1.37 1.49 1.01 1.45 84.4 21.5 49.7 33.8 N1 N2 ITT L oil pressure R oil pressure L oil filter  R oil filter UPPER EICAS DISPLAY 91 60 11.0 1.0 LOWER EICAS DISPLAY 98 126  108 118 49.7 49.7 15 15 L ENG Oil pressure R ENG Oil pressure EICAS Computers Oil filter differential pressure switch Oil tank Oil pressure transmitter Low oil pressure switch FADEC/EEC oil  temperature probe No. 3 bearing scavenge  oil temperature probe No. 1, 1$^{1}$/2,  and 2  bearing breather Engine oil pressure Oil filter in pressure Oil filter out pressure Oil quantity transmitter Figure 6-40. Oil indicating system.  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. 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. 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  Lubrication System Thermostatic Bypass Valves Lubrication System Check Valve 6-27