Boeing 737 Systems
The Boeing 737 is equipped with three hydraulic systems which power the different flight control units during flight operation. They are hydraulic system A, system B and the standby hydraulic system. Under normal conditions systems A and B supply hydraulic pressure to all flight systems connected to them. In case of a hydraulic failure, a standby hydraulic system and Power Transfer Units (PTU) is available.
Boeing 737 Hydraulic Systems

In normal operation, hydraulic system A and B power all flight systems connected to them. System A and B operate independently at 3000 psi normal pressure and have hydraulic fluid reservoirs located in the main wheel well. Both systems are almost identical to each other. The standby system is connected to hydraulic system B by a hydraulic servicing line (figure 1.1). This means that system B supplies pressure for the standby system.

Figure 1.1 - Boeing 737 Hydraulic System Overview

A hydraulic reservoir pressurization system supplies pressurized filtered air from the pneumatic system crossover manifold to main hydraulic system reservoirs in order to supply hydraulic fluid under pressure to the hydraulic pumps. System A is pressurized by the number one Engine Driven hydraulic Pump (EDP) and an AC Electric Motor Driven Pump (EMDP). Normally the number two generator powers the EMDP for system A. In the contrary system B is pressurized by the number two EDP and an AC EMDP which is powered by the number one generator.

A schematic presentation gives an overview of the units that are powered by the different hy-draulic systems. In a situation where both systems A and B do not operate, the standby system is available to supply hydraulic pressure to the rudder, the LE devices, autoslat system and thrust reversers.

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