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forward movement of the blade. The blade should be lifted from the work on its return stroke.
hafnium. A brilliant, silvery, metallic chemical element. Hafnium’s symbol is Hf, its atomic number is 72,
and its atomic weight is 178.49. Hafnium has an extremely high melting point (above 2,000°C) and is used
in the manufacture of filaments for incandescent lamps.
hail (meteorology). A form of precipitation which comes from cumulo nimbus clouds. Hail forms when
drops of water are carried by up-currents of air inside the cloud to a level where the temperature is low
enough that the water freezes into pellets of ice.
Near the top of the cloud, the ice pellets are thrown from the up-current and fall into the warmer
air below. They pick up more water and are again carried upward. Each trip up and down allows the ice
pellets, called a hailstones, to gain another layer of ice. This process continues until the hail is too heavy to
stay in the cloud. Some hailstones grow large enough to cause severe damage to people, property, or
animals on the ground.
hair hygrometer. An instrument used to measure the humidity of the air. The sensitive element in a hair
hygrometer is a bundle of human hair held under a slight tension by a spring. The length of the hair
expands or contracts as the humidity in the air changes. This change in length moves a pointer over a
calibrated scale.
hairline crack. A tiny crack visible on the surface of a piece of material.
hairspring. A flat, spirally wound spring used to provide a restraint for rotary motion of a shaft.
Hairsprings are used in D’Arsonval-type electrical instruments to provide a calibrated restraint for the
moving coil and to carry current into and out of the coil. Hairsprings are also used in spring-powered clocks
and watches to regulate the movement of the balance wheel.
HAL (height above landing) (air traffic control). The height above a designated helicopter landing area
used for helicopter instrument approach procedures.
halation (cathode-ray tube). A form of distortion that causes a blurred image of the trace on a cathode-ray
display tube. The blurring is caused by reflection from the back of the fluorescent coating when the coating
is too thick.
half-duplex operation (communications). A type of communications in which signals can be sent in
either direction, one direction at a time, but cannot be sent in both directions at the same time.
half hitch. A type of knot used for lacing wire bundles together. The bundle is tied with two half hitches
secured with a square knot.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 288
half-life. A measure of the rate of decay of radioactive material. In one half-life, the material decays, or
loses one half of its radioactivity. In the next half-life interval, the material loses one half of that which is
left. The half-life of various types of radioactive materials varies from a few microseconds (millionths of a
second) up to billions of years.
half-round file. A metalworking or woodworking file that has one flat side. The other side is
convex—thicker in the center than at the edges.
half-sectional view (technical illustration). A view of a component in which one half of it is cut away to
show the inside. The other half is not cut away, and it shows the outside.
half view (aircraft drawing). A mechanical drawing used to show a symmetrical object. The center line of
the object is shown, and break lines indicate that only one half of the object is shown.
half-wave antenna (radio antenna). A radio antenna whose electrical length is one half of the wavelength
of the frequency for which the antenna is tuned.
half-wave rectifier (electrical circuit). A form of electrical circuit that changes alternating current into
pulsating direct current. One rectifier, either a semiconductor diode or a diode electron tube, is used to
block the flow of one half of the AC wave while allowing the other half to pass through. Half-wave
rectifiers are inefficient and do not produce smooth direct current. However, they are used where economy
of components is important and pulsations in the output are of no concern.
Hall-effect generator (electronic device). An electronic device which uses a thin wafer of semiconductor
material to measure the intensity of a magnetic field. Control current flowing through the generator
produces a Hall-effect voltage whose direction is perpendicular to the current, and whose intensity is
proportional to both the control current and the strength of a magnetic field. The direction of the magnetic
field passing through the generator is perpendicular to both the current and the voltage.
halo (atmospheric phenomenon). A prismatically colored or whitish circle or arc of a circle, with the sun
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航空术语词典Dictionary of Aeronautical Terms 中(63)
