SYSTEM DESCRIPTION (cont)
(2) System Operation.
The PAS system (fig. 718) 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 deenergize, 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 pilots caution/warning panel. The SDC check valve is a oneway 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 lowpressure 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.