Glossary

Terms from aerodynamics, propulsion, operations, regulation and maintenance used on this site.

Aerodynamics and structures

Aerofoil: Cross-sectional shape of a wing, blade or control surface, which determines lift and drag.
Angle of attack: Angle between the wing chord and the direction of the airflow. Beyond a critical value, the wing stalls.
Aspect ratio: Ratio of the square of the wingspan to the wing area. A high aspect ratio reduces induced drag.
Composite material: Combination of fibres (carbon, glass) and resin. Composites make up about half the structural weight of the Boeing 787 and Airbus A350.
Delta wing: Triangular wing suited to supersonic flight, used notably on the Mirage III and Concorde.
Drag: Aerodynamic force opposing the aircraft's motion through the air. It includes friction, form and lift-induced drag.
eVTOL: Electric vertical take-off and landing aircraft, proposed in particular for urban transport.
Flight control surfaces: Movable surfaces (ailerons, elevator, rudder) that control the aircraft about its three axes.
High-lift devices: Flaps and slats extended for take-off and landing to increase lift at low speed.
Lift: Aerodynamic force perpendicular to the airflow, produced mainly by the wing, that keeps the aircraft in the air.
Mach number: Ratio of an object's speed to the speed of sound in the surrounding medium. Mach 1 is the speed of sound.
Monocoque structure: Structure in which the skin carries most of the loads. Modern metal fuselages are semi-monocoque, stiffened by frames and stringers.
Pressurisation: Keeping cabin pressure higher than at cruising altitude, for occupant comfort and safety.
Reynolds number: Dimensionless number comparing inertial and viscous forces in a flow. It governs how wind-tunnel results scale to the full-size aircraft.
Seaplane: Aircraft able to take off from and land on water, using floats or a hull.
Single-aisle: Airliner with a single aisle, such as the A320 and 737 families.
Sonic boom: Noise heard on the ground from the shock waves of an aircraft flying faster than sound.
Sound barrier: Term for the aerodynamic difficulties encountered near Mach 1 (rising drag and reduced control effectiveness).
Stall: Sudden loss of lift when the angle of attack exceeds the critical value and the airflow separates from the upper surface.
Structural fatigue: Progressive damage of a material under repeated loads, such as pressurisation cycles. The Comet accidents of 1954 revealed its importance.
Sweep: Angle at which the wings are angled back from the perpendicular to the fuselage. It delays compressibility effects near Mach 1.
Transonic: Speed range around Mach 1 where subsonic and supersonic flows coexist on the aircraft.
Widebody: Twin-aisle airliner, such as the Boeing 747 or the Airbus A300.
Wind tunnel: Facility in which air is blown around a model to measure aerodynamic forces.
Wing warping: Twisting of the wing used by the Wright brothers to control roll, before ailerons became standard.
Winglet: Upturned extension at the wingtip that reduces tip vortices and therefore induced drag.

Propulsion

Afterburner: Fuel injection downstream of the turbine to boost thrust at the cost of high fuel burn. Concorde used it for take-off and acceleration.
APU (auxiliary power unit): Small turbine that supplies electricity and compressed air on the ground and allows the engines to be started.
Bypass ratio: Ratio of cold-stream to hot-stream airflow in a turbofan. A high ratio reduces fuel burn and noise.
FADEC: Full authority digital engine control: a computer manages fuel and engine parameters.
Hydrogen propulsion: Use of hydrogen either burnt in a turbine or in a fuel cell producing electricity. No hydrogen airliner is in service.
Piston engine: Internal combustion engine in which pistons drive a crankshaft connected to the propeller.
Radial engine: Piston engine with cylinders arranged radially around the crankcase, widely used on airliners from the 1930s to the 1950s.
Rotary engine: Radial piston engine whose cylinders spin with the propeller around a fixed crankshaft, such as the 1908 Gnome.
Sustainable aviation fuel (SAF): Fuel produced from biomass, waste or synthetic processes, blended with kerosene in certified proportions.
Thrust: Force produced by the engine that drives the aircraft forward.
Thrust reverser: Device that deflects engine flow forward to help braking after landing.
Turbofan: Jet engine in which a fan accelerates a bypass airflow around the engine core. It powers almost all current airliners.
Turbojet: Jet engine whose thrust comes from expelling the hot gases leaving a gas turbine.
Turboprop: Gas turbine driving a propeller, used notably on regional aircraft such as the ATR 72.

Operations and navigation

Air traffic control: Service that keeps aircraft separated and traffic flowing, from towers and en-route control centres.
Autopilot: System that holds or follows a flight path, altitude or speed without direct pilot input.
CRM: Crew resource management: training in teamwork, communication and decision-making on the flight deck.
ETOPS: Rules allowing a twin-engine aircraft to fly beyond a set flying time from a diversion airport. ICAO now uses the term EDTO.
Flight recorders ('black boxes'): Flight data recorder (FDR) and cockpit voice recorder (CVR), protected to survive an accident and used by investigators.
Fly-by-wire: System in which pilot inputs reach the control surfaces as electrical signals via computers. Concorde had it; the A320 brought it to airliners with flight-envelope protections.
Glass cockpit: Flight deck in which electromechanical instruments are replaced by displays.
GNSS: Satellite navigation (GPS, Galileo…), used for area navigation and precision approaches.
Hub: Airport where an airline concentrates connections to link many destinations.
Human factors: Study of human capabilities and limitations applied to aircraft design, operation and maintenance.
ILS: Instrument landing system: ground radio beacons guide the aircraft laterally and vertically during approach.
Safety management system (SMS): Structured approach to identifying hazards and managing risks, required by ICAO of operators and maintenance organisations.
Sidestick: Small side-mounted control stick replacing the central control column on Airbus fly-by-wire aircraft.
TCAS: On-board collision avoidance system that alerts the crew and suggests a manoeuvre if another aircraft comes too close.

Regulation

Airworthiness: An aircraft's fitness to fly safely, in conformity with its approved design.
Airworthiness directive: Mandatory action required by an authority to correct an unsafe condition on an aircraft, engine or component.
Airworthiness review certificate (ARC): Document attesting, after a periodic review, that an aircraft's airworthiness is being maintained.
Chicago Convention: 1944 treaty on international civil aviation, setting out airspace sovereignty principles and founding ICAO.
CS-25: EASA certification specifications for large aeroplanes (transport aircraft).
DGAC: Direction générale de l'Aviation civile, the French civil aviation authority.
EASA: European Aviation Safety Agency, created in 2002 and based in Cologne (current name: European Union Aviation Safety Agency, since 2018).
FAA: Federal Aviation Administration, the civil aviation authority of the United States.
ICAO: International Civil Aviation Organization, a United Nations agency created by the 1944 Chicago Convention, based in Montreal.
ICAO Annexes: Documents gathering ICAO standards and recommended practices (licensing, airworthiness, investigation, etc.).
JAA: Joint Aviation Authorities: association of European authorities that harmonised rules (JARs) before EASA was created.
Part-21: European rule covering certification of aeronautical products and approval of design and production organisations.
Part-CAMO: Requirements for continuing airworthiness management organisations of commercial air transport aircraft.
Part-M: European rule on the continuing airworthiness of aircraft.
Type certificate: Document by which an authority attests that an aircraft model meets the applicable airworthiness requirements.

Maintenance

A, C and D checks: Traditional levels of scheduled inspection, from frequent light checks to heavy structural checks several years apart.
AMM: Aircraft maintenance manual, published by the manufacturer, describing maintenance tasks and procedures.
ATA 100: Air Transport Association specification numbering aircraft systems by chapter (for example 32 for landing gear). It has been incorporated into ATA iSpec 2200.
Borescope inspection: Inspection of an engine's interior using an optical probe inserted through dedicated ports.
CAMO: Continuing airworthiness management organisation: it plans maintenance, tracks directives and keeps aircraft records.
Certificate of release to service (CRS): Document signed by authorised staff certifying that maintenance has been performed in accordance with requirements.
Maintenance programme: Approved document setting out the maintenance tasks and intervals for a given aircraft.
MEL: Minimum equipment list: operator document stating which equipment may be temporarily inoperative at dispatch, and under what conditions.
MMEL: Master minimum equipment list, drawn up by the manufacturer and approved by the authority, from which each operator writes its MEL.
MSG-3: Industry analysis method for defining scheduled maintenance tasks based on the consequences of failures.
Non-destructive testing (NDT): Inspection methods (ultrasound, eddy current, penetrant, radiography) that detect flaws without damaging the part.
Part-145: European rule on the approval of maintenance organisations for transport aircraft.
Part-147: European rule on the approval of maintenance training organisations, which deliver basic and type training.
Part-66: European rule defining the aircraft maintenance licence (categories A, B1, B2, B3, C), its knowledge modules and required experience.
Type rating: Endorsement added to a Part-66 licence after training on a specific aircraft model, allowing certification of work on that type.