Micron . A measurement used to identify the size of particles trapped by filters. One micron is a micro meter, or one millionth of a meter. It is 0.000039 inch.

Microprocessor . A single silicon chip that contains the arithmetic and logic functions of a computer.

Milliammeter . An instrument that measures electrical current in units of thousandths of an ampere.

Millibar . A unit of pressure in the metric system. One bar is a pressure of 14.5 psi, or 29.52 in. Hg. One millibar is one thousandth of a bar, or 0.01469 psi, or 0.02952 in. Hg.

Minor alteration . Any alteration that does not fit the definition of a major repair. See major repair.

Module (modular engine construction ). The method of construction for mast modern gas turbine engine. The engine is made of several modules, or units, that can be removed and replaced or serviced independent of the rest of the engine.

Momentum . A force caused by the inertia of a moving body as it tries to keep the object moving in the same direction, at the same speed.

Motor. (verb ) The act of rotating a turbine engine using the starter, with the ignition system deactivated. An engine is motored to force air through it to purge fuel fumes.

Multiple-can combustor . A combustor used in a gas turbine engine that consists of a series of individual burner cans, each made of an inner liner and an outer case. The individual cans are arranged around the periphery of a centrifugal compressor. Hot gases flow directly from the cans into the turbine.

N1 . A symbol representing the rotational speed of the lowpressure compressor in a dual-spool gas turbine engine.

N2 . A symbol representing the rotational speed of the highpressure compressor in a dual-spool gas turbine engine.

NACA . National Advisory Committee for Aeronautics. This organization, dedicated to the technical development of aviation, has been superseded by NASA.

NACA cowling . A long-chord cowling used over a radial engine. The forward portion of this cowling has an aerodynamic shape that produces a forward pull, and the rear portion extends back to fair in with the fuselage. There is a narrow peripheral gap between the rear of the cowling and the fuselage for the cooling air to escape. Some NACA cowlings have controllable flaps over this opening to control the amount of cooling air that flows through the engine.

Nacelle . An enclosed compartment, normally in the leading edge of the wing, in which an aircraft engine is mounted.

Naphtha . A volatile, flammable liquid distilled from petroleum. It is used as a cleaning agent and solvent, and is present in some blended turbine-engine fuels.

NASA . National Aeronautics and Space Administration.

Naturally aspirated engine . A reciprocating engine that depends upon atmospheric pressure to force the air/fuel mixture into the cylinders. Naturally aspirated engines are neither supercharged nor turbocharged.

Net thrust . The thrust produced by a turbojet or turbofan engine in which the acceleration factor is the difference between the velocity of the incoming air and the velocity of the exhaust gases leaving the engine.

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.