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Hasteloy. The registered trade name of a family of nickel-base alloys used to make turbine blades capable
of operating at high temperatures.
HAT (height above touchdown) (air traffic control). The decision height or minimum descent altitude
above the highest runway elevation in the touchdown zone (first 3,000 feet of the runway). HAT is
published on instrument approach charts in conjunction with all straight-in minimums. See decision height,
MDA (minimum descent altitude).
hat channel (metal construction). A type of formed or extruded structural material having the
cross-sectional shape of a hat. Hat channels are attached to flat sheets of metal to stiffen them and give
them rigidity.
“Have numbers” (air traffic control). A phrase used by pilots to inform ATC that they have received
runway, wind, and altimeter information only.
hazardous in-flight weather advisory service. See HIWAS.
hazardous weather information (air traffic control). A meteorological summary that contains
information which is considered significant and is not included in a current hazardous weather advisory
broadcast. This includes:
• Significant meteorological information (SIGMET/WS)
• Convective significant meteorological information (convective SIGMET/WST)
• Urgent pilot weather reports (urgent PIREP/UUA)
• Center weather advisories (CWA)
• Airmen’s meteorological information (AIRMET/WA)
• Isolated thunderstorms that are rapidly developing and increasing in intensity
• Low ceilings and visibilities that are becoming widespread
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 292
haze (meteorology). An obstruction to visibility caused by fine dust or salt particles dispensed through a
portion of the atmosphere. The particles are so small they cannot be felt or individually seen with the naked
eye. Haze is distinguished from fog by its bluish or yellowish tinge.
head angle (countersunk fastener dimension). The included angle formed between the sides of the head
of a countersunk rivet or screw.
head eccentricity (rivet dimension). The amount of difference between the center of the shank of a rivet
and the center of its head.
heading indicator. A gyroscopic flight instrument that gives the pilot an indication of the heading of the
aircraft.
headless fastener. A type of friction-held fastener used to join mechanical parts. Neither end of the
fastener is enlarged to form a head. A roll pin is an example of a headless fastener. See roll pin.
head of pressure. The pressure exerted by a column of fluid that is caused by the height of the column.
headphone (audio system). A small receiver, or earphone, mounted on a band worn over the head. Most
headphones have two receivers mounted in soft cups so one can be comfortably worn over each ear.
head pressure of a fluid. The pressure exerted by a column of fluid caused by the height of the column.
head-up display. See HUD.
head wind (aircraft flight). A wind which blows from the direction the aircraft is flying. The ground
speed of an aircraft (the speed the aircraft is moving over the ground) is less than the speed through the air
by the velocity of the head wind.
hearth furnace. A type of furnace used to melt iron and steel. The metal is melted by passing hot gases
over the charge of metal on the floor, or hearth, of the furnace.
heat. A form of energy that affects the speed of movement of the molecules of which a material is made.
The more heat energy in a material, the faster its molecules move, and the amount of heat energy in a
material determines its physical condition. Water that contains very little heat energy is a solid, ice. When
heat is added to ice, the ice melts and becomes liquid water. Additional heat added to the liquid water
causes it to boil, and it becomes water vapor (steam).
Our chief source of heat energy is the sun. Heat energy from the sun reaches the earth by radiation,
and it is stored in plants and animals in the form of chemical energy. This chemical energy can be changed
back into heat energy by the use of various types of machines.
heat engine. A form of mechanical device that converts heat energy into mechanical energy. The two most
widely used types of heat engines for aircraft are reciprocating engines and turbine engines.
heater (electron tube). A small strip of resistance wire mounted inside a tubular cathode in an electron
tube. Low-voltage AC flowing through the heater causes it to get red-hot. This heats the cathode enough for
it to emit, or give off, electrons.
heat exchanger. A device used to exchange heat from one medium to another. Radiators, condensers, and
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
　
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