Stress.  A force within an object that tries to prevent an outside  force from changing its shape. Stroboscopic tachometer.  A tachometer used to measure  the speed of any rotating device without physical contact.  A highly accurate variable-frequency oscillator triggers a  high-intensity strobe light. When the lamp is flashing at the  same frequency the device is rotating, the device appears to  stand still. Stroke.  The distance the piston moves inside the cylinder. Sump (aircraft engine component).  A low point in an  aircraft engine in which lubricating oil collects and is stored  or transferred to an external oil tank. A removable sump  attached to the bottom of the crankcase of a reciprocating  engine is often called an oil pan. Sump (fuel tank component).  A low point in an aircraft  fuel tank in which water and other contaminants collect and  are held until they can be drained out. Supercharged engine.  A reciprocating engine that uses a  mechanically driven compressor to increase the air pressure  before it enters the engine cylinders. Supercharger.  An air compressor used to increase the  pressure of the air being taken into the cylinders of a  reciprocating engine. Surface roughness.  The condition of the surface of a  reciprocating engine cylinder wall that has been honed to  make it hold lubricating oil. Surface roughness is measured  in micro-inches rms. Surge.  A condition of unstable airflow, through the  compressor of a gas turbine engine, in which the compressor  blades have an excessive angle of attack. Surge usually affects  an entire stage of compression. Synthetic oil.  Oil made by chemical synthesis of a mineral,  animal, or vegetable base. Synthetic oils have appropriate  additives that give them such characteristics as low volatility,  low pour point, high viscosity index, good lubricating  qualities, low coke and lacquer formation, and low foaming. Tachometer.  An instrument that measures the rotational  speed of an object. TAI.  Thermal anti-ice. A system used to prevent the  formation of ice on an aircraft by flowing heated air inside  the structure. Thermocouple.  A device used to generate an electrical current.  A thermocouple is made of two dissimilar metal wires whose  ends are welded together to form a loop. A voltage exists in  the loop proportional to the difference in temperature of the  junctions at which the wires are joined. The amount of current  flowing in the loop is determined by the types of metals used  for the wires, the temperature difference between the junctions,  and the resistance of the wires. Thermistor.  A semiconductor material whose electrical  resistance varies with its temperature. Thermal shock.  The sudden change in engine operating  temperature that occurs when engine power is suddenly  reduced at the same time the airspeed, thus the cooling, is  increased. Thermal shock occurs when an aircraft is required  to rapidly descend to a lower altitude. Thermal expansion coefficient.  A number that relates to  the change in the physical dimensions of a material as the  temperature of the material changes. The thermal expansion  coefficient of aluminum is approximately twice that of steel. Thermal efficiency.  The ratio of the amount of useful work  produced by a heat engine, to the amount of work that could  be done by all of the heat energy available in the fuel burned. Test club.  A wide-blade, short-diameter propeller used on a  reciprocating engine when it is run in a test cell. A test club  applies a specific load to the engine and forces the maximum  amount of air through the engine cooling fins. TEL.  Tetraethyl lead. TDC.  Top dead center. The position of a piston in a  reciprocating engine when the piston is at the top of its stroke  and the wrist pin, crankpin, and center of the crankshaft are  all in line. TBO.  Time between overhauls. A time period specified by  the manufacturer of an aircraft engine as the maximum length  of time an engine should be operated between overhauls  without normal wear causing parts of the engine to be worn  beyond safe limits. TBO depends upon proper operation and  maintenance in accordance with the engine manufacturer’s  recommendations. The overhaul of an engine when it  reaches its TBO hours is not mandatory, except for certain  commercial operators that have the requirement written into  their operations manual. Tail pipe.  The portion of the exhaust system of a gas turbine  engine through which the gases leave. The tail pipe is often  called the exhaust duct, or exhaust pipe. G-27