Therapeutic mask adapter.  A calibrated orifice in the mask adapter for a continuous-flow oxygen system that increases the flow of oxygen to a mask being used by a passenger who is known to have a heart or respiratory problem. 

Thermal dimpling. See hot dimpling. 

Thermal relief valve.  A relief valve in a hydraulic system that relieves pressure that builds up in an isolated part of the system because of heat. Thermal relief valves are set at a higher pressure than the system pressure relief valve. 

Thermistor.  A special form of electrical resistor whose resistance varies with its temperature. 

Thermistor material. A material with a negative temperature coefficient that causes its resistance to decrease as its temperature increases. 

Thermocouple. A loop consisting of two kinds of wire, joined at the hot, or measuring, junction and at the cold junction in the instrument. The voltage difference between the two junctions is proportional to the temperature difference between the junctions. In order for the current to be meaningful, the resistance of the thermocouple is critical, and the leads are designed for a specific installation. Their length should not be altered. Thermocouples used to measure cylinder head temperature are usually made of iron and constantan, and thermocouples that measure exhaust gas temperature for turbine engines are made of chromel and alumel. 

Thermocouple fire-detection system.  A fire-detection system that works on the principle of the rate-of-temperature rise. Thermocouples are installed around the area to be protected, and one thermocouple is surrounded by thermal insulation that prevents its temperature changing rapidly. In the event of a fire, the temperature of all the thermocouples except the protected one will rise immediately and a fire warning will be initiated. In the case of a general overheat condition, the temperature of all the thermocouples will rise uniformly and there will be no fire warning. 

Thermoplastic resin.  A type of plastic material that becomes soft when heated and hardens when cooled. 

Thermosetting resin.  A type of plastic material that, when once hardened by heat, cannot be softened by being heated again. 

Thermostatic expansion valve (TEV). The component in a vapor-cycle cooling system that meters the refrigerant into the evaporator. The amount of refrigerant metered by the TEV is determined by the temperature and pressure of the refrigerant as it leaves the evaporator coils. The TEV changes the refrigerant from a high-pressure liquid into a low-pressure liquid. 

Thixotropic agents.  Materials, such as microballoons, added to a resin to give it body and increase its workability. 

TIG welding. Tungsten inert welding is a form of electric arc welding in which the electrode is a nonconsumable tungsten wire. TIG welding is now called GTA (gas tungsten arc) welding. 

Toe-in. A condition of landing gear alignment in which the front of the tires are closer together than the rear. When the aircraft rolls forward, the wheels try to move closer together. 

Toe-out.  A condition of landing gear alignment in which the front of the tires are further apart than the rear. When the aircraft rolls forward, the wheels try to move farther apart. 

Torque.  A force that produces or tries to produce rotation. 

Torque links. The hinged link between the piston and cylinder of an oleo-type landing gear shock absorber. The torque links allow the piston to move freely in and out of the landing gear cylinder, but prevent it rotating. The torque links can be adjusted to achieve and maintain the correct wheel alignment. Torque links are also called scissors and nutcrackers. 

Torque tube.  A tube in an aircraft control system that transmits a torsional force from the operating control to the control surface. 

Torsion rod. A device in a spring tab to which the control horn is attached. For normal operation, the torsion rod acts as a fixed attachment point, but when the control surface loads are high, the torsion rod twists and allows the control horn to deflect the spring tab. 

Total air pressure.  The pressure a column of moving air will have if it is stopped. 

TMC.  Thrust management computer. 

Toroidal coil.  An electrical coil wound around a ring-shaped core of highly permeable material. 

Total air temperature. The temperature a column of moving air will have if it is stopped. 

