives the track.
A portmanteau of transmitter-responder, the transponder emits a radio signal based on an interrogation from an outside source, either air traffic control or another aircraft. In normal operation a pilot will be instructed by an air traffic controller to set a specific code on their transponder and when this happens the terminology used will be to 'squawk' a code e.g. "Speedbird 115 squawk 4552".
Vertical speed is the rate at which a plane is ascending or descending, often shown in feet/meters per minute. Generally, vertical speed during departure will range from 1000-3000 feet per minute, while during descent, speeds can be between 600-800 feet per minute.
Visual Flight Rules (VFR)
Visual Flight Rules are a set of regulations whereby a pilot can operate an aircraft in weather which is suitable for visual reference only. The weather requirement for VFR flight is known as visual meteorological conditions (VMC). If the weather is below VMC minima a pilot can only proceed with flight using Instrument Flight Rules (IFR).