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turret lathe. A type of metal-turning lathe in which the cutting tools are mounted in a turret. For example,
a drill, a boring tool, and a chamfering tool can be mounted in the turret. The drill is moved into the work to
drill the hole. The drill is then backed out, the turret is rotated to put the boring tool in place, and the hole is
counterbored. The boring tool is then backed out and the chamfering tool is rotated into place. The
chamfering tool is moved into the work to chamfer the edges of the counterbored hole.
twelve-point socket (mechanic’s hand tool). A socket wrench with a double hex, or a 12-point, opening
broached into its open end. A 12-point socket is easier to position on a nut or bolt head than a six-point
socket.
twenty-minute rating (battery capacity rating). The ampere hour rating of an aircraft battery when the
current drawn from it will drop its voltage to a predetermined cut-off voltage in twenty minutes.
twilight. The intervals of incomplete darkness following sunset and just preceding sunrise. The time at
which evening twilight ends and morning twilight begins is determined by arbitrary convention. See civil
twilight, nautical twilight, astronomical twilight.
twin-row radial engine (reciprocating engine type). A type of radial engine with two rows of cylinders
and a single crankshaft. The cylinders of a radial engine project out from a small crankcase that holds the
crankshaft. The piston in one cylinder attaches to a master rod whose large end goes around the crankshaft
throw, and the pistons in all of the other cylinders attach to the master rod with link rods.
A twin-row radial engine has a two-throw crankshaft and two rows of cylinders and two master
rods. The cylinders in the rear row are staggered between those in the front row for improved cooling. The
most popular twin-row radial engines have 14 or 18 cylinders.
twist drill. A metal-cutting tool turned in a drill press or a hand-held drill motor. A twist drill has a straight
shank and spiraled flutes. A cutting edge is ground on the end of the spiraled flutes. The diameter of twist
drills is measured in four different ways: Fractions of an inch, millimeters, letters, and numbers.
twisted pair (electrical cable). A type of electrical cable made up of two insulated wires twisted together
and enclosed in an outer insulation. The magnetic fields caused by current flowing in the two conductors of
a twisted pair cancel each other and minimize the magnetic field radiating from the conductors.
twister (meteorology). A colloquial term for tornado.
twist stripe (flexible fluid lines). A colored stripe running along the length of a flexible hose. If this stripe
spirals around the hose, it indicates that the hose was twisted when it was installed. Twist stripes are also
called lay lines.
two-axis automatic pilot. An automatic pilot that uses a canted rate gyro, similar to that used in a turn
coordinator, to sense rotation of the aircraft about its longitudinal and vertical axes. Output from the rate
gyro controls a pair of servos in the aileron system that keeps the wings level and the airplane stable in roll
and yaw. Two-axis automatic pilots are sometimes called wing levelers.
two-part adhesive. An adhesive packaged in two containers and mixed just before it is used. A two-part
adhesive consists of resin and an accelerator to speed up the curing action of the resin.
two-spool engine. See dual-spool gas-turbine engine.
two-stroke-cycle engine (reciprocating engine). A simple form of reciprocating engine that completes its
operating cycle in two strokes of its piston — one down and one up. When the piston moves up, fuel is
pulled into the crankcase at the same time the fuel-air mixture inside the cylinder is being compressed.
When the piston is near the top of its stroke, a spark plug ignites the compressed fuel-air mixture, and the
expanding gases force the piston down. Near the bottom of its stroke, the piston uncovers an exhaust port,
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 602
and the burned gases leave the cylinder. When the piston moves farther down, it uncovers an intake port,
and fuel and air are forced from the crankcase into the cylinder.
Two-stroke-cycle engines are inefficient in their use of fuel, but their simplicity makes them
popular for powering ultralight aircraft where light weight and low cost are paramount.
two-terminal spot-type fire detection system. A fire detection system that uses individual thermoswitches
installed around the inside of the area to be protected. These thermoswitches are wired in parallel between
two separate circuits. A short or an open can exist in either circuit without disabling the system or causing a
　
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