2.  Engine Bleed Air Distribution System____________________________________
 A.  Engine bleed air is used to supply pressure to the airplane pneumatic ducts. During low engine power operation, 15th-stage bleed air is used. During high engine power operation, 8th-stage bleed air is used. The ASCTU controls the air supply components installed on the engine and in the strut area.
 B.  The high pressure shutoff valve (HPSOV) and the high pressure controller determine when to change from high pressure to intermediate pressure (IP) to satisfy the system flow requirements. The IP check valve permits flow from the IP bleed port and prevents reverse flow from the high pressure port to the IP port.
 C.  The pressure regulating valve (PRV) and the pressure controller maintain the proper precooler inlet pressure.
 D.  The fan air modulating valve and the fan air temperature sensor control the amount of fan air that goes through the precooler. This will maintain the proper temperature of the precooler outlet air.
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 E.  The pressure regulating and shutoff valve (PRSOV) regulates the pressure in the distribution manifold and keeps it at 45 psig. The PRSOV can be closed to shutoff engine bleed air. This is done to isolate the system or when there is an overtemperature condition. During engine operation the PRSOV prevents reverse flow back to the engine. It has an override function to allow reverse flow during engine start.
 3.  Bleed Air Precooler System
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 A.  The bleed air precooler system will keep the temperature of the engine bleed air to a maximum limit of 350 °F (177 °C). There is a precooler installed on each engine. The precooler is an air-to-air heat exchanger. It uses cool fan air to remove heat from the engine bleed air before it goes into the airplane pneumatic ducts (Ref 36-12-00/001).
 4.  Indicating System
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 A.  The pressure indicating system measures pneumatic duct pressure in the left wing and the right wing manifolds. It also measures the bleed air pressure of each engine (Ref 36-21-00/001).
 B.  The temperature indicating system measures the bleed air temperature of each engine (Ref 36-22-00/001).
 C.  The bleed air flow indicating system measures the bleed air flow rate of each engine (Ref 36-23-00/001).
 D.  The ASCTU monitors the pressure and the temperature in the pneumatic ducts. It sends the values to the EICAS display. If an overpressure or an overtemperature condition occurs, the ASCTU will send a warning signal to the engine bleed control module and supply status information to EICAS and the central maintenance computer (CMC).
 E.  The duct leakage detection system uses thermal switches to detect a leak or rupture in all of the engine bleed air ducts. Thermal switches are located on all the wing leading edge and the crossover ducts in the air conditioning pack bays up to the isolation valves. The system is tested with a push of the FIRE/OVHT TEST button on the P5 pilot's overhead panel, which does a check for circuit continuity in the system (Ref 26-18-00/001). Should an overheat condition occur, caused by a leak or rupture in the pneumatic duct, it will cause the thermal switches to close and the EICAS messages to come on. The applicable EICAS messages are listed as follows:
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BLD DUCT LEAK C

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BLD DUCT LEAK L

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BLD DUCT LEAK R

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 F.  There are four amber SYS FAULT lights located on the engine bleed air system control module, M7328 on the P5 overhead panel that will come on in the event of an overheat or overpressure condition. The SYS FAULT light will also come on when the high pressure shutoff valve (HPSOV) or pressure regulating valve (PRV) does not close when commanded.
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