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than ahead of the weld.
backing (meteorology). Shifting of the wind in a counterclockwise direction with respect to either space or
time. Backing is the opposite of veering and is commonly used by meteorologists to refer to a cyclonic shift
(counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere).
backing plate (sheet metal repair). A reinforcing plate used when making a sheet metal repair. Backing
plates are often called doublers.
backlash. The play, or relative movement, between the teeth of mating gears caused by improper preload
adjustment or worn teeth.
backlash check (gear measurement). A check for the proper mesh between gears. This is done by
measuring the amount of clearance between the teeth of the two gears.
One of the gears is held rigid and the amount the other gear can move is measured to determine
the amount of backlash.
backplate (brake component). A floating plate on which the wheel cylinder and the brake shoes are
attached on an energizing-type brake.
back pressure (reciprocating engine). The pressure inside the exhaust system of a reciprocating engine
that prevents complete scavenging of the burned gases from the cylinders.
Back pressure is caused by mufflers and such power-recovery devices as turbochargers and
power-recovery turbines (PRT). See turbocharger and PRT.
backsaw (woodcutting saw). A fine-toothed hand saw which has a stiff spine along its upper edge.
Backsaws are used to make straight or angled cuts across a board and are often used with a miter box to cut
an accurate angle.
backscatter (radar meteorology). Radar energy that has been reflected or scattered by a target.
Backscatter is also called an echo.
back-suction mixture control. A type of mixture control used in some float carburetors that regulates the
fuel-air mixture ratio by varying the pressure of the air above the fuel in the float bowl.
back taxi (air traffic control). A term used by air traffic controllers to taxi an aircraft on the runway
opposite to the traffic flow. The aircraft may be instructed to back-taxi to the beginning of the runway or at
some point before reaching the runway end for the purpose of departure or to exit the runway.
backup ring. A flat leather or Teflon ring installed in the groove in which an O-ring or T-seal is placed.
The backup ring is on the side of the seal away from the pressure, and it prevents the pressure
extruding the seal between the piston and the cylinder wall.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 59
backup system. A reserve, or substitute, system that can be put into effect if the main or primary system
should fail.
back voltage (counter-electromotive force). Voltage induced in a conductor when it is carrying a
changing current. As the magnetic field produced by the changing current builds up and decays, it cuts
across the conductor and induces a voltage in it.
The polarity of this induced voltage is opposite that of the voltage which caused the original
current to flow.
bacteria. Microscopic plant life that lives in water entrapped inside aircraft fuel tanks. The growth of
bacteria in jet aircraft fuel tanks forms a film of scum which holds water against the aluminum alloy
surfaces and causes corrosion to form.
bactericide. A material used to destroy bacteria.
baffle. A part of a structure used to impede, regulate, or alter the rate or direction of flow of a fluid, light,
or sound.
baffle (air-cooled engine component). A sheet metal shield used to direct the flow of air between and
around the cylinders of an air-cooled reciprocating engine.
Forcing this air to flow through the fins on the cylinders removes the maximum amount of heat
from the engine.
baffle system (fuel tanks). A series of baffles, or partitions, inside an aircraft fuel tank. These baffles have
holes in them that allow the fuel to feed to the tank outlet, but prevent the fuel from surging enough to
uncover the outlet.
Aircraft fuel tanks carry such large quantities of fuel that the sloshing (surging back and forth) of
the fuel in a partially full tank could cause severe control problems.
bag molding. A method of applying pressure to a piece of laminated plastic material so all the layers are
held in tight contact with each other. The reinforcing material is impregnated with liquid resin and laid up
over a rigid mold in as many layers as are needed.
A sheet of flexible, airtight plastic material is placed over the mold, and the edges are sealed to
form a bag over the part. The entire assembly is then placed in an autoclave, and the air pressure is
increased to force the layers of material tightly together.
If an autoclave is not used, a vacuum pump can be attached to the inside of the bag and the air
　
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