triple-spool engine (gas turbine engine). A type of turbofan engine in which the fan is the first stage of compression, or N1. The normal low-pressure compressor is N2, and the high-pressure compressor is N3. 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.