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movement stops. In the measure of temperature using the Celsius scale (the centigrade scale), zero is the
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temperature of melting ice.
In the measure of temperature using the Fahrenheit scale, zero is the temperature of a mixture of
ice and salt water.
zero adjustment (electrical measuring instrument). The adjustment of the pointer of an electrical
measuring instrument so that the needle rests over the zero mark on the dial when no current is flowing
through the moving coil of the meter.
zero beat (frequency adjustment). A method of determining the exact frequency of an alternating current.
Vibrations caused by a variable-frequency alternating current are mixed with vibrations caused by the
alternating current whose frequency is to be found. When the two vibrations mix, a third vibration is
formed whose frequency is the difference between the frequencies of the first two vibrations.
The variable frequency is changed until the third vibration disappears. At this point, the two
frequencies are exactly the same. The beat has been reduced to zero.
zero-center ammeter. An ammeter in a light aircraft electrical system located between the battery and the
main bus. This ammeter shows the current flowing into or out of the battery.
zero fuel weight (aircraft weight). The weight of an aircraft with all of the useful load except the fuel on
board.
zero gravity. A condition in which the effect of gravity is cancelled or nullified by the force of inertia.
Zero gravity is felt in a high-performance aircraft when it is flying in a parabolic, or curved, flight path, and
in a satellite when it is orbiting the earth.
zero-lash valve lifter (reciprocating engine component). A type of valve lifter in a reciprocating engine
that uses a hydraulic device to keep all clearance out of the valve train. Each time an intake or exhaust
valve is opened, a specific amount of oil leaks from the hydraulic valve lifter, and while the valve is closed,
engine oil pressure refills the lifter so it keeps all of the operating clearance out of the valve mechanism.
zero-lift line (aerodynamics). An imaginary line through an airfoil along which air can flow over both
sides of the surface of the airfoil and not produce any lift. The zero-lift line is the chord line of a
symmetrical airfoil, but it is at a negative angle of attack for an asymmetrical airfoil.
zero stage (turbine engine). A stage of compressor blades installed ahead of the front of an axial
compressor.
zinc. A bluish-white, lustrous, metallic chemical element. Zinc’s symbol is Z, its atomic number is 30, and
its atomic weight is 65.37. Zinc is used as an alloying agent for other metals and as a corrosion-resistant
coating for steel.
zinc chromate primer. An alkyd resin primer used to prepare a metal surface to take a topcoat of paint.
Moisture in contact with a coating of zinc chromate primer causes the primer to release chromate ions that
inhibit the formation of corrosion on the metal protected by the primer.
zirconium. A lustrous, grayish-white, metallic chemical element. Zirconium’s symbol is Zr, its atomic
number is 40, and its atomic weight is 91.22. Zirconium is used as an ingredient in ceramics and refractory
materials.
Z-marker (aerial navigation). A type of directional radio beacon used to mark a specific location along an
airway or along an instrument approach course. A Z-marker transmits a vertical signal on a 75-megahertz
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carrier modulated with a series of 3,000 hertz dots. The series of dots can be heard over the radio speaker,
and the Z-marker also causes the white light on the marker beacon panel to light up in a series of short
flashes, the dots.
zonal wind (meteorology). A west wind or the westerly component of a wind. Zonal wind is
conventionally used to describe large-scale flow that is neither cyclonic nor anticyclonic.
zone numbers (aircraft drawing). Numbers placed along the edges of large aircraft drawings to identify
the location of a part on the drawing. Numerals are used along the length of the drawing, and letters are
used along the width.
Zulu time. A name for Greenwich mean time, or universal time. See universal time.
Zyglo inspection (nondestructive inspection). The registered trade name for a type of nondestructive
inspection used to find flaws that extend to the surface of a metallic or nonmetallic material. The part to be
inspected is cleaned and soaked in a bath of hot penetrating liquid that contains a fluorescent dye.
After the part has soaked for the correct length of time, it is removed from the bath, and all of the
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