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Each stage of compressor is driven by its own turbine, and the N3 compressor is the only one whose speed
is governed by the fuel control. The N1 and N2 compressors rotate at their own best speed.
triple thread. A multiple screw thread in which three threads are cut parallel to each other. The starting
point of each thread is 120° from the starting point of the others. The lead, or the distance the screw moves
forward in one revolution, is three times the pitch (the distance between the threads adjacent to each other).
tritium. An isotope of hydrogen with an atomic weight of three. The nucleus of an isotope of tritium has
two neutrons as well as the proton. A normal atom of hydrogen has only one proton and no neutrons in its
nucleus.
trivalent element. A chemical element with three electrons in its valence, or outer, shell. Boron, aluminum,
gallium, and indium are trivalent elements.
tropical air (meteorology). An air mass with characteristics developed over the low latitudes. Maritime
tropical (mT) air, the principal type, is produced over the tropical and subtropical seas and is very warm
and humid. Continental tropical (cT) air is produced over subtropical arid regions and is hot and very dry.
tropical cyclone (meteorology). A general term for a cyclone that originates over tropical oceans. By
international agreement, tropical cyclones have been classified according to their intensity:
• Tropical depression — winds up to 34 knots
• Tropical storm — winds of 35 to 64 knots
• Hurricane or typhoon — winds of 65 knots or higher.
tropopause. The division between the troposphere, the lower level of the earth’s atmosphere, and the
stratosphere, the layer of the atmosphere above the troposphere.
troposphere. The lower level of the earth’s atmosphere. The troposphere extends upward from the surface
of the earth to an altitude of about 36,000 feet (11,000 meters). The temperature drops steadily in the
troposphere, and it is in the troposphere that all of the clouds and weather conditions exist.
troubleshooting. A procedure used in aircraft maintenance in which the operation of a malfunctioning
system is analyzed to find the reason for the malfunction and to find a method for returning the system to
its condition of normal operation.
trough (meteorology). An elongated area of relatively low atmospheric pressure. Troughs are usually
associated with and most clearly identified as an area of maximum cyclonic curvature of the wind flow.
A trough is the opposite of a ridge.
TRSA (terminal radar service area) (air traffic control). Airspace surrounding designated airports in
which ATC provides radar vectoring, sequencing, and separation on a full-time basis for all IFR and
participating VFR aircraft. Service provided in a TRSA is called Stage III service.
The Aeronautical Information Manual contains an explanation of TRSA, and TRSAs are depicted
on VFR aeronautical charts. Pilot participation is urged, but not mandatory.
true airspeed. The airspeed shown on the airspeed indicator after it has been corrected for nonstandard
temperature and pressure. 14 CFR Part 1: “The airspeed of an aircraft relative to undisturbed air. True
airspeed is equal to equivalent airspeed multiplied by (p0/p)1/2.”
true airspeed indicator (aircraft instrument). A special airspeed indicator that modifies the differential
pressure measured by its diaphragm to take into consideration free-air temperature and barometric pressure.
The indication of the instrument is in units of true airspeed, rather than indicated airspeed.
true altitude. The exact height of an aircraft above mean sea level.
true course (navigation). The navigational course for an aircraft measured from the geographic north pole.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 598
true north. True direction on the earth’s surface measured from the geographic north pole.
true power (electrical power). The power, measured in watts, actually available in an alternating current
circuit. True power is found by multiplying the circuit voltage by the current in phase with this voltage. It
may also be found by multiplying the circuit voltage by the total current and then multiplying this by the
power factor of the circuit.
true wind direction. The direction, with respect to true north, from which the wind is blowing.
truncated cone. A cone with its point cut off in such a way that its top is parallel with its base.
TR unit. A transformer-rectifier unit. A TR unit reduces the voltage of alternating current and changes it
into direct current.
trunnion. Projections from the cylinder of a retractable landing gear strut about which the strut pivots to
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