aircraft systems also have an oil quantity indicating system  that shows the quantity of oil during flight. One type system  consists essentially of an arm and float mechanism that rides  the level of the oil and actuates an electric transmitter on top  of the tank. The transmitter is connected to a cockpit gauge  that indicates the quantity of oil. Oil entering the engine is pressurized, filtered, and regulated  by units within the engine. They are discussed along with  the external oil system to provide a concept of the complete  oil system. As oil enters the engine, it is pressurized by a gear-type  pump.  [Figure 6-6]  This pump is a positive displacement  pump that consists of two meshed gears that revolve inside  the housing. The clearance between the teeth and housing is  small. The pump inlet is located on the left and the discharge  port is connected to the engine’s system pressure line. One  gear is attached to a splined drive shaft that extends from  the pump housing to an accessory drive shaft on the engine.  Seals are used to prevent leakage around the drive shaft. As  the lower gear is rotated counterclockwise, the driven idler  gear turns clockwise. As oil enters the gear chamber, it is picked up by the gear  teeth, trapped between them and the sides of the gear  chamber, is carried around the outside of the gears, and  discharged from the pressure port into the oil screen passage.  The pressurized oil flows to the oil filter, where any solid  particles suspended in the oil are separated from it, preventing  possible damage to moving parts of the engine. Oil under pressure then opens the oil filter check valve  mounted in the top of the filter. This valve is used mostly  with dry sump radial engines and is closed by a light spring  loading of 1 to 3 pounds per square inch (psi) when the engine  is not operating to prevent gravity-fed oil from entering  the engine and settling in the lower cylinders or sump area  of the engine. If oil were allowed to gradually seep by the  rings of the piston and fill the combustion chamber, it could  cause a liquid lock. This could happen if the valves on the  cylinder were both closed, and the engine was cranked for  start. Damage could occur to the engine. The oil filter bypass valve, located between the pressure side  of the oil pump and the oil filter, permits unfiltered oil to  bypass the filter and enter the engine if the oil filter is clogged  or during cold weather if congealed oil is blocking the filter  during engine start. The spring loading on the bypass valve  allows the valve to open before the oil pressure collapses  the filter; in the case of cold, congealed oil, it provides a  low-resistance path around the filter. Dirty oil in an engine  is better than no lubrication. The oil filter used on an aircraft engine is usually one of four  types: screen, Cuno, canister, or spin-on. A screen-type filter  with its double-walled construction provides a large filtering  area in a compact unit.  [Figure 6-6]  As oil passes through the  Oil Pump Oil Filters Relief valve Oil filter Gear-type oil pump Bypass valve Oil valve Oil pressure passage to engine Figure 6-6.  Engine oil pump and associated units.  6-7