时间：2011-03-11 23:18来源：蓝天飞行翻译 作者：航空 点击：次
|
plywood. A form of manufactured wood product made of thin layers (plies) of wood veneer glued together. The grain of each alternate ply of veneer runs at 90° or 45° to the plies next to it. Plywood is strong, lightweight, rigid, and relatively inexpensive. PMA (Parts Manufacturer Approval). An approval, granted by the FAA under 14 CFR Part 21, that allows a person to produce a modification or replacement part for sale for installation on a type-certificated product. PMB (plastic media blasting). A method of removing paint from an aircraft surface by dry-blasting it with tiny plastic beads. pneumatic altimeter. A form of aneroid barometer whose dial is calibrated in feet or meters, rather than units of pressure. A pneumatic altimeter has a barometric scale that can be adjusted to any desired reference pressure, and the pointers of the instrument measure the difference between the existing pressure and this reference pressure in terms of feet or meters. When the local altimeter setting is set on the barometric scale, the altimeter shows indicated altitude. When standard sea level pressure of 29.92 inches of mercury or 1013.2 millibars is set on the barometric scale, the altimeter shows pressure altitude. Another type of altimeter is an absolute (radar or radio) altimeter. It measures the actual distance between an aircraft and the terrain below it by measuring the time needed for an electrical signal to pass from the aircraft to the ground and back to the aircraft. See radio altimeter. pneumatic drill motor (air drill). A small, hand-held air turbine fitted with a chuck to hold a twist drill. The housing in which the turbine is mounted has a trigger-actuated air valve that controls the speed of the turbine by controlling the amount of air flowing through it. Pneumatic drill motors are used in aircraft manufacture and maintenance because they are safer than electric drill motors, and they give the operator excellent control of the drill speed. Pneumatic drill motors do not overheat when they are stalled. pneumatics (fluid power). The branch of fluid power systems that uses a compressible fluid to transfer energy. PN junction (semiconductor device). The surface in a semiconductor diode or transistor between the P-material and the N-material. When the PN junction is forward-biased (positive voltage to the P-material), holes in the P-material and free electrons in the N-material drift to the junction. Here they combine by the free electrons filling the holes. A maximum number of electrons flow across a forward-biased PN junction. When the PN junction is reverse-biased (positive voltage to the N-material), the holes and free electrons are drawn away from the junction and very few electrons cross it. PNP transistor (semiconductor device). A bipolar transistor made of P-type and N-type silicon or germanium. The base, made of N-type material, is between a collector and an emitter, both made of P-type material. A very small current flow in the emitter-base circuit controls a much larger flow of current between the emitter and collector. |
