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.
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.