system pressure regulator (hydraulic power system component). A type of hydraulic system-pressure
control valve. When the system pressure is low, as it is when some unit is being actuated, the output of the constant-delivery pump is directed into the system. When the actuation is completed and the pressure builds up to a specified kick-out pressure, the pressure regulator shifts. A check valve seals the system off and the pressure is maintained by the accumulator. The pump is unloaded and its output is directed back into the reservoir with very little opposition. The pump output pressure drops, but the volume of flow remains the same. When the system pressure drops to the specified kick-in pressure, the regulator again shifts and directs fluid into the system.
Spool-type and balanced-pressure-type system pressure regulators are completely automatic in their operation and require no attention on the part of the flight crew. A system pressure regulator is also called an unloading valve.
T
tab (auxiliary flight control). A small, movable control hinged to the trailing edge of one of an airplane’s primary flight control surfaces. Tabs can be used to help the pilot move the primary control surface, or they can be used to produce an aerodynamic load on the primary surface that trims the aircraft for a hands-off flight condition. See antiservo tab, balance tab, servo tab, spring tab, trim tab.
TACAN (electronic navigation system). An electronic navigation system, operating in the ultrahigh frequency (UHF) range. The name TACAN is an acronym for TACtical Air Navigation. TACAN is used by military aircraft to give the pilot both the distance and direction from the transmitting stations on the ground.
Civilian aircraft have equipment that use the TACAN ground facilities to show the distance to the navigational fix. The equipment used in civilian aircraft is called DME (Distance Measuring Equipment).
tachometer. An instrument used to measure the rotational speed of an object. Tachometers are usually calibrated in revolutions per minute, RPM, or percentage of the maximum rated RPM.

tachometer cable. The flexible drive used to connect the rotating magnet inside a tachometer indicator to the tachometer drive gear inside an engine. A tachometer cable is made of a core of steel wire, around which are wound two concentric coils of steel wire, wound in opposite directions.
tachometer generator. A small alternating current generator, mounted on and driven from the tachometer drive pad on an engine. The frequency of the AC produced by the tachometer generator is proportional to the speed of the engine.
tack coat (aircraft finishing system operation). A coat of finishing material sprayed on a surface and allowed to dry until the solvents evaporate. As soon as the solvents evaporate from the tack coat, a wet, full-bodied coat of the material is sprayed over it.
tackle. An arrangement of ropes and pulleys used to gain a mechanical advantage. Tackle is most frequently spoken of as block and tackle.