Aviation Maintenance Terms beginning with A
Abradable strip
A strip of material in the compressor housing of some axial-flow gas turbine engines. The tip of the compressor blade touches the abradable strip, and wears, or abrades a groove in it. This groove ensures the minimum tip clearance.
Abradable tip (compressor blade tip)
The tip of some axial-flow compressor blades constructed so that it will abrade, or wear away, upon contact with the compressor housing, which ensures the minimum tip clearance between the blade and the housing.
Absolute pressure
Pressure referenced from zero pressure or a vacuum.
Acceleration
The amount the velocity of an object is increased by a force during each second it is acted upon by that force. Acceleration is usually measured and expressed in terms of feet per second, per second (fps²).
Accessory end
The end of a reciprocating engine on which many of the accessories are mounted. Also, called the anti- propeller end.
Accumulator
A hydraulic component that stores a non-compressible fluid, such as oil, under pressure. An accumulator has two compartments separated by a flexible or movable partition with one compartment containing compressed air. When oil is pumped into the other compartment, the partition moves over, further compressing the air which holds pressure on the oil.
Adiabatic change
A physical change that takes place within a material in which heat energy is neither added to the material, nor taken away. If a container of gas is compressed, with no heat energy added to or taken from it, the gas will become hotter; its temperature will rise.
Aeromatic propeller
A patented variable-pitch propeller that has flyweights around the blade shanks and the blades angled back from the hub to increase the effects of aerodynamic and centrifugal twisting forces. This propeller automatically maintains a relatively constant rpm for any throttle setting.
Aft-fan engine
A turbofan engine with the fan mounted behind the compressor section. The blades of an aft-fan are normally extensions of the free turbine blades.
Afterburner
A component in the exhaust system of a turbojet or turbofan engine used to increase the thrust for takeoff and for special flight conditions. Since much of the air passing through a gas turbine engine is used only for cooling, it still contains a great deal of oxygen. Fuel is sprayed into the hot, oxygen-rich exhaust in the afterburner, where it is ignited and burned to produce additional thrust.
Air bleed (carburetor component)
A small hole in the fuel passage between the float bowl and the discharge nozzle of a float carburetor. Air drawn into the liquid fuel through the air bleed breaks the fuel up into an emulsion, making it easy to atomize and vaporize.
Air cooling
The removal of unwanted heat from an aircraft engine by transferring the heat directly into the air flowing over the engine components.
Air impingement starter
A turbine engine starter that basically consists of a nozzle that blows a stream of compressed air against the turbine blades to rotate the compressor for starting the engine.
Air-oil separator
A component in a turbine engine lubrication system that removes the air from the scavenged oil before it is returned to the oil tank.
Air/fuel mixture ratio
The ratio of the weight of the air to that of the fuel in the mixture fed into the cylinders of an engine.
Airworthiness Directive
Airworthiness Directives (ADs) are legally enforceable regulations issued by the FAA in accordance with 14 CFR part 39 to correct an unsafe condition in a product. Part 39 defines a product as an aircraft, engine, propeller, or appliance.
All-weather spark plug
A shielded spark plug designed for high altitude operation. The ceramic insulator is recessed into the shell to allow a resilient grommet on the ignition harness to provide a watertight seal. All weather spark plugs, also called high-altitude spark plugs, are identified by their 3/4-20 shielding threads.
Alpha control range (alpha mode)
The flight operating mode from takeoff through landing for a turbo-prop engine. Alpha mode includes operations from 95% to 100% of the engine’s rated rpm.
Altitude engine
An aircraft reciprocating engine equipped with a supercharger that allows it to maintain its rated sea- level horsepower to an established higher altitude.
Amateur-built aircraft
Aircraft built by individuals as a hobby rather than by factories as commercial products. Amateur-built or home-built aircraft do not fall under the stringent requirements imposed by the FAA on commercially built aircraft.
Ambient air pressure
The pressure of the air that surrounds an object.
Analog indicator
An indicator that shows the value of the parameter being measured by a number marked on a graduated dial aligned with a movable pointer.
Angle of attack
The acute angle between the chordline of a propeller blade and the relative wind. The angle of attack is affected by both the engine rpm and the forward speed of the aircraft.
Annual inspection
A complete inspection of the airframe and powerplant required for FAA-certificated aircraft operating under 14 CFR part 91 General Operating and Flight Rules, and not on one of the authorized special inspection programs. An annual inspection must be conducted every 12 calendar months, and it must be conducted by an aviation maintenance technician who holds an Airframe and Powerplant rating and an Inspection Authorization. The scope of an annual inspection is the same as that of a 100-hour inspection.
Annular duct
A duct, or passage, that surrounds an object. The annular fan-discharge duct surrounds the core engine.
Annular orifice
A ring-shaped orifice, normally one that surrounds another orifice.
Annulus
A ring or groove around the outside of a circular body or shaft, or around the inside of a cylindrical hole.
Annunciator panel
A panel of warning lights visible to the flight crew. The lights are identified by the name of the system they represent and are often covered with colored lenses. Red lights indicate a dangerous condition and green indicate a safe condition.
Anodizing
A hard, airtight, unbroken oxide film electrolytically deposited on an aluminum alloy surface to protect it from corrosion.
Anti-icing
Prevention of the formation of ice on a surface.
Anti-propeller end
The end of a reciprocating engine that does not attach to the propeller. Also called the accessory end.
Aramid fiber
Fiber made from an organic compound of carbon, hydrogen, oxygen, and nitrogen. It has high strength and low density. It is flexible under load and is able to withstand impact, shock, and vibration. Kevlar is a well- known aramid fiber.
Aromatic compound
A chemical compound such as toluene, xylene, and benzene that is blended with gasoline to improve its anti-detonation characteristics.
Articulating rod
See link rod.
Aspect ratio
The ratio of the length of an airfoil, such as a compressor blade, to its width.
Asymmetrical loading
The loading of a propeller disc that causes one side to produce more thrust than the other side.
Atomize
The process of breaking a liquid down into tiny droplets or a fine spray. Atomized liquids vaporize easily.
Augmentor tube
A long, specially shaped stainless steel tube mounted around the exhaust tail pipe of a reciprocating engine. As exhaust gases flow through the augmentor tube, they produce a low pressure in the engine compartment that draws in cooling air through the cylinder fins.
Automatic intake valve
An intake valve opened by low pressure created inside the cylinder as the piston moves down. There is no mechanical means of opening it.
Automatic mixture control (AMC)
The device in a fuel metering system, such as a carburetor or fuel injection system, that keeps the air/fuel mixture ratio constant as the density of air changes with altitude.
Autosyn system
The registered trade name of a remote indicating instrument system. An Autosyn system uses an electromagnet rotor, excited with 400-hertz AC, and a three- phase distributed-pole stator.
Axial bearing load
The load on a bearing parallel to the shaft on which the bearing is mounted. Thrust produces an axial load on a bearing.
Axial turbine
A turbine that is turned by a fluid flowing through it in a direction that is approximately parallel to the shaft on which the turbine wheel is mounted.
Axial-flow compressor
A type of compressor used in gas turbine engines. Air passes through the compressor in essentially a straight line, parallel to the axis of the compressor. The compressor is made of a number of stages of rotating compressor blades between stages of stationary stator vanes.
Axis of rotation
The center line about which a propeller rotates.
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