Hermetically sealed.  A complete seal, especially against the  escape or entry of air. Gross thrust.  The thrust produced by a turbojet or turbofan  engine when the engine is static or not moving. The air is  considered to have no inlet velocity, and the velocity of the  gas leaving the engine is considered to be the acceleration  factor. Ground-boosted engine.  An aircraft reciprocating engine  with a built-in supercharger that boosts the sea-level rated  horsepower of the engine. 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. Gudgeon pin.  The British name for a wrist pin, or piston  pin. See wrist pin.  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. 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. Homebuilt 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. 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. 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. 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. 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 disk which releases the  agent. The entire contents of the container is emptied in much  less than a second.  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$_{3}$(CH$_{2}$)$_{5}$CH$_{3}$, that is  used as the low reference fuel for rating the antidetonation  characteristics of aviation gasoline. Hub (propeller component).  The high-strength component  inside a propeller that attaches the blades to the engine  propeller shaft. Hybrid compressor engine.  A gas turbine engine that has  both centrifugal and axial-flow compressors.  G-14