only. If the left or right engine bleed air is secured, or if a rupture occurs in the left or right bleed-air manifold, the appropriate check valve closes. This allows   the   air-conditioning   and   pressurization subsystems  to  operate  from  the  opposite  bleed- air source. An open engine bleed-air bypass and shutoff  valve  allows  bleed  air  to  bypass  the check  valves  and  flow  from  the  left-to-right  or right-to-left   manifolds.   The   engine   bleed-air bypass  and  shutoff  valve  is  open  during  engine starting. It is also open when operating the deicing system  with  one  engine  secured. Bleed-air pressure is sensed by the bleed-air pressure transmitter located in the bleed-air supply duct  downstream  of  the  crossover  duct  isolation check  valves.  The  pressure  is  displayed  on  the bleed-air pressure indicator on the environmental panel. Bleed  air  from  the  left  and  right  manifolds flows through the crossover duct isolation check valves  to  the  bleed-air  flow  control  and  shutoff valve.  The  bleed-air  flow  control  and  shutoff  valve is   electrically   controlled   and   pneumatically actuated  to  modulate  the  bleed-air  flow  to  the  air- conditioning   and   pressurization   systems   in response  to  predetermined  flow  schedules. Two  alternate  air  supply  sources,  APU  air  and ground  start  air,  connect  to  the  left  and  right bleed-air  manifold.  The  APU  air  duct  supplies bleed  air  through  two  check  valves  to  the  left manifold.  The  ground  start  duct  supplies  high- pressure  air  through  a  check  valve  to  the  right manifold.   These   alternate   air   supply   sources are  used  primarily  for  starting  engines  and for  ground  operation  of  the  air-conditioning system. APU Bleed Air Bleed  air  flows  from  the  APU  compressor through  two  one-way  check  valves  in  the  APU duct to the left one-way check valve in the cross- breed manifold. Bleed air is also supplied to the bleed-air shutoff valve, the left side of the engine bleed-air  bypass  and  shutoff  valve,  empennage deice valve, the left wing deice valve, and the ram air anti-icing valve. With the bleed-air switch in the  ON  position,  the  left  bleed-air  shutoff  valve is   opened.   In   the   open   position,   bleed   air   is supplied to the left engine starter control valve. To provide bleed air to the right side of the cross- breed manifold, the engine bleed-air bypass and shutoff valve is opened to allow bleed air to the right bleed-air shutoff valve and to the right wing deice valve. With the bleed-air engine No. 2 switch set  to  ON,  the  right  bleed-air  shutoff  valve  is opened. In the open position, bleed air is supplied to  the  right  engine  starter-control  valve. SE Ground Start Air When support equipment is the source of air, the  ground  air  start  hose  is  connected  to  the ground  start  connection  nipple  located  in  the  right wheel  well.  External  high-pressure  air  flows through the engine starting duct check valve into the  right  cross-bleed  manifold.  Normal  flow  is through the right one-way check valve in the cross- breed manifold to the bleed-air flow control and shutoff  valve.  High-pressure  air  is  available  to  the right  bleed-air  shutoff  valve  and  the  right  wing deice  valve.  Opening  the  right  bleed-air  shutoff valve  provides  air  to  the  right  engine  starter- control  valve. To  provide  SE  air  to  the  left  cross-bleed manifold, the engine bleed-air bypass and shutoff valve is opened. When the valve is open, air flows around  the  cross-bleed  check  valves  to  the  left bleed-air shutoff valve, the left wing deice valve, the ram air anti-icing valve, and the empennage deicing  valve.  To  provide  air  to  the  left  engine starter-control  valve,  open  the  left  bleed-air shutoff  valve. SYSTEM  COMPONENTS Now that you are familiar with the operation of   the   system   as   a   whole,   let’s   look   at   its components  and  their  operation.  Knowledge  of the  individual  components  makes  troubleshooting easier and faster. To aid you in locating parts of the components, numbers within parenthesis ( ) are included that correlate to the numbers on the illustrations. High-stage  Bleed-Air  Regulator Valve The high-stage, bleed-air regulator valve is a normally  closed,  differential-pressure  regulator. 4-3