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route. Altitudes assigned by ATC during such vectors provide required obstacle clearance.
offset parallel runways. Staggered runways having center lines which are parallel.
offset rivet set (sheet-metalworking tool). A rivet set, used in a pneumatic rivet gun, in which the cup that
fits the manufactured head of the rivet is offset from the center line of the rivet set shank. Offset rivet sets
are used to drive rivets in a location where a straight set cannot be used.
offset screwdriver. A screwdriver whose blade is perpendicular to the handle. Offset screwdrivers are used
to turn screws where there is not enough clearance to allow the use of a regular screwdriver.
offset throw (crankshaft design). Crank arms on a reciprocating engine crankshaft. The arms, or throws,
to which the connecting rods and pistons are attached are offset from the center of the crankshaft to move
the pistons in and out of the cylinder. The amount of the offset determines the stroke of the engine.
off-the-shelf item. A standard production item. An off-the-shelf computer program, for example, is a
standard program that can be bought and used, rather than having a custom program written.
OFZ (obstacle free zone) (air traffic control). A three dimensional volume of airspace which provides
protection for the transition of aircraft to and from the runway. The OFZ clearing standard precludes
taxiing and parked airplanes and object penetrations, except for frangible navaid locations that are fixed by
function. Additionally, vehicles, equipment, and personnel may be authorized by air traffic control to enter
the area using the provisions of FAA Order 7110.65, Air Traffic Control, paragraph 3-1-5. The runway
OFZ, and where applicable, the inner-approach OFZ and the inner-transitional OFZ, comprise the OFZ.
ohm (Ω). The standard unit of electrical resistance. One ohm is the resistance of a column of mercury
weighing 14.4521 grams, formed into a column of uniform cross section and a length of 1,006.3
centimeters. This standard resistance is measured with the mercury having a temperature of 0°C. More
practically, one ohm is the amount of resistance that will cause a voltage drop of one volt when one ampere
of current flows through it.
Ohm’s law. The law that explains the relationship between the flow of electrons (current), electrical
pressure (voltage), and the opposition to flow (resistance). According to Ohm’s law, the amount of current
flowing in a circuit is directly proportional to the circuit voltage and inversely proportional to the circuit
resistance.
ohmmeter. An electrical measuring instrument that measures the resistance in a circuit or component and
shows its value in ohms. An ohmmeter determines the value of an unknown resistance by measuring the
current flowing through it when the voltage across it is known.
ohms-per-volt (voltmeter sensitivity). A measure of the sensitivity of a voltmeter, found by dividing one
by the amount of current (in amperes) needed to deflect the pointer of the meter full scale.
For example, a meter that requires one milliamp (0.001 amp) of current for full-scale pointer
deflection has a sensitivity of 1,000 ohms per volt.
The greater the ohms-per-volt, the more sensitive the meter.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 413
oil analysis. A method of measuring the contents in parts per million of various chemical elements in a
sample of oil. A sample of the oil is burned in an electric arc, and the resulting light is analyzed with a
spectroscope which identifies the chemical elements in the oil and gives an indication of the amount of
each element.
oil can (sheet metal problem). A condition in which a flat piece of sheet metal is stretched so it snaps in
and out between rows of rivets, in the same way the bottom of an oil can snaps in and out.
oil circuit breaker. A protection device for electrical circuits that carry large amounts of current. The
housing containing the current-carrying contacts is filled with oil which quenches the arc that forms as the
contacts open. Quenching the arc prevents damage to the contacts when they open a circuit carrying a large
amount of current.
oil control ring (reciprocating engine component). One of the cast-iron rings installed in grooves around
a piston. The oil control ring is installed in the groove below the compression rings (top rings), and it is
usually made in several sections.
The oil control ring controls the amount of oil allowed to flow between the piston and the cylinder
wall. Excess oil is drained back into the crankcase through holes drilled through the piston in the ring
groove.
oil-cooled transformer (electrical transformer). An electrical transformer whose core and coils are
　
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