Aviation Maintenance Terms beginning with M
M&D (Malfunction and Defect) report
A small postcard- like form (FAA Form 8330) used by repair stations, maintenance shops, and technicians to report an unacceptable condition to the FAA. Information on these forms provides the basis for the General Airworthiness Alerts and subsequent Airworthiness Directives.
Mach number
The ratio of true airspeed to the speed of sound.
Mach number
The ratio of the speed of an object through the air to the speed of sound under the same atmospheric conditions. An object traveling at the speed of sound is traveling at Mach one (M1.0).
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.
Magnesyn system
The registered trade name of a remote indicating instrument system. A Magnesyn system uses a permanent magnet as its rotor and a toroidal coil excited by 400-hertz AC as its stator. A small magnet in the center of the indicator coil follows the movement of a larger magnet in the transmitter coil.
Magnetic bearing
The direction to or from a radio transmitting station measured relative to magnetic north.
Magnetic field
The space around a source of magnetic flux in which the effects of magnetism can be determined.
Magnetic field
The invisible, but measurable, force surrounding a permanent magnet or current-carrying conductor. This field is produced when the orbital axes of the electrons of the atoms in the material are all in alignment.
Magnetic flux
Lines of magnetic force that are assumed to leave a magnet at its north end and return to its south end. Lines of flux tend to be as short as possible and cannot cross each other.
Magnetic particle inspection
A method of non-destructive inspection for ferrous metal components. The part being inspected is magnetized and then flooded with a solution of iron oxide suspended in a light oil, much like kerosene. Any flaw, either on the surface or just below the surface, forms a north and south pole, and the iron oxide attracted to these poles helps locate the flaw. The iron oxide is normally treated with a fluorescent dye, and the inspection is conducted in a darkened booth. When an ultraviolet light (black light) is shone on the part, the treated iron oxide shows up as a brilliant line.
Magnetic particle inspection
A method of detecting invisible cracks and other defects in ferromagnetic materials such as iron and steel. The inspection process consists of magnetizing the part and then applying ferromagnetic particles to the surface area to be inspected.
Main rotor
The rotor that supplies the principal lift to a rotorcraft.
Maintenance
Inspection, overhaul, repair, preservation, and the replacement of parts, but excludes preventive maintenance. (1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) That is not done according to accepted practices or cannot be done by elementary operations.
Maintenance
This includes inspection, overhaul, repair, preservation, and the replacement of parts, but excludes preventive maintenance.
Major alteration
An alteration not listed in the aircraft, aircraft engine, or propeller specifications that might appreciably affect weight, balance, structural strength, powerplant operation, flight characteristics, or other qualities affecting airworthiness; an alteration not done according to accepted practices, or one that cannot be done by elementary operations.
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 alteration
An alteration not listed in the aircraft, aircraft engine, or propeller specifications: (1) that might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) that is not done according to accepted practices or cannot be done by elementary operations.
Major overhaul
The disassembly, cleaning, and inspection of an engine and the repair and replacement of all parts that do not meet the manufacturer’s specification.
Major repair
A repair: (1) That, if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) That is not done according to accepted practices or cannot be done by elementary operations.
Major repair
A repair to a component that if improperly done might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; a repair not done according to accepted practices, or one that cannot be done by 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.
Major repairs
A repair that (1) if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or (2) is not done according to accepted practices, or cannot be done by elementary operations.
Malfunction or Defect Report
A report (FAA Form 8010-4) providing the FAA and industry with a very essential service record of mechanical difficulties encountered in aircraft operations. Such reports contribute to the correction of conditions or situations which otherwise will continue to prove costly and/or adversely affect the airworthiness of aircraft.
Mandrel
A precision steel bar on which a propeller is mounted for balancing. The mandrel is placed across two perfectly level knife-edge plates, and the propeller is allowed to rotate until it stops with its heavy point at the bottom.
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
Absolute pressure as measured at the appropriate point in the induction system and usually expressed in inches of mercury.
Manifold pressure
The absolute pressure of the air inside the induction system of a reciprocating engine.
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.
Manifold valve
See flow divider (reciprocating engine).
Manufacturer’s maintenance manual
A manual provided by an aircraft manufacturer that outlines the methods, techniques, and practices prescribed for each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance.
Marker beacon
A radio navigation aid used in an instrument approach to identify distance to the runway. As the aircraft crosses over the marker-beacon transmitter, the pilot receives an accurate indication of the airplane’s distance from the runway through the medium of a flashing light and an aural signal.
Mass
A measure of the amount of matter in an object. For the purpose of measuring the mass of air flowing through a turbine engine, the weight of the air, in pounds per second, is divided by the acceleration due to gravity (32.3 feet per second).
Mass
A measure of the quantity of matter in an object.
Master switch
A switch designed to control all electric power to all circuits in a system.
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.
Matrix (advanced composites)
The material that bonds the fibers together in an advanced composite structure. The matrix carries the stresses into the fibers.
Matter
Something that has mass, takes up space, and exists as a solid, liquid, or gas.
Matter
Any substance that has mass and takes up space.
Maximum engine overtorque
As it applies to turbopropeller and turboshaft engines incorporating free power turbines for all ratings except one engine inoperative (OEI) ratings of two minutes or less, is the maximum torque of the free power turbine rotor assembly, the inadvertent occurrence of which, for periods of up to 20 seconds, will not require rejection of the engine from service, or any maintenance action other than to correct the cause.
Maximum landing weight
The heaviest weight an aircraft can have when it lands. For large wide body commercial airplanes, it can be 100,000 pounds less than maximum takeoff weight, or even more.
Maximum ramp weight
The heaviest weight to which an aircraft can be loaded while it is sitting on the ground, sometimes referred to as the maximum taxi weight.
Maximum speed for stability characteristics, VFC /MFC
A speed that may not be less than a speed midway between maximum operating limit speed (VMO/MMO) and demonstrated flight diving speed (VDF/MDF), except that, for altitudes where the Mach number is the limiting factor, MFC need not exceed the Mach number at which effective speed warning occurs.
Maximum takeoff weight
The heaviest weight an aircraft can have when it starts the takeoff roll. The difference between this weight and the maximum ramp weight would equal the weight of the fuel that would be consumed prior to takeoff.
Maximum weight
The maximum authorized weight of the aircraft and its contents, and is indicated in the Aircraft Specifications or Type Certificate Data Sheet.
Maximum zero fuel weight
The heaviest weight an aircraft can be loaded to without having any usable fuel in the fuel tanks. Any weight loaded above this value must be in the form of fuel.
Mean aerodynamic chord (MAC)
The average distance from the leading edge to the trailing edge of the wing.
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.
Mechanical advantage
A ratio of the resistance force to the effort force.
Mechanical energy
This includes all methods of producing increased motion of molecules such as friction, impact of bodies, or compression of gases.
Medium-bypass ratio engine
A turbofan engine whose bypass ratio is between 2:1 and 4:1.
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.
Methanol
Alcohol made from wood.
METO horsepower
The maximum power allowed to be continuously produced by an engine. Takeoff power is usually limited to a given amount of time, such as 1 minute or 5 minutes.
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.
Microinches rms
A measure used for cylinder wall surface roughness. Twenty microinches rms means that the highest and lowest deviation from the average surface is 20 millionths of an inch.
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.
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.
Microprocessor
A single silicon chip that contains the arithmetic and logic functions of a computer.
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.
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.
Millivoltmeter
An electrical instrument that measures voltage in units of millivolts (thousandths of a volt).
Minor alteration
An alteration other than a major alteration.
Minor alteration
Any alteration that does not fit the definition of a major repair. See major repair.
Minor repair
A repair other than a major repair.
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.
Mixed number
A combination of a whole number and a fraction. For example, 53⁄8 is a mixed number.
Mode Control Panel
The Mode Control Panel, (MCP) contains controls that allow aircrew to interface with the autopilot system. The MCP can be used to instruct the autopilot to perform tasks such as; hold a specific altitude, change altitudes at a specific rate, hold a specific heading, turn to a new heading, and or follow the directions of a flight management computer. The MCP is not the autopilot, it just controls the mode in which the autopilot operates.
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.
Moisture content of wood
Weight of the water contained in the wood usually expressed in percentage of the weight of the kiln-dry wood.
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.
Molecule
The smallest particle of an element or compound that retains the chemical properties of the element or compound.
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.
Moment
In determining weight and balance, the moment is the product of a weight multiplied by its arm.
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.
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.
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.
MS flareless fittings
Designed primarily for highpressure (3,000 psi) hydraulic systems that may be subjected to severe vibration or fluctuating pressure. Using this type of fitting eliminates all tube flaring, yet provides a safe and strong, dependable tube connection. The fitting consists of three parts: a body, a sleeve, and a nut.
Multiconductor cable
Consists of two or more cables or wires, all of which are encased in an outer covering composed of synthetic rubber, fabric, or other material.
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-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.
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.
Multiplication
The process of repeated addition
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