(2) System Operation

TM 1-1520-238-T-5 Change 4 7–35 7–7. SYSTEM DESCRIPTION (cont) 7–7 (2) System Operation. The PAS system (fig. 7–18) has the following three modes of operation: primary mode (SDC air), secondary mode (bleed air), and external mode (AGPU). (a) During primary operation, the engines or APU drive the SDC. Air is drawn through a screen into the air particle separator which separates the dirt particles using centrifugal force. PAS air creates a low pressure area and ejects dirt particles through an exhaust tube. Clean air then passes to the SDC throttle inlet valve. The SDC throttle valve reduces the starting load on the APU by decreasing the air available to the SDC by 70%. The SDC throttle inlet valve solenoid is controlled by the pilot APU panel START/RUN switch through a 60 second time delay. The SDC throttle inlet valve is open except during APU start. Placing the START/RUN switch to RUN causes the solenoid to de–energize, restricting air from the air particle separator to the SDC for 60 seconds. The SDC compresses air to 30 ± 5 psi and raises the temperature to approximately 400° F (204° C). The surge valve maintains a constant pressure by monitoring supply air pressure, static air pressure and total air. A differential in pressure causes the valve to open or close as necessary, which prevents pressure surges that cause compressor stall. The overboard vent discharges excessive compressor air flow. The SDC outlet pressure switch monitors SDC pressure output. When pressure drops below 14 psi, the switch closes, lighting the SHAFT DRIVEN COMP indicator on the pilot’s caution/warning panel. The SDC check valve is a one–way valve which routes hot pressurized air to the air pressure manifold and is then distributed to the pneumatic systems. In the secondary and external modes, the SDC check valve prevents manifold air from entering the SDC. The air pressure regulating valve reduces manifold pressure to 19 ±3 psi for fuel system use. The utility air receptacle allows the use of a low–pressure air hose with pressure driven tools and test equipment. The SDC over temperature switch lights the SHAFT DRIVEN COMP indicator when the oil temperature is above 340° to 360° F (171° to 182° C). (b) During secondary operation, when SDC outlet pressure drops below 14 psi, the bleed air shutoff valve opens automatically, allowing 60 psi of fifth stage bleed air from engine 1 to enter the manifold. The bleed air check valve prevents manifold air from entering the bleed air shutoff valve during primary or external modes of operation. Air pressure is distributed in the same manner as the primary mode. (c) During external operation, high pressure air generated by an AGPU enters the manifold through the external air receptacle. The check valve prevents loss of manifold air during other modes of operation. Air pressure is distributed in the same manner as the primary mode.