Aviation Maintenance Terms beginning with M
Mach number
A measurement of speed based on the ratio of the speed of the aircraft to the speed of sound under the same atmospheric conditions. An airplane flying at Mach 1 is flying at the speed of sound.
Magnetic bearing
The direction to or from a radio transmitting station measured relative to magnetic north.
Major alteration
An alteration not listed in the aircraft, aircraft engine, or propeller specifications. It is one that might appreciably affect weight, balance, structural strength performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or that cannot be made with elementary operations.
Major repair
A repair to an aircraft structure or component that if improperly made might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or that is not done according to accepted practices, or cannot be made with elementary operation.
Manifold cross-feed fuel system
A type of fuel system commonly used in large transport category aircraft. All fuel tanks feed into a common manifold, and the dump chutes and the single-point fueling valves are connected to the manifold. Fuel lines to each engine are taken from the manifold.
Manifold pressure
The absolute pressure of the air in the induction system of a reciprocating engine.
Manifold pressure gauge
A pressure gauge that measures the absolute pressure inside the induction system of a reciprocating engine. When the engine is not operating, this instrument shows the existing atmospheric pressure.
Master switch
A switch in an aircraft electrical system that can disconnect the battery from the bus and open the generator or alternator field circuit.
Matrix
The material used in composite construction to bond the fibers together and to transmit the forces into the fibers. Resins are the most widely used matrix materials.
Mean camber
A line that is drawn midway between the upper and lower camber of an airfoil section. The mean camber determines the aerodynamic characteristics of the airfoil.
Mercerize
A treatment given to cotton thread to make it strong and lustrous. The thread is stretched while it is soaked in a solution of caustic soda.
Micro-Mesh
A patented graduated series of cloth-backed cushioned seats that contain abrasive crystals. Micro-Mesh is used for polishing and restoring transparency to acrylic plastic windows and windshields.
Micro-organism
An organism, normally bacteria or fungus, or microscopic size.
Microballoons
Tiny, hollow spheres of glass or phenolic material used to add body to a resin.
Microbial contaminants
The scum that forms inside the fuel tanks of turbine-engine-powered aircraft that is caused by micro-organisms. These micro-organisms live in water that condenses from fuel, and they feed on the fuel. The scum they form clogs fuel filters, lines, and fuel controls and holds water in contact with the aluminum alloy structure, causing corrosion.
Micron (“micro meter”)
A unit of linear measurement equal to one millionth of a meter, one thousandth of a millimeter, or 0.000039 inch. A micron is also called a micrometer.
Micronic filter
The registered trade name of a type of fluid filter whose filtering element is a specially treated cellulose paper formed into vertical convolutions, or wrinkles. Micronic filters prevent the passage of solids larger than about 10 microns, and are normally replaced with new filters rather than cleaned.
Microswitch
The registered trade name for a precision switch that uses a short throw of the control plunger to actuate the contacts. Microswitches are used primarily as limit switches to control electrical units automatically.
Mil
One thousandth of an inch (0.001 inch). Paint film thickness is usually measured in mils.
Mildew
A gray or white fungus growth that forms on organic materials. Mildew forms on cotton and linen aircraft fabric and destroys its strength.
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.
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-disc brakes
Aircraft brakes in which one set of discs is keyed to the axle and remains stationary. Between each stationary disc there is a rotating disc that is keyed to the inside of the wheel. When the brakes are applied, the stationary discs are forced together, clamping the rotating discs between them. The friction between the discs slows the aircraft.
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