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. 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. 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. Induced current.  Electrical current produced in a conductor  when it is moved through or crossed by a magnetic field. Indicated airspeed (IAS).  The airspeed as shown on an  airspeed indicator with no corrections applied. Inch-pound.  A measure of work accomplished when a force  of 1 pound moves an object a distance of 1 inch. IFR.  Instrument flight rules. 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. ICAO.  The International Civil Aeronautical Organization. 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. 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. Hypersonic speed.  Speed of greater than Mach 5 (5 times  the speed of sound). 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. G-19