Aviation Glossary & Abbreviations

The Civil Aviation Authority is the UK's specialist aviation regulator. It licenses pilots and engineers, oversees airworthiness, regulates airports and airlines, and investigates safety concerns. Since Brexit, the CAA runs the UK's own licensing system in parallel with EASA's, though the two remain closely aligned.

EASA sets common aviation safety standards across the European Union, including pilot licensing ("an EASA licence"), aircraft certification and airline oversight. Since the UK left the EU, UK-issued licences are CAA licences rather than EASA ones, though the underlying rules remain very similar — a legacy of the UK having helped write them.

The FAA regulates all aspects of civil aviation in the US — pilot licensing, airline safety, air traffic control and airport operations. A CAA or EASA licence isn't automatically valid to fly commercially in the US, and vice versa; conversion or validation is usually required.

ICAO doesn't regulate pilots or airlines directly — it sets international standards and recommended practices that member states, including the UK, agree to follow. This is why licences, phraseology and many procedures look broadly similar wherever in the world you fly.

You'll see "NAA" used when a rule applies across every EASA country rather than naming one specific regulator — for example, "an ATO is approved by its NAA" means the CAA in the UK, the DGAC in France, the LBA in Germany, and so on. Useful shorthand once you're comparing schools across borders.

Gliding isn't trained through a standard ATO or DTO — it sits under its own EU-wide rulebook, Part-SFCL, delivered in the UK through the BGA's arrangement with the CAA, including a single central Declared Training Organisation covering its member clubs. The BGA also runs gliding's own long-standing badge and rating system alongside that. See the Types of Flying Schools guide.

Schools affiliated to the BMAA train pilots toward the microlight class of the NPPL — a UK National licence, not part of the EASA-style system used for PPL/CPL/ATPL. Neither the BMAA nor its schools are an ATO or DTO.

The LAA is best known for supporting amateur-built and vintage light aircraft in the UK, but also has schools affiliated to it for microlight-class NPPL training, working alongside — and sometimes as an alternative to — the BMAA.

A PPL is usually the first "real" licence a pilot earns, typically requiring a minimum of 45 flying hours, a series of ground exams, a skills test and a Class 2 medical. It allows you to carry passengers and fly cross-country, but not to be paid to fly. Most Modular routes to a commercial career start here — see our Training Paths guide.

Introduced as an easier, cheaper entry point into recreational flying, the LAPL requires fewer hours than a PPL and only a Class 2 or LAPL-specific medical. It's restricted to smaller aircraft and doesn't convert as easily into a commercial career path as a PPL does.

The NPPL predates the LAPL and remains available in the UK for microlights, simple single-engine aircraft and some gliders. It has more relaxed medical requirements — a valid driving licence can sometimes substitute for a medical certificate — but comes with more restrictions on where and what you can fly than a full PPL.

A CPL sits above the PPL and requires significantly more flying hours, harder exams and a Class 1 medical. On its own it qualifies you for some commercial flying, such as instructing or aerial work, but airline jobs almost always also require an Instrument Rating and, in practice, a "frozen ATPL".

A "frozen ATPL" means you've passed all the ATPL theory exams and hold a CPL and Instrument Rating, but haven't yet logged the flight hours (typically 1,500) needed to "unfreeze" it into a full ATPL. Almost every airline First Officer starts their career holding a frozen ATPL, not the full licence.

Unlike the traditional route (PPL → CPL → ATPL), the MPL is designed from day one around operating as part of an airline crew, and cadets are usually tied to a specific sponsoring airline throughout. It can be faster, but it's less flexible — see the comparison on our Training Paths page.

Without an IR, a pilot must fly under Visual Flight Rules (VFR), keeping clear of cloud and maintaining visual reference to the ground. An IR permits flight under Instrument Flight Rules (IFR), including in IMC — essential for any kind of reliable airline or commercial operation where weather can't be allowed to cancel every flight.

Most PPL training happens on single-engine aircraft. Progressing towards an airline career requires a Multi-Engine (ME) rating, since almost all airliners have two or more engines — handling an engine failure safely is a core part of ME training.

Becoming an FI is a common early-career step for newly qualified commercial pilots — it builds flying hours (often towards the 1,500 needed to unfreeze an ATPL) while earning a living, and is widely respected by airlines as genuine, responsibility-bearing experience rather than just "time-building".

A TRI trains pilots who already hold a licence and are converting onto a new aircraft type — for example, a pilot moving from the A320 to the A350. It's a more senior instructor qualification than an FI, usually held by experienced line pilots.

TREs conduct the checks that keep a pilot legally current on their aircraft type — including the Licence and Operator Proficiency Checks (LPC/OPC) every commercial pilot must pass periodically. It's one of the most senior non-management roles a line pilot can hold.

Part-SFCL sits alongside Part-FCL (the rulebook behind PPL/CPL/ATPL) but is specific to sailplanes, delivered through each country's own national gliding federation rather than a standard ATO or DTO — the Types of Flying Schools guide covers how that works in the UK and across EASA countries.

The BPL sits alongside PPL and LAPL as one of the licences a Declared Training Organisation is permitted to train towards — distinct from the fixed-wing and rotary licences most of this glossary covers, but regulated the same way.

Not every doctor can sign off a pilot medical — an AME has specific aviation medicine training and CAA authorisation. Booking an initial Class 1 assessment with an AME early is one of the most important first steps for anyone considering a commercial career, since it's the clearest way to identify any medical issue before committing significant money to training. See our pilot FAQ for more on this.

A Class 1 medical is more thorough than a Class 2, covering cardiovascular health, vision, hearing, neurology and mental health in detail. It must be renewed periodically — more frequently as pilots get older — and losing it is one of the few things that can end a professional flying career outright.

A Class 2 medical is less demanding than a Class 1 and is usually valid for longer. Many people complete their PPL on a Class 2 medical before ever needing to consider whether they'd pass the stricter Class 1 standard required for a commercial career.

Only an ATO can deliver the training that leads to a CPL, ATPL or Instrument Rating. The term replaced the older "FTO" designation under EASA's regulatory framework, though "flight school" is still used informally to mean the same thing. Compare approved schools in our School Directory.

A DTO can train you to a PPL, LAPL and additional ratings like night flying, but not a CPL, ATPL or Instrument Rating — those stay ATO-only. Rather than a full approval audit, the operator declares to the regulator that it meets the rules, though the regulator can still inspect. See the Types of Flying Schools guide for how DTOs, ATOs, gliding and microlight training all fit together.

FTO was the pre-EASA name for an approved training provider. You'll still see it in older documents and some school branding, but the current regulatory term — and the one that matters for checking a school is properly approved — is ATO.

MCC training covers communication, task-sharing and decision-making between a Captain and First Officer — skills that don't come up when flying single-pilot aircraft during earlier training. It's typically completed just before or alongside a first type rating, and is a required step before joining an airline.

The ILS provides both lateral (localiser) and vertical (glideslope) guidance, allowing an aircraft to descend safely to a runway even in cloud or fog, down to a defined decision height. It remains the most widely used precision approach aid at major airports worldwide.

A VOR transmits a signal that lets suitably equipped aircraft determine which direction they are from the beacon, letting pilots navigate along defined routes between stations. It's an older technology gradually being supplemented — though not fully replaced — by satellite navigation.

Unlike a VOR, an NDB simply transmits a signal in all directions — the aircraft's ADF equipment shows the direction to the beacon, but interpreting that into a useful navigation picture takes more pilot workload. NDBs are increasingly being decommissioned as GPS navigation takes over.

DME works by timing a signal exchange between the aircraft and a ground transmitter — often co-located with a VOR or ILS — to calculate precise slant-range distance, complementing the bearing information a VOR alone provides.

ADF is the aircraft-side receiver that points towards an NDB's transmission, giving the pilot a relative bearing to the beacon. It's an older technology, largely superseded by GPS-based navigation on modern aircraft.

RNAV uses a combination of navigation sources — GPS, DME, inertial reference — to calculate the aircraft's position and follow a programmed route far more flexibly than the old beacon-to-beacon method. Most modern airspace and approach procedures are now built around RNAV capability.

GNSS covers GPS (US), Galileo (EU), GLONASS (Russia) and other satellite constellations that aircraft can use for highly accurate position information. Modern RNAV navigation is increasingly built on GNSS as its primary data source.

In aviation, GPS provides highly accurate position, altitude and time information, and underpins most modern RNAV navigation and many instrument approach procedures. Aviation-certified GPS receivers meet stricter accuracy and integrity standards than consumer devices.

A METAR is a compact, standardised snippet of text reporting wind, visibility, cloud, temperature, pressure and significant weather at an airfield. Pilots read METARs as part of flight planning to check conditions are within their — or the aircraft's — limits.

Where a METAR reports what the weather actually is right now, a TAF forecasts what it's expected to be — typically covering the next 24–30 hours. Pilots use both together when planning a flight and choosing alternate airports.

Formally "Notice to Air Missions", NOTAMs are how the aviation system communicates time-sensitive or short-notice information that isn't on a chart — anything from a temporary crane near an airport to airspace restrictions for an event. Checking NOTAMs is a mandatory part of pre-flight planning.

QNH is set so that on the ground at an airfield, the altimeter reads the airfield's actual elevation above sea level. It's the standard setting used for most enroute and approach flying, and is given to pilots by ATC or in the ATIS/METAR.

QFE is less commonly used today but still appears at some airfields — it's handy for circuit flying since the altimeter directly shows height above the runway rather than altitude above sea level. Most commercial operations use QNH instead.

MSA is calculated to provide a safety margin above the highest terrain or obstacle within a defined radius — often around a navigation point or airport — and is marked on charts so pilots always know the lowest altitude they can safely descend to if not visual with the ground.

VMC is a description of the weather itself — specific minimum visibility and distance-from-cloud figures, which vary by airspace class and altitude. It's easy to conflate with VFR, but they're not the same thing: VMC is the condition, VFR is the set of rules a pilot flies under. A pilot with an Instrument Rating can choose to fly IFR in VMC; a VFR-only pilot cannot legally continue flying once the weather drops below VMC.

VFR flight requires minimum visibility and cloud clearance — VMC — and the pilot is responsible for seeing and avoiding terrain, obstacles and other traffic visually. Most early PPL training is conducted under VFR.

Flying in IMC legally requires an Instrument Rating and following IFR. In the UK there's also a lesser qualification, historically called the "IMC Rating" and now more formally the Restricted Instrument Rating, which permits limited flight in IMC without the full IR — the older name is still what most pilots and instructors call it day to day.

IFR flight requires an Instrument Rating and follows a defined set of procedures and air traffic control clearances, since the pilot may not be able to see outside the aircraft at all. It's the rule set required in IMC, though a rated pilot can also fly IFR in good weather. Virtually all airline flying is conducted under IFR, regardless of the actual weather.

Above a defined transition altitude, pilots stop using local QNH and switch to a standard pressure setting (1013.2 hPa), reading altitude as a "Flight Level" — e.g. FL350 means roughly 35,000 feet on the standard setting. This keeps all aircraft at high altitude working from the same reference, however different their local sea-level pressure is.

ATC covers everything from clearance delivery and ground control at an airport to enroute controllers managing traffic across huge areas of airspace. Pilots are in near-constant radio contact with some form of ATC for the majority of a commercial flight.

Rather than have every aircraft ask ATC individually for wind, runway in use and QNH, airports broadcast this as a looping recorded message that pilots listen to before contacting ATC — saving radio frequency time for everyone.

TCAS monitors other transponder-equipped aircraft nearby and, if a genuine conflict develops, issues a "Resolution Advisory" telling the crew to climb or descend to avoid it — independent of, and as a backup to, air traffic control.

GPWS — and its more capable successor EGPWS, which uses a terrain database to look ahead of the aircraft's flight path — has dramatically reduced "controlled flight into terrain" accidents, where a fully functioning aircraft is flown into the ground, usually due to a loss of situational awareness.

FTL sets maximum duty periods, minimum rest requirements and limits on consecutive early/late/night duties, based on the science of fatigue risk. It's one of the most heavily regulated parts of airline operations, precisely because a tired pilot is a genuine safety risk. Our EFB includes a working FTL Calculator for logged-in premium users.

SOPs cover everything from checklist flows to how a specific approach should be briefed, ensuring any two pilots from the same airline — who may never have flown together before — can operate predictably and safely as a crew.

CRM developed after investigators found that many accidents involved fully functional aircraft where poor communication or decision-making within the crew was the real cause. It's now a mandatory, recurring part of airline training alongside technical skills, and it's closely related to ADM — the decision-making a single pilot applies within that same teamwork.

Where CRM is about a crew working together, ADM is about the judgement calls an individual pilot makes — noticing a situation is changing, working out what's actually at stake, and choosing deliberately between options rather than just going with the first idea that comes to mind. It's taught as a structured, learnable process rather than an innate talent, and it's exactly what airline selection processes are trying to see evidence of.

RVSM increases the usable capacity of busy high-altitude airspace by safely halving the vertical gap required between aircraft, but requires more accurate altimetry equipment and crew training to qualify for.

ETOPS — sometimes joked to stand for "Engines Turn Or Passengers Swim" — certifies that a twin-engine aircraft, its crew and the airline's maintenance can handle an engine failure far from land, allowing routes like transatlantic flights that would otherwise have required a three- or four-engine aircraft.

The APU lets an aircraft run its systems, lighting and air conditioning on the ground — or as a backup in flight — without the fuel cost and noise of running the main engines, and is also used to start them.

MTOW accounts for the aircraft's structural limits and the performance needed to fly safely on that day's runway length, weather and altitude. Actual take-off weight is calculated for every flight and must stay within this limit.

Not every equipment failure grounds an aircraft — the MEL, approved by the regulator for each aircraft type, defines exactly what can be broken, for how long, and what operational limitations apply as a result, keeping aircraft flying safely without every fault being treated the same way.

V1 is the last point at which a rejected takeoff can be safely stopped within the runway — past it, the crew must continue the takeoff even with a problem. VR is the speed at which the aircraft is rotated to begin the climb, and V2 is the minimum speed that guarantees safe climb performance even if an engine fails right after take-off.

Almost every professional pilot starts their airline career as a First Officer, sharing flying duties equally with the Captain but without final command authority. Progression to Captain typically takes several years and depends on seniority and vacancies — see Pilot Careers for the fuller picture.

Not every airline uses this grade, but where it exists, it typically reflects years of experience as an FO and can come with expanded responsibilities or pay, without yet holding full command.

On a commercial flight the Captain is always PIC. The PIC role also matters hugely for licence progression — many hour requirements, such as the 1,500 needed to unfreeze an ATPL, specifically require PIC time, not just any time in the aircraft.

Conducted by a TRE, usually in a simulator, the LPC tests handling of normal and emergency procedures and must be passed regularly — typically annually — to keep flying legally.

Often combined with the LPC in the same simulator session, the OPC checks a pilot against their airline's own SOPs and route or type-specific requirements, rather than the more generic licence-level standard the LPC covers.

CPD isn't a single regulatory requirement in the way LPC/OPC checks are, but is widely encouraged — covering everything from technical refreshers to leadership and human-factors training across a career. See our Student and Professional CPD Hubs.