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B.  The main reservoir is an airtight vessel consisting of a metal shell, return baffle, outlet baffle and supply, return drain, overflow, pressurization, and balance line ports fitted with attaching hoses. The baffle wall just inside the return port prevents aeration of fluid. A radial baffle at the outlet port prevents vortexing. The system A, system B, and standby reservoirs are connected together by balance lines to provide pressurization, makeup fluid, and thermal expansion space. The balance lines are also used as fill lines during fluid servicing (Ref 12-12-0).
3.  System A Engine-Driven Pumps
A.  Hydraulic system A pressure is supplied by two engine-driven hydraulic pumps. The pumps are mounted on the lower right side of engines No. 1 and 2 (Fig. 1). Each pump is a pressure compensated, variable displacement pump which can be depressurized by an electrically controlled depressurizing valve. Each pump delivers a maximum flow of about 22 gpm at approximately 3000 psi, upon demand by the hydraulic system. For a more detailed description of the engine-driven pump, refer to 29-11-31.
540 
Jun 20/85  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  29-11-0 Page 7 

4.  System A Modular Unit
A.  The modular unit is provided to manifold hydraulic fluid to various easily replaceable cartridge type components (Fig. 3). The modular unit consists of a housing with provisions for installing the cartridge type hydraulic pump pressure filters, pump low pressure warning switches, pressure relief valve, check valves, and male fittings for attaching system tubing connectors. The modular unit housing has drilled passages to provide fluid flow through the unit. The modular unit is installed on the forward bulkhead of the left wheel well.
5.  System A Hydraulic Filters
A.  Pressure Filters
(1)  A nonbypass cartridge type pressure filter in the pressure line from each engine-driven hydraulic pump filters the hydraulic fluid before it enters the various systems (Fig. 2). The pressure filters are installed in the system A modular unit, and consist of a filter bowl and a noncleanable filter element (Fig. 4).
B.  Return Filter (Fig. 4)
(1)  
A return filter installed in the return line near the reservoir filters the hydraulic fluid before it enters the reservoir. The filter consists of a filter bowl, replaceable filter element and a filter head. The filter head incorporates a bypass valve, shutoff valve, two check valves and a differential pressure indicator. The bypass valve bypasses the hydraulic fluid in the event of excessive restriction to flow through the filter element. When an excessive flow restriction causes a pressure drop across the element, the differential pressure indicator will become visible, indicating that the element is contaminated and should be replaced. The shutoff valve is provided to prevent draining the reservoir when the filter bowl is removed.

(2)  
The check valves are installed in the filter head to direct fluid flow through the filter element from the outside in, and to prevent contaminated fluid from being drawn out into the return system from the IN port of the filter when the system is depressurized and a hydraulic component is manually actuated.

559 
29-11-0 Page 8  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Oct 20/83 

F86773 F86779 F86708
505  System A Modular Unit 
Nov 15/67  Figure 3  29-11-0 
Page 9 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

System A Hydraulic Filters  500 
29-11-0  Figure 4 (Sheet 1)  May 15/67 
Page 10 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

559  System A Hydraulic Filters 
Aug 15/76  Figure 4 (Sheet 2)  29-11-0 
Page 11 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

C.  Case Drain Filters
(1)  A case drain filter, installed in each engine-driven hydraulic pump case return line, is provided to detect incipient failures of a hydraulic pump and therefore prevent contamination of the hydraulic reservoir and possible damage to the remaining hydraulic pump. The filters are installed on the right side of the engines (Fig. 1). The filter consists of a filter head, a filter element and a filter bowl. The filter element and a magnetic plug provide the check points for detection of incipient pump failures (Fig. 4).
7.  System A Hydraulic Fluid Heat Exchanger
A.  A hydraulic fluid heat exchanger is provided to cool the hydraulic fluid by transferring heat from the fluid to the fuel. The heat exchanger is in the return line common to both engine-driven pumps, and consists of two coils of tubing mounted in the bottom of the No. 1 fuel tank. Fluid circulates through the heat exchanger whenever system A engine-driven pumps are in operation. The heat exchanger fluid inlet and outlet ports are in the fuel tank wing rear spar (Fig. 5).
　
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