The terms behind the headlines, explained in plain language. AirPro News writes for people who work in aviation, aerospace and space, but every beat has its own vocabulary, and a reader from MRO does not necessarily speak launch, and vice versa. This glossary covers the terms that appear most often in our coverage. Missing something? Email contact@airpronews.com and we will add it.
A · B · C · D · E · F · G · H · I · L · M · N · O · P · R · S · T · U · V · W
A
A check / C check / D check
The traditional letter checks of scheduled airliner maintenance. An A check is a light inspection every few weeks; a C check is a heavy check every 18–24 months that takes the aircraft out of service for one to three weeks; a D check (or heavy structural check) is the deepest overhaul, every six to ten years. Many operators now use task-based programmes rather than letter checks, but the terms remain in daily use.
ACMI lease
Aircraft, Crew, Maintenance and Insurance: a wet lease in which the lessor supplies the aircraft with its crew and maintenance, and the lessee pays per block hour and provides fuel, fees and the commercial risk.
Advanced air mobility (AAM)
The umbrella term for new short-range air transport using electric or hybrid aircraft, especially eVTOLs, operating from vertiports within and between cities and regions. Urban air mobility (UAM) is the city-centre subset.
Aftermarket
The market for parts, repairs, maintenance and upgrades after an aircraft or engine is delivered. For engine and equipment manufacturers it is usually more profitable than the original sale.
Aircraft on Ground (AOG)
An aircraft that cannot fly because it is waiting for a part or repair. AOG is the highest priority in maintenance and parts logistics, because every hour costs the operator lost revenue.
Airworthiness Directive (AD)
A legally binding instruction from a regulator such as the FAA or EASA requiring operators to inspect, modify or replace something on a specific aircraft, engine or component to correct an unsafe condition.
Alcohol-to-jet (ATJ)
A sustainable aviation fuel pathway that converts ethanol or other alcohols into jet fuel. It is the main route to scale SAF beyond the limited supply of waste fats and oils used by HEFA.
ASK and RPK
Available seat kilometres and revenue passenger kilometres: the standard measures of an airline’s capacity and traffic. One seat flown one kilometre is one ASK; one paying passenger flown one kilometre is one RPK. RPK divided by ASK is the load factor.
B
Backlog
The number of aircraft (or engines) ordered but not yet delivered. A manufacturer’s backlog, often expressed in years of production, shows how far out delivery slots are booked.
Beyond visual line of sight (BVLOS)
Drone operations where the pilot cannot see the aircraft. Routine BVLOS approval is the regulatory step that turns drone delivery and inspection from trials into businesses.
Block hour
The time from an aircraft’s first movement at departure to its stop at the destination gate. Airlines, lessors and maintainers price and measure much of their business per block hour.
Book-and-claim
A system that lets a company pay for sustainable aviation fuel delivered into the fuel system at one airport and claim the emissions reduction for flights elsewhere, because SAF is chemically identical to conventional fuel once blended.
Book-to-bill ratio
New orders divided by deliveries over a period. Above 1.0 the backlog is growing; below 1.0 the manufacturer is delivering faster than it is selling.
C
Certification
The regulator’s formal approval that a design, a production organisation or a maintenance organisation meets the applicable rules. See Type certificate, Supplemental type certificate and Part 145.
CFM56 and LEAP
The two families of engines from CFM International, the GE Aerospace and Safran joint venture, that power most Boeing 737s and about half of Airbus A320-family aircraft. The CFM56 equipped earlier generations; the LEAP powers the 737 MAX and A320neo.
Collaborative combat aircraft (CCA)
Uncrewed military aircraft designed to fly alongside crewed fighters, carrying sensors or weapons and controlled by the crewed aircraft or a ground station. Also called loyal wingmen.
Composite
A material made of fibres, usually carbon, set in a resin. Composites are lighter than aluminium for the same strength and now make up around half the structure of aircraft such as the Boeing 787 and Airbus A350.
Constellation
A group of satellites working together to provide continuous coverage, from a handful in GEO to thousands in LEO for broadband services.
CORSIA
The Carbon Offsetting and Reduction Scheme for International Aviation, ICAO’s global scheme under which airlines offset growth in international emissions above a baseline, with SAF use counting against the obligation.
D
Delivery slot
A position in a manufacturer’s production schedule assigned to a specific customer aircraft. With backlogs stretching years, slots are traded, deferred and fought over as much as the aircraft themselves.
DER repair
A repair approved by an FAA Designated Engineering Representative rather than by the original manufacturer. Together with PMA parts, DER repairs give airlines and MROs alternatives to OEM parts and pricing.
Dry lease and wet lease
A dry lease supplies the aircraft only; the airline provides crew, maintenance and insurance. A wet lease (see ACMI) supplies the aircraft with crew and maintenance. Most leased airliners are on dry leases from lessors.
E
EASA
The European Union Aviation Safety Agency, responsible for certification, rulemaking and oversight of civil aviation in the EU and associated states. The FAA’s counterpart.
Engine shop visit
A scheduled or unscheduled removal of an engine for overhaul or repair in a specialised engine shop. Shop visits are the most expensive maintenance events in an aircraft’s life and the main driver of the engine aftermarket.
ETOPS
Extended Operations: the rules that allow twin-engine aircraft to fly routes more than 60 minutes from a diversion airport. An ETOPS rating (for example 180 minutes) defines how far from an alternate an aircraft may fly.
eVTOL
Electric vertical take-off and landing aircraft: battery-powered aircraft that lift off vertically like a helicopter, typically with multiple distributed rotors, and in many designs transition to wing-borne flight for cruise.
F
FAA
The Federal Aviation Administration, the United States civil aviation regulator, responsible for certification, air traffic control and safety oversight. Its certifications are recognised, with bilateral agreements, by most other regulators.
Firm order, option and letter of intent
A firm order is a signed purchase agreement, usually with a deposit; it counts in the backlog. An option is a right to buy additional aircraft later at agreed terms. A letter of intent or memorandum of understanding signals intent but is not binding; air-show announcements are often LOIs later converted to firm orders.
Freighter and P2F conversion
A freighter is an aircraft built or converted to carry cargo. A passenger-to-freighter (P2F) conversion adds a main-deck cargo door and floor to a used passenger aircraft, a large MRO business in its own right.
G
Geared turbofan (GTF)
Pratt & Whitney’s PW1000G engine family, which uses a gearbox between the fan and the turbine so each can run at its most efficient speed. It powers the A320neo (as one of two options), A220 and Embraer E2, and its durability issues have driven a large wave of shop visits.
H
Heavy maintenance
Base maintenance carried out in a hangar with the aircraft out of service for days or weeks: C and D checks, structural inspections, cabin retrofits and modifications. Contrast with line maintenance.
HEFA
Hydroprocessed Esters and Fatty Acids: the process that turns used cooking oil, animal fats and vegetable oils into sustainable aviation fuel. It is the only SAF pathway in large-scale commercial production today, and its feedstock supply is limited.
Hydrogen aircraft
Aircraft using hydrogen either in fuel cells to generate electricity for motors or burned directly in modified gas turbines. Both need cryogenic liquid hydrogen storage; both are in development, none is yet in commercial service.
I
IATA and ICAO
IATA is the airline industry’s trade association, which sets commercial standards and publishes traffic statistics. ICAO is the UN agency that sets international aviation standards and rules; national regulators implement them.
In-flight entertainment and connectivity (IFEC)
Seat-back or streaming entertainment systems and the satellite or air-to-ground links that provide internet on board. A large cabin-retrofit and supplier market.
L
Launch vehicle
A rocket that carries a payload into orbit or beyond. Vehicles are classed by payload to low Earth orbit: small (under about 2 tonnes), medium, heavy and super-heavy.
LEO, MEO and GEO
Low Earth orbit (up to about 2,000 km, where most satellites and constellations fly), medium Earth orbit (navigation constellations such as GPS) and geostationary orbit (35,786 km, where a satellite stays fixed over one point on the equator).
Lessor
A company that owns aircraft and leases them to airlines. Lessors own roughly half the world’s commercial jet fleet and are among the largest customers of Airbus and Boeing.
Line maintenance
Routine checks and minor repairs carried out at the gate or on the ramp between flights, without taking the aircraft out of service: daily and pre-flight checks, defect rectification, component swaps.
Load factor
The share of available seats filled by paying passengers: RPK divided by ASK. A load factor above 80 per cent is typical for well-run airlines.
M
MALE UAV
Medium-altitude, long-endurance uncrewed aircraft such as the MQ-9B or Eurodrone: large military drones that fly for a day or more at altitudes of around 25,000–45,000 feet, mainly for surveillance.
Minimum equipment list (MEL)
The document that specifies which items may be inoperative on an aircraft while it continues to fly, and for how long, before repair is required.
MRO
Maintenance, repair and overhaul: the industry that keeps aircraft airworthy. Segments include line, base (heavy), engine and component maintenance, performed by airlines’ own divisions, independent MROs and manufacturers’ service arms.
N
Nacelle
The housing around an engine, including the inlet, cowls and thrust reverser. Nacelles are made by specialist suppliers and maintained by dedicated MRO shops.
Narrowbody and widebody
A narrowbody (single-aisle) aircraft has one aisle and up to about 240 seats: the A320 and 737 families. A widebody (twin-aisle) has two aisles and flies most long-haul routes: the A330, A350, 777 and 787.
O
OEM
Original equipment manufacturer: the company that designed and built the aircraft, engine or component, as opposed to independent suppliers of parts, repairs or services.
P
Part 145 and Part-145
The US (FAA Part 145) and European (EASA Part-145) rules under which maintenance organisations are approved. A repair station’s Part 145 certificate defines which aircraft, engines and components it may work on.
PMA part
A part made under an FAA Parts Manufacturer Approval by a company other than the original manufacturer. PMA parts are certified equivalents to OEM parts and are usually cheaper.
Power-by-the-hour
A maintenance contract, often from the engine or component manufacturer, in which the operator pays a fixed rate per flight hour and the provider takes on the cost and risk of repairs. Also called a flight-hour agreement.
Power-to-liquid (PtL) and e-fuels
Synthetic aviation fuel made from green hydrogen and captured carbon dioxide using renewable electricity. The most scalable and lowest-emission SAF pathway in theory, and the most expensive and least developed in practice.
Powered-lift
The FAA’s aircraft category for vehicles that take off vertically but fly on wings, such as tiltrotors and most eVTOLs. The category determines how the aircraft are certified and how pilots are qualified.
Production rate
The number of aircraft a manufacturer builds per month on a given programme, such as ‘rate 38’ for the 737 MAX. Rate increases are announced years ahead and constrained by the supply chain and regulators.
R
ReFuelEU Aviation
The EU regulation requiring fuel suppliers at EU airports to blend a rising minimum share of SAF into jet fuel from 2025, with a sub-target for synthetic fuels, and requiring airlines to uplift fuel there.
Regional aircraft
Aircraft with roughly 30–150 seats used on shorter, thinner routes: regional jets such as the Embraer E-Jet and Airbus A220 and turboprops such as the ATR 72 and De Havilland Dash 8.
Remote ID
The requirement that drones broadcast their identity and position in flight, the equivalent of a digital licence plate, which regulators treat as a prerequisite for routine operations over people and beyond visual line of sight.
Reusable rocket
A launch vehicle whose first stage (and in some designs more) returns to land and flies again. Reuse is the main reason launch costs have fallen over the past decade.
Rideshare launch
A launch carrying multiple customers’ satellites on one vehicle, sold by the kilogram, which has cut the cost of reaching orbit for small satellites.
S
Sale and leaseback
A transaction in which an airline sells aircraft it owns or has on order to a lessor and leases them back, freeing capital while keeping the aircraft in its fleet.
Service bulletin (SB)
A manufacturer’s notice describing an inspection, modification or repair for an aircraft, engine or component. SBs are recommendations unless a regulator makes them mandatory through an airworthiness directive.
Slot
At congested airports, a permitted time for an aircraft to arrive or depart. Slots at airports such as Heathrow are scarce, traded and valuable. Not to be confused with a delivery slot.
Spaceport
A licensed site for launches and landings, from national ranges such as Cape Canaveral and Kourou to the commercial spaceports now being built in Europe, Australia and elsewhere.
Supplemental type certificate (STC)
A regulator’s approval for a modification to a certified aircraft, engine or component, from a cabin connectivity system to a freighter conversion. STCs are a core product of the modification industry.
Supply chain
The tiers of suppliers below the aircraft manufacturer: tier 1 firms supply major systems and structures, tier 2 supply assemblies and components to them, tier 3 supply parts and materials. Weakness anywhere in the chain limits production rates.
Sustainable aviation fuel (SAF)
Jet fuel made from renewable or waste feedstocks rather than crude oil, chemically similar enough to be blended with conventional fuel and used in existing engines. Lifecycle emissions are typically 60–80 per cent lower; supply is a small fraction of demand.
T
Teardown
Dismantling a retired aircraft or engine to recover serviceable parts for resale. The source of most used serviceable material.
Turboprop
An aircraft powered by gas-turbine engines driving propellers. More fuel-efficient than jets on short sectors and the standard for regional routes under about 90 minutes.
Type certificate (TC)
The regulator’s approval of an aircraft, engine or propeller design, confirming it meets airworthiness standards. Obtaining a type certificate for a new aircraft typically takes years of testing after first flight.
U
ULCC
Ultra-low-cost carrier: an airline built on very low base fares, high aircraft utilisation and revenue from add-on fees, such as Ryanair, Wizz Air, Spirit and Frontier.
Unmanned aircraft system (UAS)
The regulator’s term for a drone together with its control station and data link. UAV (unmanned aerial vehicle) and RPAS (remotely piloted aircraft system) refer to the same thing; ‘uncrewed’ is increasingly preferred.
Used serviceable material (USM)
Parts recovered from retired aircraft and engines, inspected and recertified for reuse. USM is cheaper than new parts and a major part of the aftermarket, especially for older fleets.
V
Vertiport
A take-off and landing facility for eVTOL aircraft, with charging and passenger handling. Vertiports range from adapted heliports to purpose-built terminals and are the infrastructure bottleneck for advanced air mobility.
W
Wide-body freighter
Large cargo aircraft such as the Boeing 777F and 747-8F and the planned Airbus A350F, used on long-haul express and general cargo routes.
Last updated: 29 September 2026. See also: events calendar · MRO hub · SAF hub · eVTOL & AAM hub · Space hub.