Horsepower.  A unit of mechanical power that is equal to 33,000 foot-pounds of work done in 1 minute, or 550 foot- pounds of work done in 1 second. 

Hot dimpling.  A process used to dimple, or indent, the hole into which a flush rivet is to be installed. Hot dimpling is done by clamping the metal between heating elements and forcing the dies through the holes in the softened metal. Hot dimpling prevents hard metal from cracking when it is dimpled. 

Hot-wire cutter.  A cutter used to shape blocks of Styrofoam. The wire is stretched tight between the arms of a frame and heated by electrical current. The hot wire melts its way through the foam. 

HRD.  High-rate-discharge. 

HSI.  Horizontal situation indicator. 

Hydraulic actuator.  The component in a hydraulic system that converts hydraulic pressure into mechanical force. The two main types of hydraulic actuators are linear actuators (cylinders and pistons) and rotary actuators (hydraulic motors). 

Hydraulic fuse.  A type of flow control valve that allows a normal flow of fluid in the system but, if the flow rate is excessive, or if too much fluid flows for normal operation, the fuse will shut off all further flow. 

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 power pack.  A small, self-contained hydraulic system that consists of a reservoir, pump, selector valves, and relief valves. The power pack is removable from the aircraft as a unit to facilitate maintenance and service. 

Hydraulics.  The system of fluid power which transmits force through an incompressible fluid. 

Hydrocarbon.  An organic compound that contains only carbon and hydrogen. The vast majority of fossil fuels, such as gasoline and turbine-engine fuel, are hydrocarbons. 

Hydroplaning.  A condition that exists when a high-speed airplane is landed on a water-covered runway. When the brakes are applied, the wheels lock up and the tires skid on the surface of the water in much the same way a water ski rides on the surface. Hydroplaning develops enough heat in a tire to ruin it. 

Hydrostatic test.  A pressure test used to determine the serviceability of high-pressure oxygen cylinders. The cylinders are filled with water and pressurized to 5⁄3 of their working pressure. Standard-weight cylinders (DOT 3AA) must by hydrostatically tested every five years, and lightweight cylinders (DOT 3HT) must be tested every three years. 

Hypersonic speed.  Speed of greater than Mach 5 (5 times the speed of sound). 

Hyperbolic navigation.  Electronic navigation systems that determine aircraft location by the time difference between reception of two signals. Signals from two stations at different locations will be received in the aircraft at different times. A line plotted between two stations along which the time difference is the same forms a hyperbola. 

Hypoxia.  A physiological condition in which a person is deprived of the needed oxygen. The effects of hypoxia normally disappear as soon as the person is able to breathe air containing sufficient oxygen. 

ICAO.  The International Civil Aeronautical Organization. 

Icebox rivet.  A solid rivet made of 2017 or 2024 aluminum alloy. These rivets are too hard to drive in the condition they are received from the factory, and must be heat-treated to soften them. They are heated in a furnace and then quenched in cold water. Immediately after quenching they are soft, but within a few hours at room temperature they become quite hard. The hardening can be delayed for several days by storing them in a subfreezing icebox and holding them at this low temperature until they are to be used. 

IFR.  Instrument flight rules. 

Inch-pound. A measure of work accomplished when a force of 1 pound moves an object a distance of 1 inch. 

Indicated airspeed (IAS).  The airspeed as shown on an airspeed indicator with no corrections applied. 

Induced current.  Electrical current produced in a conductor when it is moved through or crossed by a magnetic field. 

Induced drag.  Aerodynamic drag produced by an airfoil when it is producing lift. Induced drag is affected by the same factors that affect induced lift. 

Induction time.  The time allowed an epoxy or polyurethane material between its initial mixing and its application. This time allows the materials to begin their cure. 

