S C No. 4 No. 4 1/2 No. 5 No. 6 No. 2 No. 1 No. 3 C S S S S C C Scavenge pumps NO. 4 Bearing seal check valve Scavenge pumps Tower shaft housing S To low oil pressure switch To oil pressure transmitter Cooler bypass valve Supply oil Pressure oil Sense oil Bypass oil Scavenge oil Breather air Seal drain Chip detector Last-chance oil strainer Positive-displacement gear pump Test port To oil temperature indicator Fuel oil cooler Deaerator Oil tank To oil quantity indicator Pressure stage Scavenge stage Oil pump Filter bypass valve Deoiler Oil filter LP5 LP6 Breather  vent port To fuel pump  driver spline LP3 Accessory gearbox Oil pressure regulating valve To oil filter P switch Figure 6-45.  Typical turbine dry-sump pressure regulated lubrication system.  accessory case. The accessory gearbox oil suction pump,  located in the main accessory gearbox, scavenges oil from  the gearbox housing to the oil tank. Oil from the No. 4, No.  4 1 ⁄ 2 and No. 5 bearing accumulates in the bearing cavity and  is scavenged to the accessory gearbox.  The turbine rear bearing oil suction pump scavenges oil from  the No. 6 bearing compartment and directs the scavenged oil  through a passage in the turbine case strut. From there, it is  directed to the bearing cavity for the 4, 4 1 ⁄ 2, and 5 bearing  cavity where it joins this oil and is returned to the oil tank.  The scavenge oil passes through the deaerator as it enters  the oil tank, which separates the air mixed in the return oil.  The oil stays in the tank while the air flow into the accessory  gearbox and enters the deoiler. The breather pressurizing system ensures a proper oil  spray pattern from the main bearing oil jets and furnishes a  pressure head to the scavenge system. Breather tubes in the  compressor inlet case, the oil tank, the diffuser case, and the  turbine exhaust case are connected to external tubing at the  top of the engine. By means of this tubing, the vapor-laden  atmospheres of the various bearing compartments and the  oil tank are brought together in the deoiler in the accessory  gearbox. The deoiler separates out the oil from the air/oil  mist and vents the air back to the atmosphere.  The dry-sump variable-pressure lubrication system uses  the same basic subsystems that the regulated systems used  (pressure Scavenge breather).  [Figure 6-46]  The main  difference is that the pressure in this system is not regulated  by a regulating bypass valve. Most large turbofan engine  pressure systems are variable-pressure systems in which the  pump outlet pressure (oil pressure) depends on the engine  rpm. In other words, the pump output pressure is proportional  to the engine speed. Since the resistance to flow in the system  does not vary much during operation and the pump has only  the variable of turning faster or slower, the pressure is a  function of engine speed. As an example, oil pressure can  vary widely in this type of system, from 100 psi to over 260  psi, with the relief valve opening at about 540 psi. Typical Dry-Sump Variable Pressure  Lubrication System Breather Pressurizing System 6-30