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keep it from drifting off the runway when making an approach for a cross-wind landing.
crack (material defect). A partial separation in a piece of material caused by vibration, overloading, or
internal stresses.
crack arrester (aircraft structure). A small hole drilled in the end of a crack to prevent the crack
continuing. Drilling this hole is called stop-drilling the crack.
A crack in a piece of metal will continue to grow as long as the stresses concentrated at the end of
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the crack are greater than the tensile strength of the metal. A hole drilled at the end of a crack spreads these
stresses out over the entire circumference of the hole, rather than allowing them to concentrate at the
extremely small area of the end of the crack.
cracked gasoline. A hydrocarbon fuel (gasoline) manufactured by heating crude petroleum products under
pressure, usually in the presence of a catalyst. The heavier hydrocarbons in the crude oil are broken down
by the cracking process into products that are distilled into gasoline.
cradle (maintenance fixture). A supporting fixture used to hold an aircraft structure while it is being
repaired or stored. The cradle is padded so it will not damage the lightweight aircraft structure placed in it.
crankcase (reciprocating engine component). The housing which encloses the crankshaft, camshaft, and
many of the accessory drive gears of a reciprocating engine. The cylinders mount on the crankcase, and the
engine is attached to the airframe by the crankcase.
crankpin (reciprocating engine crankshaft component). The portion of a crankshaft to which the
connecting rod is attached. The force produced by the expanding gases pushing on the piston is applied to
the crankpin by the connecting rod. The crankpin travels in a circular path and causes the crankshaft to
rotate.
crankshaft (reciprocating engine component). The central component of a reciprocating engine, a
high-strength, alloy steel shaft with hardened and polished bearing surfaces that ride in bearings in the
crankcase.
Offset throws are formed on the crankshaft on which crankpins are ground and polished.
Connecting rods, driven by the pistons, ride on the crankpins and change the in-and-out motion of the
pistons into rotation of the crankshaft.
The propeller of an airplane or the rotor system transmission of a helicopter is driven by the
crankshaft.
crankshaft runout (reciprocating engine dimension). The amount a crankshaft is bent. Runout is
checked by supporting the crankshaft in V-blocks or rollers and mounting a precision dial indicator so its
arm rides around the end of the crankshaft as it is rotated.
As the shaft is turned, the dial indicator shows the amount, usually in thousandths of an inch, the
shaft is bent, or the amount it is run out.
crater (welding bead component). A recess, or dip, in the puddle of molten metal in the flame of a gas
welding torch or in the arc used for electric welding.
crazing. A form of heat damage that occurs in a transparent thermoplastic material. Crazing appears as a
series of tiny, hairlike cracks in the surface of the plastic. If a material is heated nonuniformly, one side will
expand more than the other, and the stresses caused by the uneven expansion produce the crazing.
Crazing destroys the strength of the material and makes it difficult to see through.
creep (gas turbine engine component defect). A condition of permanent elongation of a turbine blade.
Creep causes the turbine blades to actually grow in length when they are acted on by high temperature and
high centrifugal loading.
creeper (shop tool). A flat board with low wheels on it. A mechanic can lie on a creeper while working
under the fuselage or under a low wing of an airplane.
crepe masking tape. A form of paper tape which is crinkled, with one side coated with a pressure-sensitive
adhesive. Crepe masking tape is used to prepare a surface for painting. The crinkles allow the tape to lie flat
when it is used to mask a curved line.
Crescent wrench. The registered trade name for an adjustable open-end wrench. Because of its popular use,
the name Crescent wrench has become accepted as the generic name for this type of wrench. A Crescent
wrench has one fixed jaw and one adjustable jaw. The adjustable jaw moves in and out, parallel to the fixed
jaw. The jaws form a parallel opening that is offset by about 15° from the axis of the wrench handle.
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crewmember (aircraft flight crew). A person aboard an aircraft for the purpose of operating the aircraft in
flight. 14 CFR Part 1: “A person assigned to perform duty in an aircraft during flight time.”
　
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