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