abrasion resistant. The ability of a material to resist damage by abrasion.
abrasive. A material containing minute particles of a hard substance used to wear away a softer surface. Aluminum oxide, silicon carbide, and glass beads are abrasives commonly used in aircraft maintenance.
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 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.
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