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selector valves receive their pressurized fluid.
pressure plate (aircraft disk brake component). A strong, heavy plate used in multiple-disk brakes that
receives the force from the brake cylinders and changes it into a squeezing action on the brake disks to
produce the friction that slows the aircraft when the brakes are applied.
pressure pot (paint spray equipment). A container that holds the paint material to be sprayed under a
controlled air pressure. The material is forced by the air pressure to the spray gun, where it is mixed with
atomizing air and sprayed onto the surface being painted.
pressure reducing valve (fluid power system component). A valve in a fluid power system that reduces
the pressure of a liquid or gas from a high value to a fixed lower value.
pressure regulator (fluid power system component). A control valve in a fluid power system that
reduces the pressure of a liquid or gas from a high value to a lower value that can be adjusted by the
operator. After being set, the output pressure remains constant as the input pressure changes, within limits.
pressure relief valve (fluid power system component). A valve that relieves all pressure above a given
amount. In a hydraulic system, the pressure is relieved back to the inlet side of the pump, and in a
pneumatic system, it is relieved to the outside air.
pressure relief valve (oxygen system component). A valve in an oxygen system that relieves the pressure
if the pressure reducing valve should fail.
pressure switch. A switch used to turn an electrical device on or off when a specified fluid pressure is
reached. Some hydraulic systems have a pressure switch that senses the hydraulic system pressure. When
the pressure drops to a low value, called the cut-in pressure, the switch closes and turns the hydraulic pump
on. When the pump brings the pressure up to a higher value, called the cut-out pressure, the pressure switch
turns the pump off.
pressure transducer. A type of pressure measuring device that produces an electrical signal proportional
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 458
to the pressure being measured.
pressure vessel (aircraft structure). The portion of the structure of a pressurized aircraft that is sealed and
pressurized in flight. Normally, only the area housing the occupants and some of the baggage areas are
included in the pressure vessel.
pressurization. A form of climate control of an aircraft. An air compressor increases the pressure of the air
inside the cabin of a high-flying aircraft to a value that allows the occupants to breathe normally. Without
pressurization, supplemental oxygen would be needed when flying above an altitude of about 10,000 feet.
pressurized aircraft. An aircraft in which the cabin area is sealed off and pressurized with air from a cabin
supercharger. Passengers and crew need supplemental oxygen when the air pressure gets much below that
found at an altitude of about 10,000 feet.
To preclude the need of supplemental oxygen when flying at high altitude, the air pressure in the
cabin can be increased to a pressure that compares with an altitude of about six to eight thousand feet.
Humans are comfortable at this pressure.
pressurized ignition system (aircraft reciprocating engine). A type of ignition system used on
reciprocating engines of aircraft that fly at high altitude.
The low air pressure at high altitude allows the high voltage inside an unpressurized distributor to
jump to the wrong electrode. To prevent this, the distributor can be pressurized with compressed air from
the engine supercharger. This higher-density air is a good enough insulator to prevent the high voltage from
jumping to the wrong electrode.
pressurizing and dump valve (turbine engine fuel system component). A valve used in a turbine engine
fuel system equipped with duplex fuel nozzles. When the demand for fuel is low, fuel flows through the
primary manifold, but when the demand is high, the pressurizing valve opens and directs fuel into the
secondary fuel manifold as well.
When the engine is shut down, the dump valve drains the fuel from the manifolds. See duplex fuel
nozzles.
prestretching (control cable manufacturing process). A step in the manufacture of aircraft control cable
assemblies.
After the terminals are installed on the cables, a load of 60% of the cable breaking strength is
applied and held for a specified length of time. Prestretching prepares the cable for installation in the
aircraft.
prevailing easterlies (meteorology). The broad current or pattern of persistent easterly winds in the tropics
and in polar regions.
prevailing torque. The torque required to turn a threaded fastener before it contacts the surface it is
　
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