Inlet air pressure flows through the filters. to the shuttle valve (14), which selects the higher air pressure on each side of the butterfly (15) and routes the selected pressure to the solenoid (2) and chamber B (8). With the solenoid de-energized (as shown in figure 4-3), the opening side of the actuator (4), or chamber A (6), is vented (5) to ambient pressure through the solenoid. The re- sulting pressure differential between chambers A and B produces a force to keep the butterfly closed. A butterfly position indicator switch (3) controls a light (1) that indicates the butterfly is in a closed position. With the solenoid energized (opposite to the position shown in figure 4-3), air pressure is ported to chamber A, which opens the butterfly and keeps it open. In the event of an overpressure that causes the inlet pressure downstream (16) of the butterfly to attain the preset value of the pressure switch (7), the switch actuates and de-energizes the solenoid electrical circuit to close the valve. When the inlet pressure returns to the switch reset value, the electrical circuit closes to re-establish solenoid control. In the event of an overtemperature that causes the inlet temperature to attain the preset value of the temperature switch (9), the switch de-energizes the solenoid and closes the valve. When the temperature returns to the switch reset value, the solenoid re-establishes control. Check Valves Five check valves are used in the bleed-air system: two in the cross-bleed duct, two in the auxiliary power unit (APU) bleed-air duct, and one in the ground starting duct. These are 3-inch diameter, insert-type, spring-loaded closed split- flapper valves, which are designed to be inserted into, and contained by, the aircraft duct. Low-Stage Bleed-Air Check Valve The low-stage bleed-air check valve is installed in the engine pylon bleed-air duct on the right side of the engine. The low-stage bleed-air check valve allows bleed air from the 10th-stage engine compressor to enter the bleed-air subsystem to protect the engine when high-stage bleed air is scheduled. The low-stage bleed-air check valve consists of a main housing and two semicircular flappers hinged on a post positioned radially through the center of the housing. The low-stage bleed-air check valve permits flow in the direction indicated by the arrow, and restricts flow in the opposite direction. The flappers are spring-loaded in the closed position. Engine Bleed-Air Bypass and Shutoff Valve The engine bleed-air bypass and shutoff valve, located in the cross-bleed manifold, is normally closed. It is open for engine starting and during single-engine, wing-deicing operations. (See figure 4-4.) When the solenoid (1) is energized, the shuttle valve (7) senses the higher pressure air from the right and left pressure inlets (3 and 5) and directs it through the solenoid to chamber A (6) to open the butterfly (4). When the solenoid is de-energized, air bypasses the solenoid and enters chamber B (2) to assist the spring in closing the engine bleed-air bypass and shutoff valve. Bleed-Air Flow Control and Shutoff Valve The bleed-air flow control and shutoff valve is a normally closed valve with two flow schedules: fixed and inlet pressure regulated. (See figure 4-5.) The valve is electropneumatically controlled and pneumatically actuated. The venturi inlet (17) and throat pressure (18) are routed to the delta-P servo diaphragm (22). As the inlet pressure to the bleed-air flow control and shutoff valve is increased, regulated pressure routed to the actuator diaphragm (13) causes the butterfly (15) to open. When the resultant venturi delta-P reaches the predetermined value, as set by the calibration spring (12), the delta-P servo diaphragm moves. This causes the flexure beam (11) to lift off the servo valve and seat (20). This decreases pressure downstream of the control orifice (3), which closes the butterfly to a position that maintains the desired venturi delta-P. This delta-P corresponds to the desired high-flow setting when solenoid A (26) is de-energized. When solenoid A is energized, regulated pressure acting on the high-flow low-flow reset diaphragm (7) moves the reset lever to the low- flow stop (10) and reduces the calibration spring load on the delta-P servo diaphragm. This causes the delta-P servo diaphragm to regulate the airflow at low condition. Solenoid A is operated electrically by an altitude switch (25). As the venturi inlet pressure increases, the inlet pressure compensating piston (5) moves against the reset lever (9) and modulates the air flow to a low value. The inlet pressure compensating spring preload and rate are selected to provide a prescribed schedule. When solenoid B (27) is energized, actuator pressure is vented to ambient, and the butterfly valve closes. 4-6