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obtuse triangle. A triangle that contains an obtuse angle (an angle greater than 90°). See illustration for
oblique triangle.
occluded front (meteorology). A composite of two fronts. An occluded front forms when a cold front
overtakes a warm front or a quasi-stationary front.
oceanic airspace (air traffic control). Airspace over the oceans of the world, considered international
airspace, where oceanic separation and procedures per ICAO are applied. Responsibility for the provisions
of air traffic control service in this airspace is delegated to various countries, based generally upon
geographic proximity and the availability of the required resources.
oceanic published route (air traffic control). A route established in international airspace and charted or
described in flight information publications such as Route Charts, DOD En Route Charts, Chart
Supplements, NOTAMs, and Track Messages.
O-condition (sheet metal temper condition). Fully annealed. A fully annealed aluminum alloy sheet is
said to be in its O-condition.
octagon. A plane (flat), closed, eight-sided figure. In a regular octagon, all sides have the same length, and
all of the angles are the same.
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octahedron. A solid form that has eight plane (flat) surfaces.
octal number system. A system of numbers based on eight units (zero through seven). Octal numbers are
used in digital electronics to make long strings of zeros and ones more manageable. One octal digit replaces
three binary digits.
Decimal Octal Binary
0 0 000
1 1 001
2 2 010
3 3 011
4 4 100
5 5 101
6 6 110
7 7 111
octane rating (reciprocating engine fuel rating). The system used to rate the ability of a
reciprocating-engine fuel to resist detonation. The higher the octane number, the more resistant the fuel is
to detonation.
The fuel being rated is run in a test engine whose compression ratio can be varied. This ratio is
changed until the fuel detonates, or explodes inside the cylinder, rather than burning as it should. The
engine is left with the compression ratio that caused the fuel to detonate, and a fuel composed of a mixture
of iso-octane, a fuel that resists detonation, and heptane, a fuel prone to detonate, is fed to the engine.
The ratio of the two fuels is varied until the engine detonates as it did with the fuel being rated.
When the performance with the two fuels is matched, the ratio of the octane to heptane is noted, and the
fuel is rated with the percentage of octane used. If a mixture of 80% octane and 20% heptane causes the
engine to perform as it did with the fuel being rated, the fuel is given an octane number of 80. At one time
aviation gasoline was given a dual rating such as 80/87. The first number represented its antidetonation
rating with a lean, cruise fuel-air mixture, and the higher number, its rating with a rich, takeoff mixture.
Fuel with antidetonation characteristics better than that of iso-octane is rated in performance numbers. See
performance numbers.
octave. An interval of audible vibrations spanning eight full tones above or below another. A tone, in this
regard, is a sound with a distinct pitch.
odd harmonics. The odd multiples of a fundamental frequency. For example, frequencies that are three,
five, seven, and nine times that of a fundamental, or original, frequency are odd harmonics of the
frequency.
odometer. The portion of an automobile speedometer that indicates the distance traveled.
oersted. A unit of magnetic intensity in the centimeter-gram-second system of measurement. One oersted
is the amount of magnetic intensity equal to one gilbert per square centimeter, and this is 79.577
ampere-turns per meter.
off course (air traffic control). A term used to describe a situation where an aircraft has reported a
position fix or is observed on radar at a point not on the ATC-approved route of flight.
off-idle mixture (aircraft fuel metering system adjustment). The fuel-air mixture ratio produced by a
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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fuel metering system during the time the engine is transitioning from its idle RPM to a speed at which fuel
is metered through the main metering system.
off-route obstruction clearance altitude (air traffic control). An off-route altitude which provides
obstruction clearance with a 1,000-foot buffer in nonmountainous terrain areas and a 2,000-foot buffer in
designated mountainous areas within the United States. This altitude may not provide signal coverage from
ground-based navigational aids, air traffic control radar, or communications coverage.
off-route vector (air traffic control). A vector by ATC which takes an aircraft off a previously assigned
　
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