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8.  System A Hydraulic Valves
A.  System A Relief Valve
(1)  The cartridge type system A relief valve installed in the modular unit protects the system against damage by abnormally high pressures. (See Figs. 2 and 3). The valve is set to relieve at 3500 (+ 50) psi and reset at 3100 psi minimum. The valve is set by the manufacturer and no maintenance other than removal and installation is recommended. A reverse flow poppet allows fluid to reverse flow through the valve when return pressure exceeds pressure in the pressure network to compensate for contraction after shutdown.
B.  Hydraulic Supply (Fire) Shutoff Valve
(1)  The hydraulic supply (fire) shutoff valves are provided to stop the flow of hydraulic fluid to the engine area. The supply shutoff valve is a sliding gate type valve which is motor-driven by 28. The valves, one in each of the engine-driven pump supply lines, are mounted on the rear spar just aft of the engines (Fig. 1). The valves are controlled by engines No. 1 or No. 2 fire switches (Fig. 7). A position indicator on the valve provides a valve CLOSED or OPEN position indication.
543 
29-11-0 Page 12  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 15/73 

F86725
508  System A Hydraulic Fluid Heat Exchanger 
Jun 20/86  Figure 5 (Sheet 1)  29-11-0 
Page 13 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

System A Hydraulic Fluid Heat Exchanger  508 
29-11-0  Figure 5 (Sheet 2)  Jun 20/86 
Page 14 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

F86732
500  Hydraulic Supply (Fire) Shutoff Valve 
Feb 20/90  Figure 6  29-11-0 
Page 14A/14B 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

C.  Ground Interconnect Valve
(1)  The ground interconnect valve is provided to pressurize system A using system B hydraulic power source or when using an external source connected to the ground power module. A check valve installed on the system A side of the interconnect valve prevents the valve from being used to pressurize system B using system A power source, and also prevents loss of system A fluid through a failure of system B. The valve is motor-operated, consisting of a motor, cam-operated slide valve and three port housing (one port plugged) (Fig. 7). The valve can be operated on the ground only. For normal ground operation the valve is wired through the parking brake relay normally set contact. The valve is also wired to close automatically in case the interconnect valve control switch is not moved to the CLOSED position prior to releasing the parking brake. For this condition the valve receives power through the parking brake relay normally-off contact to the OPEN side of the valve. The valve is controlled by the ground interconnect valve switch on the forward overhead panel, and installed on the forward bulkhead of the wheel well. A manual override lever is provided on the valve for operating the valve on the ground without electrical power.
505 
Nov 15/67  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  29-11-0 Page 15 

F86743
 Ground Interconnect Valve  500 
29-11-0Page 16   Figure 7 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 15/67 

9.  System A Operation
A.  System A reservoir supplies a positive flow of fluid to the engine-driven pumps (Fig. 2). Fluid flows from the reservoir through supply shutoff valves to the pumps. Each pump supplies fluid from zero to approximately 22-1/2 gpm at 3000 psi through a pressure filter and check valve as demanded by the system. As pressure increases to 1200 (+ 250) psi, the pump low pressure warning switch is actuated, opening the circuit to the pump low pressure warning light. The pressure transmitter pressure line senses the increase and electrically positions the pressure indicator on the first officer's panel. As system pressure approaches 3000 psi the pumps automatically adjust output to the need of the system. If system pressure increases to 3500  (+ 50) psi the system relief valve will open to reduce pressure then reset at 3100 psi. For pump cooling and lubrication, fluid circulates through the pump and back to the reservoir through a case drain filter, check valve, and heat exchanger via a return filter module.
B.  If the system should malfunction, the pumps are depressurized by positioning the engine-driven hydraulic pump valve switch to OFF, thus activating the depressurization valve in the pump and completely stopping pump output (Ref 29-11-31, Fig. 1). However, hydraulic fluid from the reservoir will still flow through the pump casing for cooling and lubrication.
　
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