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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 10. Flow Limiting Valve___________________
 A.  A flow limiting valve is installed in the standby system hydraulic line downstream of the standby system modular package (Fig. 1). The valve is located on the keel beam in the left wheel well just below the standby hydraulic system reservoir. The flow limiting valve controls the rate of standby system fluid flow to the leading edge flap and slat actuators. At normal temperatures with a hydraulic pressure of 200 to 3000 psi, fluid flow to the actuators is regulated at 1.35 to 1.58 gallons per minute.
 11. Leading Edge Standby Drive Shutoff Valve________________________________________
 A.  Operation of the leading edge flap and slat alternate hydraulic system is controlled by the standby drive shutoff valve. The motor-operated, two-position valve is contained in the standby modular package. The valve is located on the wheel well aft bulkhead above the keel beam. The standby drive shutoff valve connects the standby system pressure line to the leading edge flap and slat actuators. The shutoff valve is actuated by arming the alternate flap master switch and by moving the alternate flap drive control switch to DOWN. This positions the standby drive shutoff valve to allow standby pressure to extend the leading edge flaps and slats. Moving the alternate flap master switch to OFF closes the standby drive shutoff valve, thus connecting the leading edge flap and slat actuator standby pressure lines to standby return. However, the leading edge flap and slat actuators will remain extended until system B hydraulic pressure is restored.
 12. Operation_________
 A.  The leading edge flaps and slats are controlled by the trailing edge flap system. When the trailing edge flaps are retracted, the leading edge flap and slat control valve is closed. With the control valve in this position, the leading edge flap and slat actuators are held in the retracted position by system B hydraulic pressure. In addition, internal mechanical locks keep the leading edge slat actuators retracted in the absence of extend pressure (Fig. 4). When the trailing edge flaps are extended to 1- to 5-unit positions, the control valve moves to the intermediate position. In this position, the leading edge flaps fully extend and all leading edge slats (1 thru 6) extend to the intermediate position. Extension of the trailing edge flaps to the 10-unit range and beyond fully opens the control valve. In this position, hydraulic fluid is ported to fully extend all leading edge slats (1 thru 6) while the leading edge flaps remain extended. If system B hydraulic pressure drops below 2000 psi, the flap and slat actuator blocking valves will hydraulically lock the actuators in the position at which the loss of pressure occured. This locking action prevents aerodynamic blowback of the leading edge flaps and slats.
 B.  Retraction of the trailing edge flaps also retracts the leading edge flaps and slats. As the trailing edge flaps retract to the 10- to 5-unit range, the leading edge flap and slat control valve moves to intermediate position. This retracts all leading edge slats (1 thru 6) to the intermediate position, while leading edge flaps remain fully extended. Further retraction of the trailing edge flaps to the FLAP UP position closes the leading edge flap and slat control valve, retracting all leading edge flaps and slats.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 C.  The hydraulic system power transfer unit will automatically activate and supply standby hydraulic power if system B engine-driven pump fails, airplane is in the air mode and trailing edge flaps are not up (Ref 29-00-00, MP).
 D.  The standby hydraulic system will extend the leading edge flaps and slats if system B hydraulic pressure is lost. The standby system is activated by the alternate flap master switch on the overhead panel. Actuation of the master switch to ARM energizes the standby system. Moving the alternate flap drive control switch to DOWN completes a circuit which opens the leading edge flap standby drive shutoff valve. Standby system hydraulic pressure opens the standby system blocking valve in each leading edge flap and slat actuator and extends the leading edge devices simultaneously. The leading edge flaps and slats will remain extended until system B hydraulic pressure is restored.
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