Inconel.  The registered trade name for an alloy of chromium,  iron, and nickel. Inconel is similar to stainless steel, but  cannot be hardened by heat treatment. Hybrid spark plug.  A fine-wire spark plug that has a  platinum center electrode and iridium ground electrodes. Hydraulic lock.  A condition in which oil drains into the  lower cylinders of a reciprocating engine and leaks past the  piston rings to fill the combustion chamber. If the oil is not  removed before the engine is started, it can cause serious  damage. Inductive reactance.  An opposition to the flow of AC or  changing DC caused by inductance in the circuit. Inductive  reactance, whose symbol is XL, causes a voltage drop, but  it does not use power nor produce heat. Inertia.  The tendency of a body to resist acceleration. A body  at rest will remain at rest or a body in motion will stay in  motion in a straight line unless acted on by an outside force. Hydromechanical.  Any device that combines fluid pressures  with mechanical actions to achieve a desired result. In a  hydromechanical fuel control used for a turbine engine,  hydraulic servos are used in conjunction with the mechanical  linkages. Inertia starter.  A starter for a large reciprocating engine  that uses energy stored in a rapidly spinning flywheel to  turn the crankshaft. Ice bridging.  A spark plug failure that occurs when starting  a reciprocating engine in extremely cold weather. When a  cylinder fires, the fuel-air mixture is converted into carbon  dioxide and water vapor. The water vapor condenses on the  spark plug electrodes and forms ice that bridges the electrode  gap and prevents the plug firing until the ice is melted. This  normally requires removing the spark plugs from the engine. Inlet guide vanes.  A set of stator vanes in front of the first  stage of compression in a gas turbine engine. The inlet guide  vanes deflect the air entering the compressor in the correct  direction for optimum operation. Inlet guide vanes may be  fixed, or their angle may be controlled hydraulically by fuel  from the fuel control. Integral fuel tank.  An aircraft fuel tank made by sealing off  part of the structure so fuel can be carried in the structure itself. IDG.  Integrated drive generator. An AC generator installed  on turbine engines. An IDG incorporates a brushless, three- phase AC generator and a constant-speed drive in a single  component. Intercooler.  An air-to-air heat exchanger installed between  a turbosupercharger and the carburetor. Intercoolers decrease  the temperature of compressed air to prevent detonation. Igniter.  The component in a turbine-engine ignition system  that provides a high-energy spark for igniting the fuel-air  mixture in the combustion chamber for starting. IHP.  Indicated horsepower. The theoretical horse-power a  reciprocating engine develops. Interference angle (poppet valve dimension).  The  difference between the valve seat and the valve face angles.  Normally, the valve seats are ground with between 0.5º and 1º  greater angle than the valve face. This allows the face to touch  the seat with a line contact that provides the best sealing. IMEP.  Indicated mean effective pressure. The average  pressure existing inside the cylinder of a reciprocating engine  during its power stroke. Impulse coupling.  A spring-loaded coupling between a  magneto shaft and the drive gear inside the engine. When the  engine is rotated for starting, the impulse coupling locks the  magnet so it cannot turn. The spring in the coupling winds  up as the crankshaft continues to turn, and when the piston is  near top center, the coupling releases and spins the magnet,  producing a hot and retarded spark. Interference fit.  A type of fit used when assembling certain  mechanical devices. The hole is made smaller than the part  that fits into it. The material containing the hole is heated to  expand the hole, and the part that fits into the hole is chilled  to shrink it. The parts are assembled, and when they reach  the same temperature their fit is so tight they will not loosen  in service. Internal-combustion engine.  A form of heat engine in which  the fuel and air mixture is burned inside the engine to heat  and expand the air so it can perform useful work. In-line engine.  A reciprocating engine with all of the  cylinders arranged in a straight line. Incandescent.  Glowing because of intense heat. Internal timing.  The adjustment of the breaker points of a  magneto so they will begin to open at the time the magnet  is in its E-gap position. G-15