manifold valve. See flow divider.
manometer. An instrument used to measure low values of air pressure. A manometer consists of a long glass tube mounted in front of a calibrated scale. The tube is filled with a liquid, and air pressure forces the liquid up in the tube where its height is measured against the scale.
Different ranges of pressure can be measured by using different liquids: water is used for low
pressure, ethylene dibromide for medium pressure, and mercury for higher pressure. Differential pressure can be measured by forming the tube into the shape of the letter U and applying the two pressures to the two ends of the tube.
manual depressurization valve (aircraft pressurization system component). A manual back-up for the automatic cabin pressure outflow valve. If the automatic outflow valve should malfunction, the cabin pressure can be controlled with the manual depressurization valve.
manufactured rivet head. The head of a rivet that was formed when the rivet was manufactured. The head formed when the rivet is driven is called the shop head. Rivets used in aircraft construction have several different shapes of manufactured heads. The most common are: the universal head (MS20470), the 100° countersunk head (MS20426), the round head (AN 430), and the flat head (AN 442).

manufacturer’s maintenance or service manual. A document issued by the manufacturer of an aircraft, engine, or component and approved by the FAA that details procedures to be followed for the maintenance of the specific device.
manufacturing. The process in which raw materials are changed into finished and usable products. An aircraft is manufactured when the raw aluminum, iron, copper, and organic chemicals are changed into the finished product.
MAP (manifold absolute pressure). The absolute pressure of the air inside the induction system of a reciprocating engine, measured in inches of mercury. The manifold pressure of a nonsupercharged engine can never be higher than the pressure of the surrounding air, but supercharging can increase the manifold pressure above that of the atmosphere.
MAP (missed approach point) (air traffic control). A point prescribed in each instrument approach at which a missed approach procedure shall be executed if the required visual reference does not exist.
Marconi antenna (radio antenna). A quarter-wavelength, vertically polarized antenna, normally used for transmitting and receiving radio communications in the higher frequency bands. Aircraft communications in the very-high-frequency (VHF) band normally uses a Marconi antenna whose length is one fourth of the wavelength of the center frequency to be transmitted.
One conductor from the transmitter is connected to the quarter-wave antenna, and the other conductor is connected to the metal structure of the aircraft on which the antenna is mounted. This metal structure serves as a quarter-wavelength reflector. A Marconi antenna is nondirectional (it radiates its signal equally well in all directions).