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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. 

## Breather Pressurizing System 

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. 

## Typical Dry-Sump Variable Pressure Lubrication System 

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. 

