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. 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. MFD.  Multi-function display. MHz (megahertz).  1,000,000 cycles per second. 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. 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. 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. 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. 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. 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. 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. MEK.  Methyl-ethyl-ketone is an organic chemical solvent  that is soluble in water and is used as a solvent for vinyl and  nitrocellulose films. MEK is an efficient cleaner for preparing  surfaces for priming or painting. Micro-organism.  An organism, normally bacteria or fungus,  or microscopic size. 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. MIG welding.  Metal inert gas welding is a form of electric  arc welding in which the electrode is an expendable wire.  MIG welding is now called GMA (gas metal arc) welding. 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.  G-22