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.

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 sealevel 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.

APC . Absolute pressure controller.

APU . Auxiliary power unit. A small turbine- or reciprocatingengine-powered generator, hydraulic pump, and air pump. APUs are installed in the aircraft and are used to supply electrical power, air, and hydraulic pressure when the main engines are not running.

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 wellknown aramid fiber.