Aviation Glossary A–Z

Surveillance technology in which equipped aircraft broadcast position and other information. ADS-B Out supports ATC surveillance; ADS-B In can display traffic and weather information when supported by the aircraft equipment.

A height measured above the ground directly beneath or near the referenced point, rather than above mean sea level.

An FAA en route ATC facility responsible for a large geographic area. Centers manage IFR traffic between terminal areas and provide other ATC services as workload and rules permit.

A regulatory classification describing operating, communication, equipment and weather requirements in a portion of airspace. U.S. classes are A, B, C, D, E and G.

An FAA rule requiring specified actions to correct an unsafe condition in an aircraft, engine, propeller or appliance when the directive applies.

Barometric pressure value used to calibrate a pressure altimeter so it indicates altitude according to the applicable reference.

ATC authorization for an aircraft to execute a specified instrument approach procedure. The clearance does not remove the pilot’s responsibility to comply with applicable procedure and operating requirements.

Authorization issued by ATC for an aircraft to proceed under specified traffic conditions. A clearance is not authorization to violate a regulation.

For aviation weather reporting, the height above the surface of the lowest broken or overcast layer, or vertical visibility into an obscuration, when applicable.

Airspace of defined dimensions within which ATC service is provided according to the airspace classification. It does not mean every VFR aircraft is continuously controlled.

Pressure altitude corrected for nonstandard temperature. Higher density altitude reduces aircraft and engine/propeller performance and can increase takeoff distance.

Radio navigation equipment that provides slant-range distance between an airborne interrogator and a compatible ground station.

An altitude reference expressed in hundreds of feet using the standard altimeter setting, used above the transition altitude structure prescribed for the operation.

A graphical advisory for weather hazards generally below SIGMET severity. For the contiguous U.S., G-AIRMETs replaced legacy text AIRMETs in January 2025.

Generic term for satellite navigation systems. GPS is the U.S. constellation; approved aircraft equipment can use satellite navigation for en route, terminal and approach operations as authorized.

Instrument Flight Rules. IFR operations use the regulatory and ATC framework for instrument flight, including an IFR clearance when required.

Ground-based precision approach system that provides lateral localizer and vertical glideslope/glidepath guidance to equipped aircraft.

Altitude reference based on mean sea level. Aeronautical charts and ATC altitude assignments commonly use MSL unless another reference is specified.

Standard coded surface aviation weather observation. It reports observed conditions such as wind, visibility, weather, sky condition, temperature/dew point and altimeter setting.

Operator-specific FAA-authorized document and procedures defining when certain inoperative equipment may be deferred under specified conditions. It is not simply a list of equipment considered optional.

The integrated U.S. system of airspace, airports, navigation and surveillance facilities, ATC services, procedures, people and supporting infrastructure.

Navigation aid: a facility, signal or equipment used to assist aircraft navigation. Examples include VOR and DME.

Time-critical aeronautical information about a facility, service, procedure, hazard or other condition important to flight operations. Pilots should use the current official NOTAM system and briefing sources.

Navigation framework that specifies required aircraft/system performance rather than requiring navigation to be tied to one particular sensor or ground aid.

Pilot report of actual weather encountered in flight. PIREPs are especially valuable for turbulence, icing, cloud tops/bases and conditions not fully represented by surface observations.

Area Navigation. Capability allowing an aircraft to fly desired paths within the coverage/capability of navigation systems rather than only directly from one ground-based aid to another.

Required Navigation Performance. A PBN specification that adds onboard navigation-performance monitoring and alerting to the required accuracy/performance criteria.

Weather advisory concerning significant hazards to the safety of all aircraft, including specified severe conditions.

Airspace where certain activities must be confined or where restrictions/limitations may apply to nonparticipating aircraft. Examples include restricted areas, prohibited areas, warning areas and MOAs, each with different rules.

FAA approval of a major change to the type design of a previously type-certificated aircraft, engine or propeller.

Terminal Aerodrome Forecast. Coded forecast of expected weather conditions at an airport during a defined valid period.

Temporary restriction on aircraft operations in a defined area for a stated purpose, time and altitude range. Current TFR information should be checked through official FAA sources.

Terminal Radar Approach Control facility. Provides radar approach/departure and related terminal ATC services in a defined area, often serving multiple airports.

FAA design approval showing that a product type design complies with applicable airworthiness requirements.

Compact aircraft code used by ATC and flight-planning systems. It is not always identical to the manufacturer model or marketing name.

Visual Flight Rules. Regulatory framework for flight under visual conditions and applicable airspace/weather requirements.

VHF Omnidirectional Range. Ground-based radio navigation system that provides azimuth/course information to equipped aircraft.

Wide Area Augmentation System. FAA augmentation of GPS that improves accuracy, integrity and availability for approved aviation navigation and approaches.

Rotating airflow generated behind an aircraft as a consequence of lift, with greatest concern behind heavier aircraft and during takeoff/landing. FAA separation standards account for wake categories and operational conditions.