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abrasive blasting. A method of removing carbon and other contaminants from machine parts. In abrasive
blasting, the parts are sprayed with a high-velocity blast of air containing fine particles of abrasive material
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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such as sand, aluminum oxide, or glass beads.
abrasive tip (turbine blade). A turbine blade with a hardened insert at the tip that is able to cut into the
turbine shroud ring. See abradable shroud.
abscissa. A coordinate representing the distance from the Y-, or vertical, axis in a plane Cartesian
coordinate system.
The abscissa is measured along the X-, or horizontal, axis and the ordinate along the Y-, or
vertical, axis.
absolute altimeter. An electronic altimeter used to indicate the exact height of an aircraft above the terrain.
See radio altimeter.
absolute altitude. The actual distance between an aircraft and the terrain over which it is flying. Absolute
altitude is measured with an electronic altimeter.
absolute ceiling. The maximum height above sea level at which an aircraft can maintain level flight under
standard atmospheric conditions.
absolute humidity. The actual amount of water vapor present in a specific volume of air. If one cubic
meter of air contains 100 grams of water, the absolute humidity of the air is 100 grams per cubic meter.
absolute instability (meteorology). The state of a layer of air within the atmosphere in which the vertical
distribution of temperature is such that a parcel of air, if given an upward or downward push, will move
away from its initial level without further outside force being applied.
absolute pressure. Pressure measured relative to zero pressure, or a vacuum. Absolute pressure is
measured with a barometer, and in aviation usage is often expressed in inches of mercury. Manifold
pressure in a reciprocating engine is an example of an absolute pressure.
absolute pressure controller (reciprocating engine control). A type of turbocharger controller which
limits the maximum discharge pressure the turbocharger compressor can produce while the aircraft is flying
below its critical altitude.
absolute pressure gage. A pressure measuring instrument that measures pressure referenced from a
vacuum.
An aneroid barometer is one of the more accurate types of absolute pressure gages. It measures the
changes in the dimensions of an evacuated bellows as it is affected by the pressure of the ambient air.
absolute pressure regulator (pneumatic system component). A regulator valve at the compressor inlet in
an aircraft high-pressure pneumatic system.
Regulating the inlet air pressure prevents excessive speed variation and/or compressor
overspeeding.
absolute temperature. Temperature referenced from absolute zero, the temperature at which all molecular
movement has ceased.
There are two absolute temperature scales, Kelvin and Rankine. The Kelvin scale uses the same
size increments as the Celsius scale, and the Rankine scale uses the same size increments as the Fahrenheit
scale.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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See temperature.
absolute value. The numerical value of a number without considering whether its sign is plus or minus. For
example, positive eight (+8) has the same absolute value as negative eight (–8).
absolute vorticity (meteorology). The swirling motion, or vorticity, imparted to the atmosphere by the
combination of the rotation of the earth and the circulation of the air relative to the earth.
absolute zero. The temperature at which all molecular movement inside a material stops. Absolute zero is
0° Kelvin, 0° Rankine , –273° Celsius, and –460° Fahrenheit.
absorptance (electromagnetic radiation). The ratio of the total unabsorbed radiation to the total amount
of radiation falling on the object whose absorptance is being measured.
abvolt. A basic unit of electromotive force in the electromagnetic-centimeter-gram-second system. One
abvolt is equal to 10-8 volts in the absolute meter-kilogram-second-ampere system. The abbreviation for
abvolt is aV.
AC (Advisory Circular). Information published by the FAA explaining the Federal Aviation Regulations
and describing methods of performing certain maintenance and inspection procedures. Compliance with
ACs is not mandatory, and the information in the ACs is not necessarily approved data.
AC (alternating current). Electrical current in which the electrons continually change their rate of flow
and periodically reverse their direction.
ACARS (aircraft communication addressing and reporting system). A two-way communication link
between an airliner in flight and the airline’s main ground facilities.
　
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