Aviation Maintenance Terms beginning with O
Oblique view
A view that is similar to an isometric view except with two of the three drawing axes always at right angles to each other.
Ohm’s Law
Explains the relationship between voltage, current, and resistance in an electrical circuit, and states that current flow in an electrical circuit is directly proportional to the amount of voltage applied to the circuit.
Ohmmeter
A current measuring instrument that provides its own source (self-excited) of power.
Ohm
The standard unit used to measure resistance.
One-hundred-hour inspection
A complete inspection that is required for all aircraft operated for hire every 100 hours. An annual inspection must be conducted by an A&P mechanic with Inspection Authorization.
Operating center of gravity range
The center of gravity for an aircraft loaded and ready for flight.
Orthographic projection
A method of showing all six possible views of an object: front, top, bottom, rear, right side, and left side.
Outside calipers
Used for measuring outside dimensions, such as the diameter of a piece of round stock
Oblique shock wave
A pressure wave that forms on a sharp- pointed object when air flows past it at a supersonic speed.
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.
Odometer
The portion of an automobile speedometer that indicates the distance traveled.
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.
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.
On-speed condition
The speed condition in which the engine is turning at the rpm for which the propeller governor is set.
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.
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.
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.
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.
Oblique shock wave
A shock wave that forms on a sharp- pointed object moving through air at a speed greater than the speed of sound. Air passing through an oblique shock wave is slowed down, but not to a subsonic speed, and its static pressure is increased.
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.
Octane rating
A rating of the anti-detonation characteristics of a reciprocating engine fuel. It is based on the performance of the fuel in a special test engine. When a fuel is given a dual rating such as 80/87, the first number is its anti-detonating rating with a lean fuel-air mixture, and the higher number is its rating with a rich mixture.
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-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.
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 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.
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.
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.
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.
Ozone
An unstable form of oxygen produced when an electric spark passes through the air. Ozone is harmful to rubber products.
Operational control
With respect to a flight, means the exercise of authority over initiating, conducting or terminating a flight.
Open circuit
An incomplete or broken electrical circuit.
Open-grained wood
Common classification of painters for woods with large pores, such as oak, ash, chestnut, and walnut. Also known as “coarsetextured.”
Operational check
An operational test to determine whether a system or component is functioning properly in all aspects in conformance with minimum acceptable manufacture design specifications.
Optical fiber
Any filament or fiber made of dielectric materials that guides light whether or not it is used to transmit signals.
Orifice
Opening through which gas or air flows. It is usually the final opening controlled by a valve.
Oxidizing
Combining oxygen with any other substance. For example, a metal is oxidized when the metal is burned, i. e. , oxygen is combined with all the metal or parts of it oxidizing flame.
Oxidizing flame
An oxy-fuel gas flame having an oxidizing effect due to excess oxygen.
Oxygen cutting
Cutting metal using the oxygen jet which is added to an oxygen-acetylene flame.
Oxygen regulator
Manually-adjustable device used to reduce cylinder pressure to torch pressure and to keep the pressure constant. They are never to be used as fuel gas regulators.
Own-ship
From the perspective of a flight crew, or of the ASSAP and CDTI functions used by that flight crew, the own-ship is the ASA participant (aircraft or vehicle) that carries that flight crew and those ASSAP and CDTI functions.
Overpressure
The tire is required to withstand, for at least 3 seconds without rupture, a minimum test inflation pressure of at least 3.0 times the rated inflation pressure at ambient temperature.
Overhaul
Complete teardown and reassembly associated with major repair or maintenance. The terms overhaul and reconditioning are synonymous for fixed pitch propellers.