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gas (NH3). The ammonia breaks down into nitrogen and hydrogen, and the nitrogen unites with some of
the alloying agents in the steel to form an extremely hard surface.
Nitriding, which is used to harden crankshaft bearing surfaces and cylinder walls, is done at a
lower temperature than other forms of case hardening, and it does not warp the parts being hardened.
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nitrogen. A colorless, odorless, tasteless, inert, gaseous chemical element. Nitrogen’s symbol is N, its
atomic number is 7, and its atomic weight is 14.0067. Nitrogen makes up about 78% of the volume of the
earth’s atmosphere and is used as an ingredient in fertilizers and explosives.
nobelium. A radioactive, synthetic chemical element in the actinide series. Nobelium’s symbol is No, its
atomic number is 102, and its atomic weight is 254.
noble. Inactive or inert. In the electrochemical series of metals, the metal in a combination that does not
corrode is the more noble.
noble gas. Another name for inert gas. The noble (inert) gases are helium, neon, argon, krypton, xenon, and
radon.
noctilucent clouds (meteorology). Clouds of unknown composition which occur at great heights, probably
around 75 to 90 kilometers. They resemble thin cirrus, but usually have a bluish or silvery color, although
sometimes orange to red.
nodal suspension system (helicopter rotor suspension system). A method of vibration dampening, used
in certain Bell helicopters to reduce the vibration of the main rotor system.
no-gyro approach (air traffic control). A radar approach or vector provided by ATC to an aircraft in case
of a malfunctioning gyro compass or directional gyro. Instead of providing the pilot with headings to be
flown, the controller observes the radar track and issues control instructions such as “turn right/left” or
“stop turn,” as appropriate.
noise (electronic equipment). Any unwanted electrical interference. Noise may come from static
electricity or from signals fed into the circuit by inductive or capacitive coupling from other circuits.
no-load current (electrical transformer current). Current flowing in the primary winding of a
transformer connected to a source of AC voltage when no current is flowing in the secondary winding.
No-load current is also called excitation current, or idling current.
Nomex. A patented nylon material that is used to make the honeycomb core for certain types of aircraft
structural sandwich-type materials. Nomex is also used to make fire-resistant fabrics.
nominal value. A stated value that may be different from an actual value. For example, an electrical
resistor may have a nominal resistance of 1,000 ohms, with a tolerance of plus or minus ten percent. Its
nominal value is 1,000 ohms, but its actual value may be anywhere between 900 and 1,100 ohms.
nonabrasive (material characteristic). The characteristic of a material that keeps it from scratching
another material when it is rubbed against it.
nonapproach control tower (air traffic control). A control tower that authorizes aircraft to land or
takeoff at the airport controlled by the tower or to transit the Class D airspace.
The primary function of a nonapproach control tower is the sequencing of aircraft in the traffic
pattern and on the landing area. Nonapproach control towers also separate aircraft operating under
instrument flight rules clearances from approach controls and centers. They also provide ground control
services to aircraft, vehicles, personnel, and equipment on the airport movement area.
nonatomizing spray gun (paint spray gun). A type of spray gun that propels the material in a solid stream,
rather than in the form of tiny droplets.
noncomposite separation (air traffic control). Air traffic separation in accordance with minimums other
than the composite separation minimum specified for the area concerned.
nondestructive inspection. Any type of inspection that does not damage the system or component being
inspected. X-ray is a system of nondestructive inspection that allows the inside of a device to be inspected
without its having to be opened or in any way damaged.
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nondimensional number. A pure number which has no dimensional value assigned to it. Ratios, such as
Mach numbers and Reynolds numbers, are examples of nondimensional numbers.
nondirectional antenna (radio antenna). A radio antenna that receives or transmits signals from all
directions with equal strength.
nondirectional beacon (air traffic control). An L/MF or UHF radio beacon transmitting nondirectional
signals with which the pilot of an aircraft equipped with direction finding equipment can determine his
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航空术语词典Dictionary of Aeronautical Terms 中(146)
