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interval (MLDI) for the adapted vertex for each arrival aircraft, based on runway configuration, airport
acceptance rate, airport arrival delay period, and other metered arrival aircraft.
ACLT is either the vertex time of arrival (VTA) of the aircraft, the tentative calculated landing
time, or the actual calculated landing time (TCLT/ACLT) of the previous aircraft plus the arrival aircraft
interval (AAI), whichever is later. ACLT will not be updated in response to the aircraft’s progress.
acorn nut. A nut with a domed top. The threaded hole does not go completely through the nut, but the end
of the threads is covered by the dome.
Acorn nuts, also called cap nuts, are used on the bolts to produce a finished, smooth appearance.
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
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 10
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.
active infrared detection. The method of detection in which a beam of infrared rays is transmitted toward
a suspected target. Reflected rays from the target are used to detect and identify it.
active runway (air traffic control). Any runway or runways currently being used for takeoff or landing.
When multiple runways are used, they are all considered to be active runways.
actuating cylinder (fluid power system component). A cylinder and piston arrangement used to convert
hydraulic or pneumatic fluid pressure into work. Fluid under pressure moves the piston that does the work.
actuating horns. Levers attached to aircraft control surfaces to which control cables are connected to move
the surfaces.
actuator (fluid power system component). A device which transforms fluid pressure into mechanical
force.
Actuators may be linear, rotary, or oscillating, and they may be actuated by either hydraulic or
pneumatic pressure.
ACU (acceleration control unit) (turbine engine fuel control). The section in a turbine engine fuel
control that schedules the rate of increase of the fuel to the nozzles when the engine controls call for
acceleration. The ACU prevents the engine from stalling during acceleration.
acute angle. An angle of less than 90°. An acute angle is also called a closed angle.
AD (airworthiness directive). A regulatory notice sent out by the FAA to the registered owner of an
aircraft informing him or her of the discovery of a condition that prevents the aircraft from continuing to
　
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