Half-wave rectifier . An electrical rectifier circuit that converts AC into pulsating DC. Only one alternation of each cycle is present in the output.

Halogenated hydrocarbon . A hydrocarbon compound in which one or more hydrogen atoms have been replaced with atoms of one of the halogen elements such as fluorine, chlorine, or bromine.

Head of pressure . Pressure exerted by a column of fluid and created by the height of the column.

Heat engine . A mechanical device that converts the chemical energy in a fuel into heat energy. The heat energy is then converted into mechanical energy and useful work.

Heli-Coil insert . The registered trade name of a special helical insert used to restore threads stripped from a bolt hole, or to reinforce the threads in an aluminum casting. The damaged threads are drilled out and new threads are cut with a special oversize tap. A coil of stainless steel wire, with a cross section in the shape of a diamond, is screwed into the hole and serves as the new threads. Heli-Coil inserts are also used to provide durable threads in soft metal castings. Some spark plug holes in aluminum alloy cylinder heads are fitted with Heli-Coil inserts to minimize the wear caused by repeated removal and installation of the spark plugs.

Helical spline . A spline that twists, or winds, around the periphery of a shaft. Helical splines are used to change linear motion into rotary motion of the shaft on which the splines are cut.

Helical spring . A spring wound in the form of a helix, or coil.

Helix . A spiral.

Heptanes . An organic compound, CH (CH ) CH, that is 3 2 5 3 used as the low reference fuel for rating the antidetonation characteristics of aviation gasoline.

Hermetically sealed . A complete seal, especially against the escape or entry of air.

Hertz . A unit of frequency equal to one cycle per second.

High-bypass ratio engine . A turbofan engine whose bypass ratio is 4:1 or greater.

High-pressure compressor . The second-stage compressor in a dual-spool gas turbine engine. The high pressure compressor is called the N2 compressor and is the one that is rotated by the starter for starting, and the one whose rpm is controlled by the fuel control.

High unmetered fuel pressure . Pressure in a Teledyne- Continental fuel injector pump that is adjusted by the variable orifice.

Home-built aircraft . See amateur-built aircraft.

Honing (cylinder wall treatment ). Scratching the surface of the cylinder wall with an abrasive to produce a series of grooves of microscopic depth and uniform pattern. The honed pattern holds oil to lubricate the cylinder walls.

Horsepower . The most commonly used unit of mechanical power. One horsepower is equal to 33,000 foot-pounds of work done in one second.

Hot section . The portion of a gas turbine engine that operates at a high temperature. The hot section includes the combustion, turbine, and exhaust sections.

Hot-tank lubricating system . A turbine engine lubricating system in which the oil cooler is located in the pressure subsystem. The oil is returned to the tank without being cooled.

HRD fire extinguisher . A fire extinguisher that carries the extinguishing agent in a sealed sphere or cylinder. When the agent-discharged switch is closed, an ignited powder charge drives a cutter through a frangible disc which releases the agent. The entire contents of the container is emptied in much less than a second.