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of hydrogen with one neutron in the nucleus) than normally occurs in water.
hecto. The metric prefix that means one hundred (1 x 102). Hecto is used as a prefix, as in hectometers
(100 meters), hectoliters (100 liters), etc.
hedge-hop (aircraft flight). To fly an aircraft very near the surface of the earth. This flight is below the
level of many obstructions on the ground, and the aircraft must be pulled up to pass over these obstructions.
Hedge-hopping is both dangerous and illegal in all except a few special instances, such as agricultural
application.
heel of a bend (rigid fluid lines). The outside of a bend. The metal in the heel of a bend in rigid tubing has
been stretched and is thinner than in other parts of the tubing.
height above airport. See HAA.
height above landing. See HAL.
height above touchdown. See HAT.
height-velocity envelope (helicopter operation). A chart produced by the manufacturer of a helicopter
showing the conditions of airspeed related to height that produces conditions unsafe for an autorotative
landing.
Heliarc welding. A name used for inert-gas arc welding. There are two basic types of Heliarc welding:
TIG (tungsten inert-gas) welding and MIG (metal inert-gas) welding.
TIG welding uses a small-diameter nonconsumable tungsten rod as the electrode from which the
arc jumps to the metal being welded. MIG welding uses a consumable soft steel rod as an electrode. This
rod melts and adds metal to the molten pool to reinforce the weld.
helical potentiometer (electrical component). A multi-turn potentiometer, or variable resistor. The
resistance element is made in the form of a spiral, or helix, and the wiper is moved over the element by
turning a multi-turn screw. Most helical potentiometers require ten turns of the screw to move the wiper
from one end of the resistance element to the other.
helical spline. A spline that twists, or winds, around the periphery of a shaft. Helical splines are used to
change linear motion of the device that rides on the splines into rotary motion of the shaft on which the
splines are cut. They are also used to change rotary motion of the shaft into linear motion of the device that
rides on it.
Helical splines are cut onto the shaft of some aircraft engine starter motors to move the starter gear
into mesh with the flywheel gear (linear motion) when the starter shaft is spun (rotary motion).
helical spring. A coil spring wound in the form of a spiral, or helix. Helical springs can be wound in such a
way that they can absorb either tensile or compressive loads.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 295
Heli-Coil insert. The registered trade name for a special helical insert used to restore threads that have
been stripped from a hole. The damaged threads are drilled out, and new threads are cut with a special
oversize tap. A stainless steel coil of wire whose cross section is in the shape of a diamond is screwed into
the hole and left there to serve as the new threads.
Heli-Coil inserts are also used to provide durable threads in soft metal castings. Some spark plug
holes in cast-aluminum cylinder heads are fitted with Heli-Coil inserts to minimize the wear caused by
repeated removal and installation of the spark plugs.
helicopter. A heavier-than-air flying machine that is supported in the air by aerodynamic lift produced by
an engine-driven rotor. Since the rotor is engine driven, a helicopter does not need forward motion through
the air for the rotor to produce lift.
14 CFR Part 1: “A rotorcraft that, for its horizontal motion, depends principally on its
engine-driven rotors.”
helipad. A small designated area, usually with a prepared surface, on a heliport, airport landing/takeoff
area, apron/ramp, or movement area used for takeoff, landing, or parking of helicopters.
heliport. 14 CFR Part 1: “An area of land, water, or structure used, or intended to be used, for the landing
and takeoff of helicopters.”
helium. A colorless, odorless, tasteless, inert gaseous chemical element. Helium’s symbol is He, its
atomic number is 2, and its atomic weight is 4.0026. Because helium will not combine with other chemical
elements, it is used to force oxygen away from the molten metal in certain types of arc welding, and to fill
some types of electron tubes to prevent oxygen from contacting the hot filaments.
Because of its lightness and because it will not burn, helium is used to inflate lighter-than-air
aircraft.
helix. A screw-like, or spiral, curve.
henry. The basic unit of inductance. One henry, H, is the amount of inductance needed to produce an
electromotive force of one volt, when the current in the inductor changes at the rate of one ampere per
　
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