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In the acce ssory case, the vent (or breather) is a screen- protected opening that allows accumulated air pressure within the accessory case to escape to the atmosphere. The scavenged oil carries air into the accessory case and this air must be vented. Otherwise, the pressure buildup within the accessory case would stop the flow of oil draining from the bearing, forcing this oil past the bearing oil seals and into the compressor housing. If in enough quantity, oil leakage could cause burning and seal and bearing malfunction. The screened breat hers are usually located in the front center of the accessory case to prevent oil leakage through the breather when the aircraft is in unusual flight attitudes. Some breathers may have a baffle to prevent oil leakage during flight maneuvers. A vent that leads directly to the bearing compartment may be used in some engines. This vent equalizes pressure around the bearing surface so that the lower pressure  at the first compressor stage does not cause oil to be forced past the bearing rear oil seal into the compressor. 

## Lubrication System Check Valve 

Check valves are sometimes installed in the oil supply lines of dry-sump oil  systems to prevent reservoir oil from seeping 

(by gravity) through the oil pump elements and high-pressure lines into the engine after shutdown. Check valves, by stopping flow in an opposite direction, prevent accumulations of undue amounts of oil in the accessory gearbox, compressor rear housing, and combustion chamber. Such accumulations could cause excessive loading of the accessory drive gears during starts, contamination of the cabin pressurization air, or internal oil fires. The check valves are usually the spring-loaded ball-and-socket type constructed for free flow of pressure oil. The pressure required to open these valves varies, but the valves generally require from 2 to 5 psi to permit oil to flow to the bearings. 

## Lubrication System Thermostatic Bypass Valves 

Thermostatic bypass valves are included in oil systems using an oil cooler. Although these valves may be called different names, their purpose is always to maintain proper oil temperature by varying the proportion of the total oil flow passing through the oil cooler. A cutaway view of a typical thermostatic bypass valve is shown in Figure 6-41 . This valve consists of a valve body, having two inlet ports and one outlet port, and a spring-loaded thermostatic element 

