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manifold pressure. 14 CFR Part 1: “Absolute pressure as measured at the appropriate point in the
induction system and usually expressed in inches of mercury.”
manifold pressure gage. A pressure gage 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.
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
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Page 373
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).
mare’s tail (meteorology). A cirrus cloud with a curve on one end and a long flowing tail on the other.
marine grommet (aircraft fabric covering component). A small plastic or metal reinforcing ring
attached to the fabric covering at low points in the structure and at the trailing edges of the wing and tail
surfaces. Grommets are used to reinforce the drain or vent holes cut in the fabric at these locations. A
marine grommet, also called a seaplane grommet, differs from ordinary grommets because of a shield
placed over the hole. This shield keeps the spray during takeoff and landing from getting into the structure.
marker beacon (radio navigation equipment). A low-powered, 75-MHz, fixed-frequency radio
transmitter that directs its signal vertically upward in a small, fan-shaped pattern. Marker beacons are
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located along the flight path used when approaching an airport for landing, and are identified by their
modulation frequency and keying code. When received by compatible airborne equipment, marker beacons
indicate to the pilot, both aurally and visually, when the aircraft is directly over the facility.
When the aircraft is directly above the outer marker the pilot hears a series of 400-hertz dashes
　
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