Aviation Terms and Acronyms

Acronym/Term	Description
A/C	Shorthand for aircraft.
AC 90-100	See FAA AC 90-100
AC 90-101	See FAA AC 90-101
ACARS	Aircraft Communications Addressing and Reporting System – Created in 1978 by ARINC this system provides a communications link between aircraft and ground stations. Data can be transmitted over this link to/from aircraft avionics using a Communications Management Unit (CMU). Airlines use ACARS to send/receive crew messages, aircraft operating statistics and numerous other pieces of information.
AD	Airworthiness Directive – A notification generated by an aviation regulator regarding a safety issue.
AEO	All Engines Operative - Refers to the aircraft condition when all engines are operating normally.
AFM	Aircraft Flight Manual - Document created by aircraft manufacturer to describe operation of an aircraft and its performance characteristics.
AGL	Above Ground Level – A measure of altitude with reference to the elevation of local terrain rather than the more common reference of mean sea level.
AHRS	Attitude Heading Reference System – A three axis reference system that provides aircraft with heading, attitude and yaw information, typically for the primary flight display. Unlike IRS/INS/IRU systems, AHRSs have not been approved for navigation.
AIP	Aeronautical Information Publication – Documentation of information necessary for air navigation which is published by governments around the world per requirements established by ICAO. They have three sections: general information, enroute navigation and airport (aerodrome) procedures and are revised every 28 days.
AIRAC Cycle	Aeronautical Information Regulation and Control Cycle - The 28 calendar day cycle by which Aeronautical Information Publication (AIP) is updated, as published in ICAO Annex 15 – Aeronautical Information Services.
ALT	The elevation of an object from a known reference level or datum, called zero level. Most often this level is defined as the mean sea level, but it can vary. Also Altitude.
Altitude	The height difference between an object and a known reference level or datum, called zero level. Most often this level is defined as the mean sea level, but it can vary. Abbreviated ALT.
AMC	Acceptable Means of Compliance – EASA term for method of satisfying regulatory requirements.
AMSL	Above Mean Sea Level - Altitude.
ANP	Actual Navigation Performance – Term used by Boeing as a measure of the current estimated navigational performance. ANP is the flight management computer’s estimate of the quality (integrity and accuracy) of its position determination. Note: The RTCA DO-236 Term is Estimated Position Uncertainty or EPU.
ANPE	Actual Navigation Performance Effect - A component of the Vertical Error Budget (VEB) that accounts for the possibility of an “along-track” inaccuracy which would cause an aircraft on a descent path to be higher or lower than the coded path. Also known as Horizontal Coupling Effect.
ANSP	Air Navigation Service Provider – The organization responsible for managing air traffic, such as the FAA’s Air Traffic Organization, Nav Canada, Airservices Australia, Deutche Flugsicherung GmbH, etc.  See ATC, ATM and ATS.
A/P	Autopilot – System used to automatically guide aircraft.
APU	Auxiliary Power Unit – Turbine powered source of electrical generation, cabin heating/cooling and hydraulic pressure commonly used by aircraft on the ground. Can serve as a back-up provider of the above if main engines are not capable of doing so.
AR	Authorization Required – the suffix on an RNP procedure indicating that prior authorization is required. See RNP AR.
ARINC	ARINC Incorporated – ARINC is a company specializing in aviation communications and systems engineering. The ARINC name was originally derived from the term Aeronautical Radio, Incorporated.
ARINC 424	Worldwide standard for formatting and interchange of navigation data, consisting of text records with 132 fixed columns.
ARP	Airport Reference Point – Latitude and longitude of an airport. Also known as Aerodrome Reference Point
ASL	Above Sea Level - The distance between an object and an imaginary line at sea level.
ATC	Air Traffic Control – The activity of or organization managing air traffic. See ANSP, ATM and ATS.
ATM	Air Traffic Management – The activity of or organization managing air traffic. See ANSP, ATC and ATS.
ATO	Air Traffic Organization – A major branch of the U.S. FAA responsible for air traffic services. This is the ATC, ATS or ANSP for the U.S.
ATS	Air Traffic Services – The organization responsible for managing air traffic. Same as ANSP, ATC and ATM.
AWO	All Weather Operations – Flight operations that can be conducted regardless of weather conditions usually meaning that they are conducted under instrument flight rules (IFR).
BEA	Bureau d’Enquetes Accidents – French accident investigation board.
BR	Basic Regulation – EASA term for wide ranging requirements.
CAAC	General Administration of Civil Aviation of China – The Chinese aviation regulator.
CAASD	Center for Advanced Aviation System Development - A Federally funded research and development center run by the MITRE organization.
CASA	Civil Aviation Safety Authority - Aviation regulatory agency. Australia and Korea are examples of nations using the CASA abbreviation for their regulatory agency.
CASR 173	Civil Aviation Safety Regulation Part 173 – Australian CASA regulation covering the requirements for the certification of designers of instrument approach and departure procedures, including: the qualifications and training required for persons engaged in instrument flight procedure design; the procedures to be used by organizations in the conduct of design work; and provisions for on-going maintenance of procedures. Naverus is a CASR 173 certified procedure developer in Australia.
Category I	Classification of, and requirements for, instrument flight procedures that permit aircraft to descend to no lower than 60 meters (200 feet) above the runway threshold provided weather conditions meet the minimum visibility requirements typically at least 800 meters (1/2 mile) or a runway visual range of at least 550 meters (1/3 mile). Note: Since 1985 the U.S. FAA classifies any approach that is not Category II or Category III as Category I. Accordingly, approaches such as Localizer (LOC), Very high frequency Omni-directional Radio range (VOR), Non-Directional Beacon (NDB), RNAV, RNP and others are considered Category I by the FAA. Most other states limit Category I to ILS or MLS based straight-ins with lateral and vertical guidance.
Category II	Classification of, and requirements for, instrument flight procedures that that permit aircraft to descend to between 30 meters (100 feet) and 60 meters (200 feet) above the ground provided weather conditions meet minimum visibility requirements, typically runway visual range of at least 350 meters (1/5 mile).
Category III A	Classification of, and requirements for, instrument flight procedures that that permit aircraft to descend to less than 30 meters (100 feet) above the ground provided weather conditions allow a runway visual range of at least 200 meters (1/8 mile).
Category III B	Classification of, and requirements for, instrument flight procedures that that permit aircraft to descend to less than 15 meters (50 feet) above the ground provided weather conditions allow a runway visual range between 50 meters (1/32 mile) and 200 meters (1/8 mile).
Category III C	Classification of, and requirements for, instrument flight procedures that that permit aircraft to descend to all the way to touchdown in zero visibility weather conditions. Category III C systems must use the autopilot to automatically land the aircraft.
CDA	Continuous Descent Approach – A fuel-saving and noise-reducing approach that relies on the energy an airplane has from cruise level to “glide” at idle power all the way down to a runway. Naverus Tailored RNP approaches can incorporate CDAs. This type of approach differs from older “dive and drive” approaches in which an aircraft reduces power to descend to the initial descent altitude where it increases power to maintain that altitude, then reduces power for the descent to the next lower altitude where it increases power settings again, repeating this pattern multiple times all the way down to the runway.
CDU	Control Display Unit – Cockpit instrument that allows flight crews to input and observe system settings.
Certification	The process of obtaining regulatory approval.
CF	Certification – EASA abbreviation for certification.
CFIT	Controlled Flight into Terrain - When a normally operating aircraft unintentionally strikes terrain or water. These accidents typically occur near airports while aircraft are maneuvering at lower altitudes and result from a loss of situational awareness. CFIT is the most common type of commercial airliner accidents
CNS	Communication, Navigation, and Surveillance – The three disciplines of air traffic management.
CofA	Certificate of Airworthiness – EASA term for regulatory determination of an aircraft’s ability to perform.
DA	Decision Altitude – The altitude at which a pilot conducting an instrument approach along a glide path must decide whether to continue to a normal landing (if the runway environment is in sight) or execute a missed approach (aborting the landing if the landing environment is not in sight or a normal landing is not possible).  The DA is measured from mean sea level. Decision Altitudes are used for precision approaches (such as ILS, MLS, PAR, LAAS, and RNP) where a glide path angle is specified.  See also DA(H) and DH.
DA(H)	Decision Altitude (Height) - Same as Decision Altitude with the addition of height above touchdown zone elevation shown in parenthesis. A DA(H) of 1679’ (250’) would be a DA of 1679’ with a DH 250’ above the runway touchdown zone elevation (subtracting the DH from the DA will find that the runway touchdown zone is at an elevation of 1429’ MSL). See also DA and DH.
db	Decibel – sound measurement unit.
DCA	Department of Civil Aviation – Aviation regulatory agency. Thailand refers to its aviation regulatory agency using this term.
DH	Decision Height - Decision Altitude expressed as height above the touchdown zone elevation of a runway. See also DA and DA(H).
EASA	European Aviation Safety Agency – The organization formed by the European Union in 2003 to oversee aviation safety, regulation and certifications. JAA responsibilities have been transferred to this organization.
EFIS	Electronic Flight Information System – Cathode Ray Tube (CRT) or Liquid Crystal Display (LCD) cockpit displays of flight information.  More efficient at displaying information to crews, aircraft equipped with these are said to have “glass cockpits”.
EGPWS	Enhanced Ground Proximity Warning System.
Engine Out	A condition of flight where one engine has failed.  Also called OEI (One Engine Inoperative).
EPR	Environmental Protection Requirements – European term for environmental regulations.
FAA	Federal Aviation Administration of the United States – Organization responsible for air traffic control and aviation regulation.
FAA AC 90-100	U.S. FAA Advisory Circular 90-100 “U.S. Terminal and En Route Area navigation (RNAV) Operations” – FAA document that outlines requirements to conduct RNAV operations.
FAA AC 90-101	U.S. FAA Advisory Circular 90-101 “Approval Guidance for RNP Procedures with SAAAR” - FAA document that outlines requirements for approval to conduct RNP operations. This document contains materials related to both airworthiness and operations and is the basis for evaluating RNP applications in the United States. The ICAO PBN Manual serves a similar purpose with significant amounts of identical text.
FAA Order 8260.52	FAA Order describing the U.S. criteria for RNP procedure design.  The ICAO RNP AR Manual is almost identical.
FAF	Final Approach Fix – The point where the last segment of an instrument approach begins. The final segment spans the FAF to the runway threshold.
FANS	Future Air Navigation System – A precursor to RNP, FANS is set of requirements and methods for increasing air traffic capacity.
FAS	Final Approach Segment – The last segment of an approach procedure, beginning with the FAF and ending at the runway threshold. The FAS is usually aligned with the extended runway centerline.
FCL	Flight Crew Licensing – EASA term for pilot licensing.
FCP	Flight Control Panel – Panel of controls that allow crew to interface with aircraft systems.
FMA	Flight Mode Annunciator
FMC	Flight Management Computer – The core computer of the Flight Management System that controls navigation.
FMS	Flight Management System - An integrated system, consisting of an airborne sensor, receiver and Flight Management Computer (FMC) with both navigation and aircraft performance databases, which provides performance and navigation guidance to a display and to an automatic flight control system (e.g., autopilot).
GA	General Aviation – Refers to all U.S. flight activities that are not military or scheduled airline.
GBAS	Ground Based Augmentation System – Any system which uses a ground based transmitter to supplement signals received from a satellite navigation system. LAAS is a type of GBAS defined in the US.
GLS	GNSS Landing System – The equipment and system aboard an aircraft that combines GPS with GBAS to provide extraordinarily precise guidance on approach.
GM	Guidance Material – EASA instructions on how to meet regulatory requirements.
GMT	Greenwich Mean Time – Former term for UTC.
GNSS	Global Navigation Satellite System - Generic reference for a satellite based navigation system. The U.S. GPS is one.
GPS	Global Positioning System - A satellite based navigation system deployed by the U.S. Department of Defense that provides positioning service anywhere in the world. GPS provides highly precise position and time information on a continuous basis to an unlimited number of properly equipped users. The system is only modestly affected by weather, and references location based on the Worldwide Geodetic System of 1984 (WGS84). GPS accuracy and integrity are measured continuously by the FAA and other government organizations. Measured accuracy performance (difference between estimated position and actual position) for the system is about 3 meters horizontally and 5 meters vertically 95% of the time, and double those numbers 99.99% of the time. Without augmentation, aircraft using GPS for RNP operations use only the horizontal information, relying on highly developed and accurate altimetry systems for vertical information. GPS is one several satellite systems that are referred to as a GNSS or Global Navigation Satellite System. It is currently the only fully functional system and has been operational since 1978 and available globally since 1994. Galileo, a system developed by the European Union as an alternative to GPS, will become operational in 2010.
HAT	Height Above Touchdown zone - The height in feet of the Decision Height (DH) or Minimum Descent Altitude (MDA) above the TouchDown Zone Elevation (TDZE).
HIGH	Honeywell Inertial GPS Hybrid – A Honeywell developed algorithm that tightly integrates GPS and IRS into a hybrid navigation solution adequate to achieve 100% worldwide availability of RNP 0.1 without the use of differential corrections.
JAA	Joint Aviation Authorities – An association of European aviation regulators.  The roles and responsibilities of the JAA have been transitioned to EASA, which was formed by the European Union in 2003.
IATA	International Air Transport Association – Trade organization representing airline interests worldwide.
ICAO	International Civil Aviation Organization – Organization created by treaty among nations to foster international aviation.
ICAO IFPP	International Civil Aviation Organization Instrument Flight Procedures Panel – ICAO group responsible for establishing ICAO standards for instrument flight procedures.  Prior to May 3, 2007 this panel was referred to as ICAO OCP.
ICAO OCP	International Civil Aviation Organization Obstacle Clearance Panel – ICAO group responsible for establishing ICAO standards for instrument flight procedures.  Effective May 3, 2007 this panel is referred to as ICAO IFPP.
ICAO PBN Manual	International Civil Aviation Organization Performance Based Navigation Manual – ICAO recommendations for steps an operator should take in order to be granted regulatory approval to conduct RNP operations.  Also known simply as PBN manual.
ICAO RNP AR Manual	International Civil Aviation Organization Required Navigation Performance Approval Required Manual – ICAO criteria for designing RNP procedures.  Also known as RNP AR Manual.
IFR	Instrument Flight Rules – A method of flying that requires the pre-filing of a flight plan and involves surveillance by and instructions from air traffic control.  IFR is the only legal way to fly in poor weather in which visibility is restricted.  All commercial airline operations are conducted using IFR.
ILS	Instrument Landing System - A ground based radio system designed to provide a pilot, flying an ILS-equipped aircraft, with precision lateral and vertical guidance for the final approach in landing.  ILS sends two radio beams up from the runway: the Localizer provides lateral guidance to line the aircraft up with the runway and the Glide slope provides a vertical reference.  Equipment and operating procedures combine to determine Category I, II or III performance capabilities.
IMC	Inertial Navigation System - A gyroscope based system that senses aircraft lateral acceleration thereby providing navigation information.  Modern INSs use GPS inputs to correct gyroscopic drift errors.  INSs also provide reliable position location for RNP operations in the event GPS signals become unexpectedly unavailable.  Also called IRS or IRU.
IPC	Illustrated Parts Catalogue – Reference document providing listing and images of aircraft, engine or system components.
IRS	Inertial Reference System - A system that senses minute changes in aircraft acceleration thereby providing navigation information relative to a known starting point.  Modern IRSs use GPS inputs to update the small drift errors introduced over hours of flight time.  IRSs also provide reliable position location for RNP operations in the event GPS signals become unexpectedly unavailable.  Also called INS or IRU.
IRU	Inertial Reference Unit – See IRS.
JPDO	Joint Planning and Development Office - A US inter-agency organization chartered by the President to lead the development of next generation air traffic management system.  The JPDO is made up of members from the U.S. Department of Transportation, Department of Defense, Department of Commerce, Homeland Security Department, FAA, NASA and the White House Office of Science and Technology.
KIAS	Knots Indicated Airspeed - The aircraft’s velocity through the air as indicated to the pilot and measured by simple ram air pressure.  This speed varies from true airspeed based on temperature and altitude and is almost always less than true airspeed.
KT	Knot – Nautical mile(s) per hour.  See also NM.  A quick conversion to mph is to add 15% to knots so (100 knots is 115 mph).  To convert roughly to km/h, double knots and subtract 15% (100 knots is 185 km/h).
LAAS	Local Area Augmentation System – A ground-based augmentation to GPS that focuses its service on the airport area (approximately a 20-30 mile radius) for precision approach, departure procedures and terminal area operations.  It broadcasts its correction message via a VHF radio data link from a ground-based transmitter.  LAAS will yield the extremely high accuracy, availability, and integrity necessary for Category II, and III precision approaches.  Its demonstrated accuracy is less than 1 meter in both the horizontal and vertical axis.  Also see GBAS.
LNAV	Lateral Navigation - A Flight Management Computer function that calculates, displays, and provides lateral guidance to a profile or path.
LPV	Localizer Performance with Vertical guidance – WAAS based approach that provides lateral and vertical guidance.
MDA	Minimum Descent Altitude – The lowest altitude to which a crew may descend in an attempt to obtain visual reference to the runway when flying a non-precision approach.  If the runway is not in sight at the Missed Approach Point (MAP) at or above the MDA, crew must execute a missed approach.
MDH	Minimum Descent Height - The lowest altitude (height) a crew may safely descend to in an attempt to obtain visual reference to the runway when flying a non-precision approach.  If runway is not sighted, crew must execute a missed approach at the Missed Approach Point (MAP).
MEL	Minimum Equipment List – Outlines the systems that must be operable for commencement of a flight.
Missed Approach	That portion of an instrument approach that provides for extraction of the aircraft to a safe altitude following a decision, at the DA, to abort the landing attempt and execute a missed approach.
MITRE	A private, not-for-profit organization founded to provide engineering and research assistance to the U.S. Federal government.
MITRE CAASD	MITRE Center for Advanced Aviation System Development - A Federally funded research and development center run by the MITRE organization.
MMR	Multi-Mode Receiver
NAS	National Air Space – The air regulated and controlled by aviation authorities.
NAV	Navigation
ND	Navigation Display – The CRT or LCD display showing an aircraft’s position and planned route of flight.
NDB	Non Directional Beacon – A radio beacon transmitting from a fixed location that aircraft can tune in to determine their location.
NextGen	NextGen is the FAA’s shorthand name for their plans for the future air traffic management system in the US.  Formerly known as NGATS (Next Generation Air Transportation System).
NGATS	Next Generation Air Transportation System – Replaced by NextGen.
NM	Nautical Mile – The standard measure for aviation navigation.  It is 1.150779 statute miles, 6,076.1155 feet or exactly 1,852 meters.
Normal Operations	When all systems essential for flight are operating normally.  In this context, means all engine operation with nominal environmental conditions and fully operational navigation system.  See also All Engines Operative (AEO).
NOTAM	Notice To Airmen – Notifications containing information important to aviation.
OEI	One Engine Inoperative - A condition of flight where one engine has failed.  Same as EO (Engine Out)
OEM	Original Equipment Manufacturer – The builder of equipment or sometimes as in the case of aircraft manufacturers Airbus, Boeing, Bombardier and Embraer the assembler of components manufactured by themselves and others.
PBN Manual	See ICAO PBN Manual
PFD	Primary Flight Display
P-RNAV	Precision Area Navigation - European RNAV operations which satisfy a required track-keeping accuracy of ±1 NM for at least 95% of the flight time.
Public RNP	RNP approach procedures designed according to the standards set out by FAA Order 8260.52 or the soon to be published ICAO RNP AR Manual.  Public RNP is capable of serving a wide variety of equipment operated by users who have received approval to conduct RNP operations.  Public RNP standards currently exist only for approach procedures, and do not, by design minimize path distance or contemplate engine-out and certain other considerations that may be important to some operators.
RAIM	Receiver Autonomous Integrity Monitoring - An algorithm which verifies the integrity of the position estimate using the inherent redundancy available when more than 4 satellites are viewable and making certain conservative assumptions about atmospheric effects.  In general aviation, a RAIM alert is issued whenever the integrity of the position estimate is worse than 0.30 nm at a 99.9% confidence level.
RAPT	Regional Airspace Procedures Team - In the US, the RAPT is an integrated team comprised of members from Air Traffic, Aviation System Standards (AVN), Flight Standards, and Airports. The RAPT evaluates requests for instrument procedures that may have an impact on airspace, airports, or flight procedures.
RNAV	Area Navigation –The ability of an aircraft to navigate (make position estimates and set courses) based on a variety of sensors and signals in range.  Without RNAV, aircraft have to navigate directly from one point and directly to another, and those points must be within radio range.  Using RNAV, aircraft are able to steer directly to a destination or steer along a preprogrammed path.  Older aircraft RNAV systems estimate position by triangulating radio beacons while the most modern aircraft use GPS to perform that function.  Airline equipment generally backs up the externally referenced position estimate with an inertial reference system estimate (IRS or IRU)  In general RNAV is less precise than RNP, does not offer curved paths and does not provide integrity monitoring and alerting.
RNP	Required Navigation Performance – A method of aircraft navigation that utilizes modern flight computers, GPS (Global Positioning System) and innovative new procedures. Aircraft using RNP precisely fly predetermined paths loaded into their flight computers. Accurate navigation performance is ensured through continual monitoring with alerts if position becomes uncertain, one feature of RNP representing its advancement over RNAV. Over the last ten years RNP has been refined for use in procedures to approach and depart airports. These procedures can be designed with paths that reduce flight distances and lower thrust settings resulting in millions of dollars in fuel savings for an airline. They also bring noise and emissions reductions benefiting airport neighbors and the environment. The accuracy and all-weather capability of RNP result in enormous flight safety enhancements as well.
RNP AR	Required Navigation Performance Authorization Required – The formal name of an RNP procedure referencing the fact that the operation must be approved by a regulator.  RNP is high performance and requires certain features in the aircraft, the procedure, and in the airline operation to assure maximum safety and benefit.  The U.S. FAA formerly used RNP SAAAR for this term.
RNP AR Manual	See ICAO RNP AR Manual
RNP SAAAR	Required Navigation Performance Special Aircrew and Aircraft Authorization Required – A U.S. FAA description for RNP that emphasizes the requirement for approval of both aircraft and crews.  All RNP procedures require approval therefore they are all “SAAAR.”  The FAA is moving to adopt the ICAO terminology which is RNP AR (RNP Authorization Required).
RNP type	The RNP type is a designator associated with each RNP SAAAR minima line on an approach procedure. This designator invokes all of the requirements associated with the applicable operation which must be met in order to execute the procedure to those minima.  Typical high performance, terminal area values are 0.30, 0.11, or 0.10 nm.
ROT	Runway Occupancy Time – The time an aircraft is on a runway.  Typically measured from touchdown until it taxis off or from taxiing onto a runway until the aircraft leaves the ground during take-off.  Average ROT is a necessary variable in calculating a runway’s capacity.
RPAT	RNP Parallel Approach Transition – An FAA-proposed solution taking advantage of RNP to provide guidance to aircraft approaching closely spaced parallel runways.
SAAAR	Special Aircraft and Aircrew Authorization Required – The FAA suffix for certain procedures to indicate that operators and aircraft require approval before using them.  The ICAO simply uses AR (Authorization Required) which the FAA has indicated they will adopt in place of SAAAR.
Stabilized Approach	A stabilized approach is defined by the aircraft being on a positive lateral and vertical path with predictable changes to the energy state that are readily observed by the pilot and fully managed and contained by the total system.
TAWS	Terrain Awareness and Warning System – A computer and sensors system which warns crews about imminent collision with the ground.  Other similar systems are called EGPWS (Enhanced Ground Proximity Warning Systems).
TDZE	TouchDown Zone Elevation - The highest centerline elevation in the touchdown zone which is the first 3000’ of a runway or the first third of a runway, whichever is less, measured from the threshold in the direction of landing.
TERPS	Terminal Instrument Procedures – Criteria used for designing navigation procedures to land or depart airport runways.  ILS is one example of a TERPS based procedure.
Terrain	The natural surface of the earth excluding obstacles.
Terrain Obstacle	Point on the natural surface of the earth which is considered a potential hazard to the safe passage of aircraft in the type of operation for which the terrain and obstacle data are used.
TGL-10	Temporary Guidance Leaflet number 10 – Document issued by the European JAA (whose responsibilities are now handled by EASA) that provides guidance material for the approval of aircraft and operations where P-RNAV is required.  This document outlines equipment functions, performance requirements and operational procedures that operators must address in order to obtain operations approval.
UAV	Unmanned Aerial Vehicle – Remotely piloted aircraft originally developed for military purposes and now finding more peacetime applications.
UTC	Coordinated Universal Time – The time which aviation worldwide references which insures that all pilots are using the same time reference irrespective of time zone.  UTC is a compromise acronym between French and English speakers.  Times expressed in UTC are denoted with a Z suffix which aviators call Zulu as part of the phonetic alphabet.  The time reference for UTC is in England as was formerly called Greenwich Mean Time or GMT.  The UTC reference is constant and does not change with adjustments for summer time or time zones – it is up to the user of a UTC time reference to convert to local time.
VHF	Very High Frequency – A type of radio with the initials referring to the range of frequencies used.   VHF radios are used for communication, navigation and a variety of other purposes.
VLJ	Very Light Jet
VMC	Visual Meteorological Conditions – Weather that offers enough visibility so that air crews can navigate and avoid other aircraft by looking outside cockpit window.  Typically means clouds with a base at 1000’ or higher and at least 3 miles of visibility.
VNAV	Vertical Navigation - A function of the Flight Management Computer that calculates, displays, and provides vertical guidance to a profile or path.  Baro-VNAV refers to one such system that uses altimetry to measure vertical position.  As augmented GNSS systems come into wider use, VNAV will likely migrate to use satellite positioning for vertical position.
VOR	VHF Omni-directional Radio range - A ground-based electronic NAVAID that transmits very high frequency navigation signals 360° in azimuth.
WAAS	Wide Area Augmentation System – U.S. based GPS augmentation system that uses ground-based stations to send position corrections to a constellation of geo-stationary satellites which in turn relay the corrections to WAAS/GPS receivers.
WGS 84	World Geodetic System of 1984 - Reference frame of the Earth used by the GPS system.  Coordinates reported by GPS (latitudes, longitudes, and elevation) are in reference to a best fit ellipsoid developed with sophisticated laser and radar techniques.  Coordinates used for GPS navigation must be surveyed using the WGS 84 reference to assure fidelity.
Z	Zulu Time – Another term for the Coordinated Universal Time (UTC) which worldwide aviation references.
4DT	Four Dimension Trajectory – The concept of adding a fourth dimension (time) to the three traditional dimensions managed by air traffic control (longitude, latitude, altitude). This system issues landing clearances based on runway threshold arrival time. These times can be transmitted to an aircraft long before they are near the airport so any speed adjustments can be made and crews can plan optimal approach profiles.
8260.52	See FAA Order 8260.52
90-100	See AC 90-100
90-101	See AC 90-101