Aviation Maintenance Terms beginning with B
Back course
The reciprocal of the localizer course for an ILS (Instrument Landing System). When flying a back-course approach, the aircraft approaches the instrument runway from the end on which the localizer antennas are installed.
Backhand welding
Welding in which the torch is pointed away from the direction the weld is progressing.
Backplate (brake component)
A floating plate on which the wheel cylinder and the brake shoes attach on an energizing- type brake.
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.
Balance cable
A cable in the aileron system of an airplane that connects to one side of each aileron. When the control wheel is rotated, a cable from the flight deck pulls one aileron down and relaxes the cable going to the other aileron. The balance cable pulls the other aileron up.
Balance panel
A flat panel hinged to the leading edge of some ailerons that produces a force which assists the pilot in holding the ailerons deflected. The balance panel divides a chamber ahead of the aileron in such a way that when the aileron is deflected downward, for example, air flowing over its top surface produces a low pressure that acts on the balance panel and causes it to apply an upward force to the aileron leading edge.
Balance tab
An adjustable tab mounted on the trailing edge of a control surface to produce a force that aids the pilot in moving the surface. The tab is automatically actuated in such a way it moves in the direction opposite to the direction the control surface on which it is mounted moves.
Balanced actuator
A linear hydraulic or pneumatic actuator that has the same area on each side of the piston.
Banana oil
Nitrocellulose dissolved in amyl acetate, so named because it smells like bananas.
Bank (verb)
The act of rotating an aircraft about its longitudinal axis.
Barometric scale
A small window in the dial of a sensitive altimeter in which the pilot sets the barometric pressure level from which the altitude shown on the altimeter is measured. This window is sometimes called the “Kollsman” window.
base
The electrode of a bipolar transistor between the emitter and the collector. Varying a small flow of electrons moving into or out of the base controls a much larger flow of electron between the emitter and the collector.
Base
The electrode of a bipolar transistor between the emitter and the collector. Varying a small flow of electrons moving into or out of the base controls a much larger flow of electrons between the emitter and the collector.
Bead (tire component)
The high-strength carbon-steel wire bundles that give an aircraft tire its strength and stiffness where it mounts on the wheel.
Bead seat area
The flat surface on the inside of the rim of an aircraft wheel on which the bead of the tire seats.
Bearing strength (sheet metal characteristic)
The amount of pull needed to cause a piece of sheet metal to tear at the points at which it is held together with rivets. The bearing strength of a material is affected by both its thickness and the diameter of the rivet.
Beehive spring
A hardened-steel, coil-spring retainer used to hold a rivet set in a pneumatic rivet gun. This spring gets its name from its shape. It screws onto the end of the rivet gun and allows the set to move back and forth, but prevents it being driven from the gun.
Bend allowance
The amount of material actually used to make a bend in a piece of sheet metal. Bend allowance depends upon the thickness of the metal and the radius of the bend, and is normally found in a bend allowance chart.
Bend radius
The radius of the inside of a bend.
Bend tangent line
A line made in a sheet metal layout that indicates the point at which the bend starts.
Bernoulli’s principle
The basic principle that explains the relation between kinetic energy and potential energy in fluids that are in motion. When the total energy in a column of moving fluid remains constant, any increase in the kinetic energy of the fluid (its velocity) results in a corresponding decrease in its potential energy (its pressure).
Bezel
The rim that holds the glass cover in the case of an aircraft instrument.
Bias-cut surface tape
A fabric tape in which the threads run at an angle of 45° to the length of the tape. Bias-cut tape may be stretched around a compound curve such as a wing tip bow without wrinkling.
Bilge area
A low portion in an aircraft structure in which water and contaminants collect. The area under the cabin floorboards is normally called the bilge.
Bipolar transistor
A solid-state component in which the flow of current between its emitter and collector is controlled by a much smaller flow of current into or out of its base. Bipolar transistors may be of either the NPN or PNP type.
Black box
A term used for any portion of an electrical or electronic system that can be removed as a unit. A black box does not have to be a physical box.
Bladder-type fuel cell
A plastic-impregnated fabric bag supported in a portion of an aircraft structure so that it forms a cell in which fuel is carried.
Blade track
The condition of a helicopter rotor in which each blade follows the exact same path as the blade ahead of it.
Bleeder
A material such as glass cloth or mat that is placed over a composite lay-up to absorb the excess resin forced out of the ply fibers when pressure is applied.
Bleeding dope
Dope whose pigments are soluble in the solvents or thinners used in the finishing system. The color will bleed up through the finished coats.
Bleeding of brakes
The maintenance procedure of removing air entrapped in hydraulic fluid in the brakes. Fluid is bled from the brake system until fluid with no bubbles flows out.
Blimp
A cigar-shaped, nonrigid lighter-than-air flying machine.
Blush
A defect in a lacquer or dope finish caused by moisture condensing on the surface before the finish dries. If the humidity of the air is high, the evaporation of the solvents cools the air enough to cause the moisture to condense. The water condensed from the air mixes with the lacquer or dope and forms a dull, porous, chalky-looking finish called blush. A blushed finish is neither attractive nor protective.
Bonding
The process of electrically connecting all isolated components to the aircraft structure. Bonding provides a path for return current from electrical components, and a low-impedance path to ground to minimize static electrical charges. Shock-mounted components have bonding braids connected across the shock mounts.
Boost pump
An electrically driven centrifugal pump mounted in the bottom of the fuel tanks in large aircraft. Boost pumps provide a positive flow of fuel under pressure to the engine for starting and serve as an emergency backup in the event an engine-driven pump should fail. They are also used to transfer fuel from one tank to another and to pump fuel overboard when it is being dumped. Boost pumps prevent vapor locks by holding pressure on the fuel in the line to the engine-driven pump. Centrifugal boost pumps have a small agitator propeller on top of the impeller to force vapors from the fuel before it leaves the tank.
Boundary layer
The layer of air that flows next to an aerodynamic surface. Because of the design of the surface and local surface roughness, the boundary layer often has a random flow pattern, sometimes even flowing in a direction opposite to the direction of flight. A turbulent boundary layer causes a great deal of aerodynamic drag.
Bourdon tube
A pressure-indicating mechanism used in most oil pressure and hydraulic pressure gages. It consists of a sealed, curved tube with an elliptical cross section. Pressure inside the tube tries to straighten it, and as it straightens, it moves a pointer across a calibrated dial. Bourdon-tube pressure gauges are used to measure temperature by C measuring the vapor pressure in a sealed container of a volatile liquid, such as methyl chloride, whose vapor pressure varies directly with its temperature.
Brazing
A method of thermally joining metal parts by wetting the surface with a molten nonferrous alloy. When the molten material cools and solidifies, it holds the pieces together. Brazing materials melt at a temperature higher than 800 °F, but lower than the melting temperature of the metal on which they are used.
British thermal unit (BTU)
The amount of heat energy needed to raise the temperature of one pound of pure water 1 °F.
Bucking bar
A heavy steel bar with smooth, hardened surfaces, or faces. The bucking bar is held against the end of the rivet shank when it is driven with a pneumatic rivet gun, and the shop head is formed against the bucking bar.
Buffeting
Turbulent movement of the air over an aerodynamic surface.
Bulb angle
An L-shaped metal extrusion having an enlarged, rounded edge that resembles a bulb on one of its legs.
Bulkhead
A structural partition that divides the fuselage of an aircraft into compartments, or bays.
Bungee shock cord
A cushioning material used with the nonshock absorbing landing gears installed on older aircraft. Bungee cord is made up of many small rubber bands encased in a loose-woven cotton braid.
Burnish (verb)
To smooth the surface of metal that has been damaged by a deep scratch or gouge. The metal piled up at the edge of the damage is pushed back into the damage with a smooth, hard steel burnishing tool.
Burr
A sharp rough edge of a piece of metal left when the metal was sheared, punched, or drilled.
Bus
A point within an electrical system from which the individual circuits get their power.
Buttock line
A line used to locate a position to the right or left of the center line of an aircraft structure.
Butyl
Trade name for a synthetic rubber product made by the polymerization of isobutylene. Butyl withstands such potent chemicals as phosphate ester-base (Skydrol) hydraulic fluids.
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