Millivoltmeter.  An electrical instrument that measures voltage in units of millivolts (thousandths of a volt). 

Mist coat. A very light coat of zinc chromate primer. It is so thin that the metal is still visible, but the primer makes pencil marks easy to see. 

Moisture separator.  A component in a high-pressure pneumatic system that removes most of the water vapor from the compressed air. When the compressed air is used, its pressure drops, and this pressure drop causes a drop in temperature. If any moisture were allowed to remain in the air, it would freeze and block the system. 

Mold line.  A line used in the development of a flat pattern for a formed piece of sheet metal. The mold line is an extension of the flat side of a part beyond the radius. The mold line dimension of a part is the dimension made to the intersection of mold lines and is the dimension the part would have if its corners had no radius. 

Mold point. The intersection of two mold lines of a part. Mold line dimensions are made between mold points. 

Moment. A force that causes or tries to cause an object to rotate. The value of a moment is the product of the weight of an object (or the force), multiplied by the distance between the center of gravity of the object (or the point of application of the force) and the fulcrum about which the object rotates. 

Monel. An alloy of nickel, copper, and aluminum or silicon. 

Monocoque.  A single-shell type of aircraft structure in which all of the flight loads are carried in the outside skin of the structure. 

MSDS. Material Safety Data Sheets. MSDS are required by the Federal Government to be available in workplaces to inform workers of the dangers that may exist from contact with certain materials. 

MSL.  Mean sea level. When the letters MSL are used with an altitude, it means that the altitude is measured from mean, or average, sea level. 

MTBF.  Mean time between failures. 

Multimeter.  An electrical test instrument that consists of a single current-measuring meter and all of the needed components to allow the meter to be used to measure voltage, resistance, and current. Multimeters are available with either analog-or digital-type displays. 

Multiple-disk brakes.  Aircraft brakes in which one set of disks is keyed to the axle and remains stationary. Between each stationary disk there is a rotating disk that is keyed to the inside of the wheel. When the brakes are applied, the stationary disks are forced together, clamping the rotating disks between them. The friction between the disks slows the aircraft. 

Nailing strip.  A method of applying pressure to the glue in a scarf joint repair in a plywood skin. A strip of thin plywood is nailed over the glued scarf joint with the nails extending into a supporting structure beneath the skin. The strip is installed over vinyl sheeting to prevent it sticking to the skin. When the glue is thoroughly dry, the nailing strip is broken away and the nails removed. 

Nap of the fabric. The ends of the fibers in a fabric. The first coat of dope on cotton or linen fabric raises the nap, and the fiber ends stick up. These ends must be carefully removed by sanding to get a smooth finish. 

Naphtha. A volatile and flammable hydrocarbon liquid used chiefly as a solvent or as a cleaning fluid. 

NDB.  Non-directional beacons. 

Negative pressure relief valve (pressurization component). A valve that opens anytime the outside air pressure is greater than the cabin pressure. It prevents the cabin altitude from ever becoming greater than the aircraft flight altitude. 

Neutral axis (neutral plane).  A line through a piece of material that is bent. The material in the outside of the bend is stretched and that on the inside of the bend is shrunk. The material along the neutral plane is neither shrunk nor stretched. 

Neutral flame. An oxyacetylene flame produced when the ratio of oxygen and acetylene is chemically correct and there is no excess of oxygen or carbon. A neutral flame has a rounded inner cone and no feather around it. 

Noise (electrical).  An unwanted electrical signal within a piece of electronic equipment. 

Nomex.  A patented nylon material used to make the honeycomb core for certain types of sandwich materials. 

Nonenergizing brake. A brake that does not use the momentum of the aircraft to increase the friction. 

Nonvolatile memory.  Memory in a computer that is not lost when power to the computer is lost. 

