transverse wave. A type of mechanical wave in which the material being vibrated moves in a direction perpendicular to the direction the wave is moving. Transverse waves are formed when a rock is thrown into a pond of water. The surface of the water moves up and down as the wave moves across the pond.
trapezoid. A plane, closed, four-sided figure. Only two of the sides of a trapezoid are parallel to each other.

trend monitoring (turbine engine maintenance). A system for monitoring the performance of a turbine engine by routine comparison of performance parameters with a base line of the same parameters established when the engine was new or newly overhauled. EGT, RPM, fuel flow, and oil consumption are monitored on every flight, and the difference between the current indication and the base line is plotted. Any deviation from a normal increase or decrease warns the technician of an impending problem.
triac (electronic component). A form of thyristor, similar to a silicon controlled rectifier, except it controls current flow in either direction. A triac acts as two SCRs connected in parallel, with the anode of one connected to the cathode of the other. A triac does not conduct until a pulse of voltage is applied to its gate. But once it begins to conduct, it can be turned off only by reducing the voltage across it to a value low enough to stop the current flowing through it. See thyristor.

triangle. A closed, plane, three-sided figure. The sum of the three angles in a triangle is always 180°.

triangulation (navigation). A method of determining the location of an aircraft by radio bearings. Lines
are drawn from two or more VOR stations along the radials on which the aircraft is located. The location on the chart where these lines cross is the location of the aircraft.