Oil analysis.  A method of measuring the contents in parts  per million of various chemical elements in oil. A sample  of the oil is burned in an electric arc, and the resulting  light is analyzed with a spectroscope which identifies the  chemical elements in the oil and gives an indication of the  amount of each element. This type of oil analysis is called a  spectrometric oil analysis program, or SOAP. Neutral position.  The position of the magnet in a magneto  when its poles are between the pole shoes and no lines of  flux are flowing through the magnetic circuit. Newton.  The unit of force needed to accelerate a mass of  one kilogram one meter per second per second. One newton  is equal to 1000,000 dynes, or 2.248 × 10$^{-1}$ pound. Nichrome.  The registered trade name for an alloy of nickel and  chromium. Nichrome wire is used for making electrical heater  elements and precision wire-wound resistors. Nichrome’s  resistance is approximately 65 times that of copper. Oil dilution.  A method of temporarily decreasing the  viscosity of the lubricating oil to make it possible to start  a reciprocating engine when the temperature is very low.  Before shutting the engine down, enough gasoline from the  fuel system is mixed with the lubricating oil in the engine to  dilute it so the starter can turn the engine over when the oil is  cold and viscous. When the engine starts and the oil warms  up, the gasoline evaporates. Nitriding.  A method of case hardening steel. Steel is placed  in a retort (a sealed, high-temperature furnace), and heated to  a specified temperature while surrounded by ammonia gas  (NH$_{3}$). The ammonia breaks down into nitrogen and hydrogen,  and the nitrogen unites with some of the alloying elements in  the steel to form an extremely hard surface. Nitriding hardens  crankshaft bearing surfaces and cylinder walls in reciprocating  engines. It takes place at a lower temperature than other forms  of case hardening, and does not cause warping. Oil-damped bearing.  A type of roller bearing installation  in a gas turbine engine in which the outer race is installed  in an oil damper compartment whose inside diameter is a  few thousandths of an inch larger than the outside diameter  of the outer race. Oil under pressure fills the oil damper  compartment and allows the bearing to compensate for sight  misalignment and to absorb vibrations of the shaft. Normal category airplane.  An aircraft that is certificated  under 14 CFR part 23 that is not certificated under the  acrobatic, utility, or commuter category. On-condition maintenance.  A maintenance program that  closely monitors the operating condition of an engine and  allows major repairs or replacements to be made when engine  performance deteriorates to a specific level. Normal shock wave.  A type of pressure wave that forms at  right angles to a surface when air moves at the speed of sound. Notch sensitivity.  A measure of the loss of strength of a  material caused by the presence of a notch, or a V-shaped cut. On-speed condition.  The speed condition in which the engine  is turning at the rpm for which the propeller governor is set. Nozzle guide vanes.  See turbine inlet guide vanes. Oblique shock wave.  A pressure wave that forms on a sharp- pointed object when air flows past it at a supersonic speed. One-hundred-hour inspection.  An inspection required by  14 CFR part 91, section 91.409 for FAA-certificated aircraft  operated for hire or used for flight instruction for hire. A  100-hour inspection is identical in content to an annual  inspection, but can be conducted by an aviation maintenance  technician who holds an Airframe and Powerplant rating, but  does not have an Inspection Authorization. See 14 CFR part  43, Appendix D for list of the items that must be included in  an annual or 100-hour inspection. Octane rating.  A system used to rate the antidetonation  characteristics of a reciprocating engine fuel. Fuel with an  octane rating of 80 performs in a laboratory test engine the  same as the fuel made of a mixture of 80% iso-octane and  20% heptanes. Operating cycle.  One complete series of events in the  operation of a turbine engine that consists of starting the  engine, taking off, landing, and shutting the engine down. Odometer.  The portion of an automobile speedometer that  indicates the distance traveled. Offset throw (crankshaft design).  Crank arms on a  reciprocating engine crankshaft. The arms, or throws, to  which the connecting rods and pistons are attached are offset  from the center of the crankshaft to move the pistons in and  out of the cylinder. The amount of the offset determines the  stroke of the engine. Optoelectronic device.  An electronic device that produces,  modulates, or senses electromagnetic radiation in the  ultraviolet, visible light, or infrared portions of the energy  spectrum. G-19