Shaft Driven Compressor (SDC)

TM 1-1520-238-10 Change 9 2-55 2.39.1 Shaft Driven Compressor (SDC). The SDC is mounted on the main transmission accessory case and uses outside air. The air passes through a particle separa- tor and then through a butterfly-type throttle valve. This valve is closed by a time-delay solenoid when the APU START switch is engaged. The valve remains closed for 60 seconds after the APU START switch is released. The purpose of the time-delay solenoid is to reduce starting loads on the APU during external power starts. It is not ac- tivated during a battery start. The shaft driven compressor will compress air at 3:1 ratio and route it through a surge valve. This valve, mounted on top of the SDC, will open or close as necessary to keep SDC air pressure constant with variable system demand. Pressurized air then passes through a spring-operated check valve and into the PAS manifold. This check valve prevents pressurized air, from sources other than the SDC, from reverse routing back to the SDC. The SDC is lubricated by the main trans- mission accessory oil pump and has its own scavenge. If the SDC oil temperature exceeds 182 �C (360 �F), or if the SDC fails, the pilot SHAFT DRIVEN COMP caution light illuminates. 2.39.2 No. 1 Engine Bleed Air. The No. 1 engine pro- vides an alternative source of pressurized air in case of SDC failure. The engine compressor bleed air selector valve opens automatically if SDC output falls below 10 psi. Simultaneously, a spring-operated check valve opens. When closed, this valve prevents pressurized air from go- ing to the engine compressor when the SDC is operating. Each engine provides bleed air to its engine inlet to pre- vent ice formation and to keep the engine cooling louvers closed while the engine is operating. The bleed air used in these last two instances is independent of the pressurized air system. 2.39.3 External Air Source Receptacles. The recep- tacle for external air source connection (fig 2-46) is on the lower portion of No. 1 engine nacelle. When the air source is connected and operating, a normally closed check valve to the pressurized air system opens to allow air to pressurize the manifold.