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arch (structure). A form of structure with curving sides and a wedge-shaped keystone at the top. An arch
supports weight by transmitting the force caused by its weight through its curving sides into the structure
supporting it. The forces supported by the arch are transmitted into the sides through the keystone.
Archimedes’ principle. The principle of buoyancy, propounded by the Greek mathematician Archimedes.
Archimedes’ principle states that an object immersed in a fluid loses as much of its own weight as
the weight of the fluid it displaces. The sciences of hydrostatics and aerostatics are based on Archimedes’
principle.
arcing. Sparking between a commutator and brush or between switch contacts when the flow of current is
interrupted.
Arcing occurs when induction tries to keep the current flowing across the broken contact.
arc lamp. A source of light produced by an electric arc which is an intense and continuous spark. The arc
is produced when electrons flow through ionized gases between two electrodes.
arc sine. The trigonometric function that means “the angle whose sine is.” For example, arc sin 0.707 = 45°,
means that the angle whose sine is 0.707 is 45°. Arc sine is usually written sin-1. See sine.
arc tangent. A value used in trigonometry that is the inverse, or opposite, of the tangent. This is the same
as one divided by the tangent. The tangent of an angle is the ratio of the length of the side opposite the
angle, to the length of the side adjacent, or next to, the angle.
If the sides of a triangle that form a right angle are two inches and three inches, the tangent of one
of the angles is 2/3, or 0.667. The arc tangent is the angle whose tangent is 0.667, and this is 33.7°. This is
normally written as Tan-1 0.667. See tangent.
arc welding. A form of electric welding in which the intense heat needed to melt the metal is produced by
an electric arc.
arctic air. The air in a mass formed over an arctic area of ice and snow. The air has the characteristics of
this type of area.
arctic front (meteorology). The surface discontinuity between very cold (arctic) air flowing directly from
the arctic region and another less cold and, consequently, less dense air mass.
area. A measurement of the amount of surface of an object. Area is usually expressed in such units as
square inches or square centimeters for small surfaces or in square feet or square meters for larger surfaces.
area control center (ICAO). An ICAO term for an air traffic control facility primarily responsible for
ATC services being provided IFR aircraft during the en route phase of flight. The U.S. equivalent is an air
route traffic control center (ARTCC).
area navigation (electronic navigation). A form of aircraft electronic navigation, commonly called
RNAV, in which VORTAC stations can be electronically moved to any convenient location. A station that
has been “moved” is called a way point, and the flight path goes from one way point to the next.
14 CFR Part 1: “A method of navigation that permits aircraft operations on any desired course
within the coverage of station-referenced navigation signals or within the limits of self-contained system
capability.”
area navigation high route. 14 CFR Part 1: “An area navigation route within the airspace extending
upward from, and including, 18,000 feet MSL to flight level 450.”
area navigation low route. 14 CFR Part 1: “An area navigation route within the airspace extending
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 44
upward from 1,200 feet above the surface of the earth to, but not including, 18,000 feet MSL.”
area-rule fuselage. The fuselage of a supersonic airplane built so the combined frontal area of the fuselage
and the wing form a smooth curve from nose to tail. An airplane with an area-rule fuselage has the
minimum transonic drag.
argon. A colorless, odorless, inert, gaseous chemical element. Argon has an atomic number of 18 and an
atomic weight of 39.998. The symbol for argon is Ar.
Argon is commonly used to fill electric light bulbs and the envelopes of electron tubes. The argon
takes the place of air, which contains oxygen, and by keeping oxygen away from the hot filaments in the
bulbs and tubes, the filaments will not burn out, or oxidize.
ARINC (Aeronautical Radio Incorporated). A corporation whose principal stockholders are the airlines.
Its function is to operate certain communication links between airliners in flight and the airline ground
facilities. ARINC also sets standards for communication equipment used by the airlines.
arithmetic. The basic foundation of all the systems of mathematics.
Operations in arithmetic are: addition, subtraction, multiplication, and division of both whole
　
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