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PAR (precision approach radar). A type of radar used at an airport to guide an aircraft through the final
stages of landing. The radarscope has two plots: one showing the position of the aircraft along an extended
center line of the runway, and the other showing the position of the aircraft as it descends along the glide
slope to the runway.
The radar operator directs the pilot to change heading or adjust the descent rate to keep the aircraft
on a path that allows it to touch down at the correct spot on the runway.
parabola. A curve formed by points that are an equal distance from a straight line and from a fixed point.
parabolic microphone. An extremely sensitive microphone mounted at the focus of a parabolic sound
reflector. The reflector is aimed at the source of sound to be picked up, and the parabolic surface reflects
the sound waves, concentrating them at the microphone.
parabolic reflector (light reflector). A light reflector whose surface is made in the form of a parabola. The
filament producing the light is located at the focus of the parabola, and the light is reflected from the
parabolic surface as a beam of parallel rays.
parachute. A large umbrella-shaped device, made of lightweight cloth, used to slow an object falling
through the air. Parachutes are folded into a pack and worn by a person who jumps from an aircraft. When
the pack is opened, the parachute canopy is pulled out by the air and opens. The parachute creates so much
drag that the wearer descends at a rate slow enough to prevent injury when contact is made with the
ground.
Cargo parachutes are used to lower equipment at locations where it is impractical for airplanes or
helicopters to land.
14 CFR Part 1: “A device used or intended to be used to retard the fall of a body or object
through the air.”
par-al-ketone. A heavy wax-like grease, used to protect cables, bolts, and other types of fittings from rust
and corrosion when they are exposed to water. Par-al-ketone is also used to protect metal objects that are to
be stored for long periods of time.
parallax. An optical (viewing) error caused by an object appearing to be in different locations when it is
viewed from different angles. Parallax causes a pointer-type instrument to give an inaccurate indication
unless it is viewed from directly above the pointer. Moving your head as you read the dial can give
different indications.
Precision instruments have a mirror built into the dial to prevent parallax error. When reading such
an instrument, move your eye until the pointer is directly over its reflection. When viewed from this
position there is no parallax.
parallel access (digital computers). A method of accessing data from a computer in which all bits of the
data are transferred at the same time. Parallel access is different from serial access, in which the data is
transferred one bit at a time.
parallel bus system (electrical power system). An electrical power system in which all of the operating
generators are connected to a single power bus.
parallel electrical circuit. A circuit in which the components are connected in such a way that there is
more than one complete path for electrons to pass from the negative terminal of the power source back to
its positive terminal.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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parallel ILS approaches (air traffic control). Approaches to parallel runways by IFR aircraft which,
when established inbound toward the airport on the adjacent final approach courses, are radar-separated by
at least two miles.
paralleling circuit. A circuit in a multiengine aircraft electrical system that causes the generators or
alternators to share the electrical load equally.
paralleling generators (aircraft electrical system operation). The procedure in which the output voltages
of the generators in a multiengine aircraft electrical system are adjusted so that they share the electrical load
equally. If one generator carries more than its share of the load, its output voltage is decreased and the
output of the other generators is increased until all share the load equally.
paralleling relay. A relay in a multiengine aircraft electrical system that controls a flow of control current
which is used to keep the generators or alternators sharing the electrical load equally. The relay opens
automatically to shut off the flow of paralleling current any time the output of either alternator or generator
drops to zero.
parallel lines. Lines that run in the same direction and are at all points separated by the same distance.
Parallel lines never cross.
parallel offset route (air traffic control). A parallel track to the left or right of the designated or
　
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