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relationship of the aircraft to the glide slope.
hub of a propeller. The portion of a propeller that attaches to the propeller shaft of the engine. The
propeller blades fasten into the hub.
HUD (head-up display). A special type of flight instrumentation that allows the pilot of an aircraft to
watch the instruments while looking ahead of the aircraft for the approach lights or the runway. Head-up
displays increase the safety of flight during the transition from instrument flight to visual flight during a
landing.
hue (color characteristic). The characteristic of light, caused by its wavelength, that gives a color its name.
Red, blue, and green are hues.
humidity. A term used to indicate the presence of water vapor, or moisture, in the air.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 306
hung start (gas turbine engine operation). An improper start of a gas turbine engine in which the engine
starts, but fails to accelerate to a self-sustaining speed. See false start.
hunting (automatic flight control system). An undesirable condition in an automatic flight control system
in which the system oscillates back and forth on either side of a stable condition. The control system seeks,
or hunts for, its stable condition, but each time it approaches it, it overshoots and must seek it again.
hunting (helicopter rotor). The oscillation of a helicopter rotor blade back and forth about its lead-lag
hinge as the blade is rotating.
hurricane (meteorology). A violent tropical cyclone caused by air flowing into a low-pressure area. Winds
blow in a hurricane at a speed of 74 miles per hour (64 knots) or greater. This is a Beaufort force 12 (the
highest force). Hurricanes bring rain, thunder, and lightning, as well as the devastating winds. In the
northern hemisphere, the air in a hurricane moves in a counterclockwise direction, while in the southern
hemisphere, it moves in a clockwise direction.
hybrid compressor engine. A gas turbine engine that has both centrifugal and axial-flow compressors.
hybrid computer. A computer that has both analog and digital circuits and functions.
hybrid spark plug. A fine-wire spark plug that has a platinum center electrode and iridium ground
electrodes.
hydraulic actuator. The component in a hydraulic system that converts hydraulic pressure into mechanical
force. The two main types of hydraulic actuators are linear (cylinders and pistons) and rotary (hydraulic
motors).
hydraulic brake. An aircraft brake in which the friction is applied to the wheel by a hydraulically actuated
unit. The friction is produced in a disk brake when the rotating disk is clamped tightly between friction
linings held in a hydraulically operated clamp.
hydraulic fuse. A device in a hydraulic system that automatically shuts off the flow of fluid if a line should
break. There are two types of hydraulic fuses: one type shuts off the flow after a given volume of fluid
flows, and the other type shuts off the flow any time the rate of flow becomes excessive.
hydraulic lock (reciprocating engine condition). A condition in which oil drains into the lower cylinders
of an engine and leaks past the piston rings to fill the combustion chamber. If this oil is not removed before
the engine is started, it can cause serious damage.
The propellers on all radial, inverted V-, and inverted in-line engines must be pulled through by
hand before they are cranked with the starter to be sure that none of the cylinders have hydraulic lock.
hydraulic motor. A hydraulic actuator that converts fluid pressure into rotary motion. Hydraulic motors
have an advantage in aircraft installations over electric motors, because they can operate in a stalled
condition without the danger of a fire. Hydraulic motors may be of the gear, piston, or vane type.
hydraulic powerpack. A small, self-contained hydraulic system that consists of a reservoir, pump, selector
valves, and relief valves. The powerpack is removable from the aircraft as a unit to facilitate maintenance
and service.
hydraulic pump. A fluid-power pump used to force fluid through a hydraulic system. Hydraulic pumps
may be engine driven, or they may be operated by an electric motor or by hand.
hydraulics. The branch of science or technology that deals with the transmission of power through an
incompressible fluid, such as oil or water.
hydraulic system. A fluid power system that transmits power or force through an incompressible fluid,
such as oil. A complete hydraulic system contains a reservoir to hold the fluid, a pump to move the fluid,
valves to control the flow of fluid, and actuators to produce the mechanical force. All of the components
are connected together with either rigid or flexible fluid lines.
　
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