acoustical liners (turbine engine component). Sound absorbing liners used in an engine nacelle or around the tail pipe to reduce the amount of noise produced by the engine.
acoustics. The science of the production, transmission, reception, and effects of sound.
AC plate resistance (electron tube characteristic). The ratio of a small change in the plate voltage to the small change in the plate current it causes. The AC plate resistance, measured in ohms, is actually the
internal resistance of the tube to the flow of alternating current.
acrobatic category airplane. An airplane certificated under 14 CFR Part 23 for flight without restrictions other than those shown to be necessary as the result of a required flight test. An acrobatic category airplane is stressed for a limit maneuvering load factor of +6.0, and –3.0.
acrobatic flight. An intentional maneuver involving an abrupt change in an aircraft’s attitude, an abnormal attitude, or abnormal acceleration not necessary for normal flight. Loops, spins, and rolls are normally considered to be acrobatic flight.
acrylic lacquer. An aircraft finishing material consisting of an acrylic resin vehicle and certain volatile solvents.
acrylic resin. A transparent, thermoplastic material used for aircraft windshields and side windows. Lucite., Plexiglas., and Perspex. are registered trade names for materials made from acrylic resin.
actinium. A radioactive chemical element found in uranium ores. Actinium’s symbol is Ac, and its atomic number is 89. Actinium is used as a source of alpha rays.
activated charcoal. Powdered or granulated charcoal. Because activated charcoal is full of tiny pores, it has an extremely large surface area for its volume. Activated charcoal is used as a filter for liquids and as a medium to absorb gases.

active clearance control. See ACC.
active component (electrical component). An electrical component that can control current or voltage for switching or amplification. Vacuum tubes, transistors, and magnetic amplifiers are active components.
active current. The current in an AC circuit in phase with the voltage. It is active current that produces true power in an AC circuit.
active detection systems. Radar, radio, and sonar systems that require the transmission of energy. Active systems are distinguished from passive systems that receive but do not transmit energy.