Gust penetration speed.  The speed that gives the greatest  margin between the high and low Mach speed buffets.  Hydraulics.  The branch of science that deals with the  transmission of power by incompressible fluids under  pressure.  Gyroscopic precession.  An inherent quality of rotating  bodies, which causes an applied force to be manifested 90º  in the direction of rotation from the point where the force  is applied.  Hydroplaning.  A condition that exists when landing on a  surface with standing water deeper than the tread depth of  the tires. When the brakes are applied, there is a possibility  that the brake will lock up and the tire will ride on the  surface of the water, much like a water ski. When the tires  are hydroplaning, directional control and braking action  are virtually impossible. An effective anti-skid system can  minimize the effects of hydroplaning.  Hand propping.  Starting an engine by rotating the propeller  by hand.  Heading.  The direction in which the nose of the aircraft is  pointing during flight.  Hypoxia.  A lack of sufficient oxygen reaching the body  tissues. Heading bug.  A marker on the heading indicator that can  be rotated to a specific heading for reference purposes, or to  command an autopilot to fly that heading.  Igniter plugs.  The electrical device used to provide the spark  for starting combustion in a turbine engine. Some igniters  resemble spark plugs, while others, called glow plugs, have  a coil of resistance wire that glows red hot when electrical  current flows through the coil.  Heading indicator.  An instrument which senses airplane  movement and displays heading based on a 360º azimuth,  with the final zero omitted. The heading indicator, also called  a directional gyro, is fundamentally a mechanical instrument  designed to facilitate the use of the magnetic compass. The  heading indicator is not affected by the forces that make the  magnetic compass difficult to interpret.  Impact ice.  Ice that forms on the wings and control surfaces  or on the carburetor heat valve, the walls of the air scoop, or  the carburetor units during flight. Impact ice collecting on the  metering elements of the carburetor may upset fuel metering  or stop carburetor fuel flow.  Headwind component.  The component of atmospheric  winds that acts opposite to the aircraft’s flightpath.  Inclinometer.  An instrument consisting of a curved glass  tube, housing a glass ball, and damped with a fluid similar  to kerosene. It may be used to indicate inclination, as a level,  or, as used in the turn indicators, to show the relationship  between gravity and centrifugal force in a turn.  High performance aircraft.  An aircraft with an engine of  more than 200 horsepower.  Horizon.  The line of sight boundary between the earth and  the sky.  Horsepower.  The term, originated by inventor James Watt,  means the amount of work a horse could do in one second.  One horsepower equals 550 foot-pounds per second, or  33,000 foot-pounds per minute.  Indicated airspeed (IAS).  The direct instrument reading  obtained from the airspeed indicator, uncorrected for  variations in atmospheric density, installation error, or  instrument error. Manufacturers use this airspeed as the basis  for determining airplane performance. Takeoff, landing, and  stall speeds listed in the AFM or POH are indicated airspeeds  and do not normally vary with altitude or temperature.  Hot start.  In gas turbine engines, a start which occurs with  normal engine rotation, but exhaust temperature exceeds  prescribed limits. This is usually caused by an excessively  rich mixture in the combustor. The fuel to the engine must  be terminated immediately to prevent engine damage.  Indicated altitude.  The altitude read directly from the  altimeter (uncorrected) when it is set to the current altimeter  setting.  Induced drag.  That part of total drag which is created by  the production of lift. Induced drag increases with a decrease  in airspeed.  Hung start.  In gas turbine engines, a condition of normal  light off but with rpm remaining at some low value rather than  increasing to the normal idle rpm This is often the result of  insufficient power to the engine from the starter. In the event  of a hung start, the engine should be shut down.  G-9 I H