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level 450. The routes are referred to as “J” routes, with numbers to identify the designated route.
jet silencer. A noise suppressor used with a gas turbine engine. A jet silencer does not actually silence a jet
engine; rather, it converts some of the low-frequency vibrations into higher frequency vibrations that are
more easily dissipated as they mix with the surrounding air.
jet stream (meteorology). A high-velocity narrow stream of winds usually found near the upper limit of
the troposphere. The jet stream normally blows from west to east, and the winds in the stream may reach
velocities of up to 250 miles per hour.
jettison. To drop something from an aircraft or space craft in flight. Fuel is jettisoned when it is dumped
from the aircraft fuel tanks to lower the weight of the aircraft to a weight approved for landing.
jet wake. The stream of hot, high-velocity gases that come from the exhaust of a gas turbine engine.
jewel bearing. A cup-type bearing in which a hardened steel pivot rides. Jewel bearings used in many
types of indicating instruments are normally made of an extremely hard glass.
jeweler’s file. A small, fine-cut metalworking file used by instrument technicians to repair aircraft
instruments. Jeweler’s files are available in many shapes.
jeweler’s rouge. A very fine, dark red abrasive made of ferric oxide. Jeweler’s rouge is used for polishing
hard metal surfaces.
JFET (junction field effect transistor). A high-impedance semiconductor device that controls the flow of
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electrons between its source and drain by a voltage placed on its gate. The function of a JFET is similar to
that of a bipolar transistor, except the control is a small change in voltage rather than a small change in
current.
jig (manufacturing fixture). A strong, heavy framework used to hold a component while it is being
assembled. Parts are assembled in jigs to assure their interchangeability. Airplane wings, for example, are
assembled in jigs, and any wing having a given part number can be interchanged with another wing having
the same part number.
If the parts were not assembled in a jig, they might look the same, but they would probably not
interchange; the bolt holes would likely not line up.
jigsaw. A type of saw that uses a very narrow blade moved back and forth across the work being cut. A
jigsaw is used to cut small-radius curves in wood, metal, or plastic.
Jo-bolt (special aircraft fastener). A patented, high-strength structural fastener used where strength
requirements are high. The Jo-bolt is passed through a close-tolerance hole, and a sleeve is pulled up over
its tapered shank to form a blind head inside the structure.
joggle (sheet metal). A small offset near the edge of a piece of sheet metal.
It allows one sheet of metal to overlap another sheet while maintaining a flush surface.
Johansson block. An accurately ground gage block used as a reference in precision machining operations.
A Johansson block (Jo block) is usually ground to an accuracy of at least 0.000 01 inch (0.25 micrometer).
Johnson bar (aircraft controls). A type of brake control used on some early airplanes. A Johnson bar is a
long handle located between the pilot’s and copilot’s seats, easily accessible to both.
Pulling straight back on the Johnson bar applies both brakes equally. Pulling the bar back
diagonally applies the brake on the side to which the bar is pulled.
jointer (woodworking tool). A woodworking power tool used to smooth the edges of a board or piece of
wood.
joule. A measure of work in the kilogram-meter-second system of units. One joule is the amount of work
done when a force of one newton moves an object one meter in the direction in which the force is applied.
A joule is also known as a newton-meter. One joule is equal to 0.1020 meter-kilogram, or 0.7375
foot-pound of work.
journal (bearing surface). A hardened and polished surface on a rotating shaft that rides in a plain bearing.
The bearing journal provides a smooth bearing surface and wears very little.
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joystick (airplane control). An old name for the control stick used to control an airplane. Movement of the
joystick to the front or back moves the elevators to control the pitch of the airplane. Movement from side to
side moves the ailerons to rotate the airplane about its longitudinal axis (to roll the airplane).
The term joystick has been changed to control stick or, more simply, to just “stick.”
JP-4 (gas turbine engine fuel). A gas turbine engine fuel composed of approximately 65% gasoline and
　
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