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reaches the oil seal.
oil sump (engine lubricating system component). An engine component designed to hold the lubricating
oil. In a wet-sump engine, the entire supply of oil is held in the sump. In a dry-sump engine, the sump is
small and holds the oil only until the scavenger pump can move it into an external oil tank.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 415
oil tank pressurizing valve (turbine lubricating system component). A check valve in the oil tank vent
system that maintains an air pressure of between 3 and 6 psi on the oil. This pressure assures a positive
flow of oil to the engine-driven oil pump at all altitudes.
oil temperature indicator (aircraft engine instrument). An engine instrument that shows the pilot or
flight engineer the temperature of the oil as it enters the engine.
oil temperature regulator (reciprocating engine component). The device used to keep the temperature
of the oil entering the engine within allowable limits. This type of regulator controls the temperature by
directing hot oil through the core of the oil cooler and cold oil around the cooler or between the core and
the shell.
oil wiper ring (reciprocating engine component). One of the cast-iron rings installed in the grooves
around a piston. The oil wiper ring is the bottom ring on a piston, and its purpose is to direct oil up between
the piston and the cylinder wall for lubrication and sealing. The amount of oil allowed to remain between
the piston and the cylinder wall is determined by the oil control ring. See oil control ring.
oleo strut (aircraft shock absorber). A type of hydropneumatic (oil-air) shock absorber used on aircraft
landing gear. The shock of the landing impact is absorbed by oil flowing through a metering orifice
(restricting hole). The smaller shocks from taxiing are taken up by a cushion of compressed air.
Omega navigation system. A worldwide, electronic, hyperbolic navigation system. A master and one or
more slave stations transmit a CW (continuous wave) signal simultaneously on the same VLF
(very-low-frequency) carrier. The receiving aircraft measures the phase difference between these signals as
they are received, and plots, on a hyperbolic chart, the line of position on which this phase difference can
occur. The lines of position from two Omega stations cross at the location of the aircraft.
OMNI (electronic navigation system). A term used for the VOR, Very-high-frequency Omni-Range,
navigation system. See VOR.
omni bearing selector (electronic navigation instrument). A component in the VOR system on which
the pilot selects the radial from the VOR station along which he wishes to fly. The omni bearing selector
(OBS) shifts the phase of the reference signal inside the instrument, so the needle on the course deviation
indicator (CDI) will center when the aircraft is on the selected radial.
omnidirectional antenna (radio antenna). A type of radio antenna that has the same field strength in all
horizontal directions. An omnidirectional antenna is also called a nondirectional antenna.
omnidirectional microphone. A microphone that picks up sounds equally well from all directions around
it. An omnidirectional microphone is also called a nondirectional microphone.
on-condition maintenance (aircraft maintenance). A type of aircraft maintenance in which parts are
replaced only when their condition is such that they appear to be no longer airworthy. On-condition
maintenance differs from life-limited maintenance, in which a part is replaced at the end of a specified
number of operational hours or operational cycles, regardless of its apparent condition.
on-course indication (air traffic control). An indication on an instrument which provides the pilot a
visual means of determining that the aircraft is located on the center line of a given navigational track, or an
indication on a radarscope that an aircraft is on a given track.
one-hundred-and-eighty-degree ambiguity (navigation system error). An error inherent in some types
of radio direction finding equipment in which the equipment cannot distinguish whether the signal being
received is ahead of the aircraft or behind it. Automatic direction finding (ADF) equipment solves this
problem by using a sense antenna.
one-hundred-hour inspection (aircraft maintenance inspection). An inspection required by 14 CFR
91.409 for FAA-certificated aircraft that are operated for hire, or are used for flight instruction for hire.
A 100-hour inspection is identical in content to an annual inspection, but it can be conducted by an
aircraft mechanic who holds an Airframe and Powerplant rating, but does not have an Inspection
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
　
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