Revised 18-Jul-2001
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"A" System
System consists of a single reservoir . Hydraulic power is supplied to the A system from two engine driven pumps fitted to engines No1 and No2 . The pump switches control solenoid operated blocking valves, with the switches on, normal pump output is supplied to the system. A small portion of the fluid that enters the pump is circulated through the pump case for cooling and lubrication. As long as the engines are operating regardless of the blocking valve position this small amount of fluid leaves the pump and is cooled by a heat exchanger in the No3 fuel tank heat exchanger before being returned to the system. 
fluid shutoff valves controlled by there respective switches control the fluid in the engine pump supply lines, when closed fluid flow to the pump is isolated. (includes lubricating and cooling fluid). pulling the fire switch on engine 1 or 2 will close these shutoff valves, the fire switch also deactivates the low pressure warning light.
System A and therefore system B and stand by reservoirs are pressurized by engine bleed air from engines 1 & 2 this is applied to the reservoir to ensure a positive supply of fluid to the pumps. 
A balance line connects the system A reservoir to the system B reservoir at a level of 2.5 gall.


A system supplies the heavy load items landing gear, trailing edge flaps.
Fluid Quantity................4.4 Gall (gear down)
                      ................3.8 Gall (gear up)
™it's normal to see a reduction in the fluid quantity indication in flight, due to the cold soak characteristics of the system
System Pressure............2,800 - 3100 PSI (red line @ 3,500)
Balance line to "B" system...........2.5 Gall
Operating time with the "A" fluid SOV closed 5 Min's
™ A system pumps max demand flow rate is 22 gal min 100% N2

Systems Operated:
Nose Wheel Steering
 Landing Gear
 Tail Skid
 Trailing Edge Flaps
 Ground Spoilers
 Leading Edge Devices
 Lower Rudder
 Outboard Spoilers

"B" System
"B" system again contains a single reservoir supplying two electric AC pumps. It's reservoir is more complicated that system "A" in that it has an auxiliary  reservoir separated by a baffle which is open at the top ("little b"), this is reserved for alternate extension of the leading edge slats and flaps using stand by hydraulic power (more about that later).
Each pump is controlled by it's individual selection switch, with the switch ON normal hydraulic output pressure is provided to the system. As in the "A" system a portion of the fluid is used for cooling and lubrication of the pump but is returned via a heat exchanger in the no1 fuel tank. if the pressure falls below the preset level a warning light will illuminate, sensors monitor the fluid temperature and if activated will bring on an overheat light. A relay in the overheat sensing circuit will cause the overheat light to go out when the affected pump is turned off. A simplified view of the system is the same as the above diagram but with no blocking valve or shut off valve (items 2 & 4).

Fluid Quantity....................Full if system "A" quantity over 2.5 gall
System Pressure............2,800 - 3100 p.s.i (red line @ 3,500)
Baffle level (little B)............1.1 gall
Do not operate a system "B" pump with less than  1,300 lbs in fuel tank No1 
Any one hydraulic system "B" pump should not be operated more than 5 times in 5 min's. After this period the pump must be left on for 5 min's or off for 30 min's.

Systems Operated:
Inboard Spoilers
Upper Rudder
Aft Airstairs

Stand-By System
The standby system reservoir supplies fluid to an AC electric motor driven pump. Positioning the standby rudder switch or the alternate flap master to on will provide power to the standby motor, which in turn supplies fluid pressure to the standby power module. System pressure is indicated by a light. There is a overheat sensing to monitor fluid temperature which will illuminate the overheat light for the standby system.
Fluid Quantity...............0.3 gall
System pressure........... 2,200 psi @ 1.14 gall per min for leading edge devices ( the is no pressure gauge for the stand-by system only a green light.
                         ............ 2,975 to 3,075 @ 3 gll per min for lower rudder
Systems Operated:
Lower Rudder Stand-By Actuator
Leading Edge Devices (Extension Only)

Standby Rudder
Positioning  the standby rudder switch to on , the system "A" rudder power switch is selected off. With "A" system off hydraulic fluid is isolated from the lower rudder power unit, standby system pressure repositions a selector valve to allow the standby system to power the lower rudder standby power control unit.

Alternate flap
Positioning the alternate flaps master switch on  arms the alternate flap switches (2). Moving either switch to down opens the leading edge flaps and slats sov. The standby system pressure then drives a hydraulic motor portion of a pump assembly. The pump portion of this assembly uses fluid from "little b" to extend the leading edge devices.


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