Gimbal.  A support that allows a gyroscope to remain in an upright condition when its base is tilted. 

Glass cockpit.  An aircraft instrument system that uses a few cathode-ray-tube displays to replace a large number of mechanically actuated instruments. 

Glaze ice.  Ice that forms when large drops of water strike a surface whose temperature is below freezing. Glaze ice is clear and heavy. 

Glide slope.  The portion of an ILS (Instrument Landing System) that provides the vertical path along which an aircraft descends on an instrument landing. 

Goniometer.  Electronic circuitry in an ADF system that uses the output of a fixed loop antenna to sense the angle between a fixed reference, usually the nose of the aircraft, and the direction from which the radio signal is being received. 

Gram. The basic unit of weight or mass in the metric system. One gram equals approximately 0.035 ounce. 

Graphite. A form of carbon. Structural graphite is used in composite structure because of its strength and stiffness. 

Greige (pronounced “gray”).  The unshrunk condition of a polyester fabric as it is removed from the loom. 

Ground effect.  The increased aerodynamic lift produced when an airplane or helicopter is flown nearer than half wing span or rotor span to the ground. This additional lift is caused by an effective increase in angle of attack without the accompanying increase in induced drag, which is caused by the deflection of the downwashed air. 

Ground.  The voltage reference point in an aircraft electrical system. Ground has zero electrical potential. Voltage values, both positive and negative, are measured from ground. In the United Kingdom, ground is spoken of as “earth.” 

Ground-power unit (GPU).  A service component used to supply electrical power to an aircraft when it is being operated on the ground. 

Guncotton. A highly explosive material made by treating cotton fibers with nitric and sulfuric acids. Guncotton is used in making the film base of nitrate dope. 

Gusset.  A small plate attached to two or more members of a truss structure. A gusset strengthens the truss. 

Gyro (gyroscope). The sensing device in an autopilot system. A gyroscope is a rapidly spinning wheel with its weight concentrated around its rim. Gyroscopes have two basic characteristics that make them useful in aircraft instruments: rigidity in space and precession. See rigidity in space and precession. 

Gyroscopic precession.  The characteristic of a gyroscope that causes it to react to an applied force as though the force were applied at a point 90° in the direction of rotation from the actual point of application. The rotor of a helicopter acts in much the same way as a gyroscope and is affected by gyroscopic precession. 

Halon 1211.  A halogenated hydrocarbon fire-extinguishing agent used in many HRD fire-extinguishing systems for powerplant protection. The technical name for Halon 1211 is bromochlorodifluoromethane. 

Halon 1301.  A halogenated hydrocarbon fire-extinguishing agent that is one of the best for extinguishing cabin and powerplant fires. It is highly effective and is the least toxic of the extinguishing agents available. The technical name for Halon 1301 is bromotrifluoromethane. 

Hangar rash.  Scrapes, bends, and dents in an aircraft structure caused by careless handling. 

Hardwood.  Wood from a broadleaf tree that sheds its leaves each year. 

Heading indicator.  A gyroscopic flight instrument that gives the pilot an indication of the heading of the aircraft. 

Heat exchanger.  A device used to exchange heat from one medium to another. Radiators, condensers, and evaporators are all examples of heat exchangers. Heat always moves from the object or medium having the greatest level of heat energy to a medium or object having a lower level. 

Helix.  A screw-like, or spiral, curve. 

Hertz.  One cycle per second. 

Holding relay.  An electrical relay that is closed by sending a pulse of current through the coil. It remains closed until the current flowing through its contacts is interrupted. 

Homebuilt aircraft. Aircraft that are built by individuals as a hobby rather than by factories as commercial products. Homebuilt, or amateur-built, aircraft are not required to meet the stringent requirements imposed on the manufacture of FAA-certified aircraft. 

