Frequency. The number of cycles (on/off) completed per unit of time. Usually expressed in Hertz.

Fretting corrosion. Occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion.

Friction. The opposition to movement between objects.

Fuel grade. The rating system used for aviation gasoline. It rates fuel according to its antidetonation characteristics.

Fuse. A protective device containing a special wire that melts when current exceeds the rated value for a definite period.

Fuselage stations (FS). Reference locations, usually given in inches, used to determine forward and aft positions on an aircraft. FS − 0 is the datum.

Heat. The total kinetic energy of the molecules of any substance.

Henry. The basic unit of inductance, symbolized with the letter H. An electric circuit has an inductance of one henry when current changing at the rate of one ampere per second induces a voltage of one volt into the circuit.

Hermaphrodite caliper. Generally used as a marking gauge in layout work. It should not be used for precision measurement.

Hidden lines. Indicates invisible edges or contours.

Horsepower. A measure of power equal to 550 foot-pounds per second or 33,000 foot-pounds per minute and 746 Watts.

Hot start. Occurs when the engine starts, but the exhaust gas temperature exceeds specified limits. This is usually caused by an excessively rich air-fuel mixture entering the combustion chamber.

Humidity. The amount of water vapor in the air.

Hung start. Occurs when the engine starts normally, but the rpm. remains at some low value rather than increasing to the normal starting rpm. This is often the result of insufficient power to the starter, or the starter cutting off before the engine starts self-accelerating.

Hydrometer. An instrument for determining the specific gravity of liquids.

Hypotenuse. The side of a right triangle that is opposite the right angle. The hypotenuse is the longest side of a right triangle.

Improper fraction. A fraction with the numerator equal to or greater than the denominator.

Inside calipers. Calipers with outward curved legs for measuring inside diameters, such as diameters of holes.