Octane rating. A system used to rate the antidetonation characteristics of a
reciprocating engine fuel. Fuel with an octane rating of 80 performs in a
laboratory test engine the same as the fuel made of a mixture of 80% iso-octane
and 20% heptanes.
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.
Oil analysis. A
method of measuring the contents in parts per million of various chemical
elements in 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. This type of oil
analysis is called a spectrometric oil analysis program, or SOAP.
Oil dilution. A
method of temporarily decreasing the viscosity of the lubricating oil to make
it possible to start a reciprocating engine when the temperature is very low.
Before shutting the engine down, enough gasoline from the fuel system is mixed
with the lubricating oil in the engine to dilute it so the starter can turn the
engine over when the oil is cold and viscous. When the engine starts and the
oil warms up, the gasoline evaporates.
Oil-damped bearing. A type of roller bearing installation in a gas turbine engine in
which the outer race is installed in an oil damper compartment whose inside
diameter is a few thousandths of an inch larger than the outside diameter of
the outer race. Oil under pressure fills the oil damper compartment and allows
the bearing to compensate for sight misalignment and to absorb vibrations of
the shaft.
Oleo shock absorber. A shock absorber used on aircraft landing gear. The initial
landing impact is absorbed by oil transferring from one compartment in the
shock strut into another compartment through a metering orifice. The shocks of
taxiing are taken up by a cushion of compressed air.
On-condition maintenance. A maintenance program that closely monitors the operating
condition of an engine and allows major repairs or replacements to be made when
engine performance deteriorates to a specific level.
One-hundred-hour inspection. An inspection required by 14 CFR part 91, section 91.409 for
FAA-certificated aircraft operated for hire or used for flight instruction for
hire. A 100-hour inspection is identical in content to an annual inspection,
but can be conducted by an aviation maintenance technician who holds an
Airframe and Powerplant rating, but does not have an Inspection Authorization.
See 14 CFR part 43, Appendix D for list of the items that must be included in
an annual or 100-hour inspection.
On-speed condition. The speed condition in which the engine is turning at the rpm for
which the propeller governor is set.
Open angle. An
angle in which sheet metal is bent less than 90°.
Open assembly time. The period of time between the application of the glue and the
assembly of the joint components.
Open wiring. An
electrical wiring installation in which the wires are tied together in bundles
and clamped to the aircraft structure rather than being enclosed in conduit.
Open-center selector valve. A type of selector valve that functions as an unloading valve as
well as a selector valve. Open-center selector valves are installed in series,
and when no unit is actuated, fluid from the pump flows through the centers of
all the valves and returns to the reservoir. When a unit is selected for
actuation, the center of the selector valve is shut off and the fluid from the
pump goes through the selector valve into one side of the actuator. Fluid from
the other side of the actuator returns to the valve and goes back to the
reservoir through the other selector valves. When the actuation is completed,
the selector valve is placed in its neutral position. Its center opens, and
fluid from the pump flows straight through the valve.
Open-hydraulic system. A fluid power system in which the selector valves are arranged in
series with each other. Fluid flows from the pump through the center of the selector
valves, back into the reservoir when no unit is being actuated.
Operating cycle. One complete series of events in the operation of a turbine engine
that consists of starting the engine, taking off, landing, and shutting the
engine down.
Optoelectronic device. An electronic device that produces, modulates, or senses
electromagnetic radiation in the ultraviolet, visible light, or infrared
portions of the energy spectrum.
Orifice check valve. A component in a hydraulic or pneumatic system that allows
unrestricted flow in one direction, and restricted flow in the opposite
direction.
O-ring. A widely used type of
seal made in the form of a rubber ring with a round cross section. An O-ring
seals in both directions, and it can be used as a packing or a gasket.
Ornithopter. A
heavier-than-air flying machine that produces lift by flapping its wings. No
practical ornithopter has been built.
Oscilloscope. An
electrical instrument that displays on the face of a cathode-ray tube the
waveform of the electrical signal it is measuring.
Otto cycle. The
constant-volume cycle of energy transformation used by reciprocating engines. A
mixture of fuel and air is drawn into the cylinder as the piston moves to the
bottom of its stroke. The mixture is compressed as the piston moves upward in
the cylinder, and when the piston is near the top of its stroke, the mixture is
electrically ignited and burns. The burning mixture heats and expands the air
inside the cylinder and forces the piston down, performing useful work. The
piston then moves back up, forcing the burned gases out of the cylinder.
Outflow valve (pressurization component). A valve in the cabin of a pressurized aircraft
that controls the cabin pressure by opening to relieve all pressure above that
for which the cabin pressure control is set.
Overboost. A
condition of excessive manifold pressure in a reciprocating engine.
Overboosting occurs when the supercharger is operated at too high a speed.
Overrunning clutch. A type of clutch that couples an input shaft with an output shaft.
When the input shaft is driven, the output shaft rotates with it. When the
output shaft is driven, the output shaft rotates with it. But when the output
shaft is driven, the input shaft does not turn.
Overspeed condition. A speed condition in which the engine is turning at an rpm higher
than that for which the propeller governor is set.
Overvoltage protector. A component in an aircraft electrical system that opens the
alternator field circuit any time the alternator output voltage is too high.
Oxidizing flame. An oxyacetylene flame in which there is an excess of oxygen. The
inner cone is pointed and often a hissing sound is heard.