With the X bleed selector in AUTO, the engine bleed valves will _________________ when the APU bleed valve is open and the APU air is supplied to the pneumatic system.

With the X bleed selector in AUTO, the engine bleed valves will _________________ when the APU bleed valve is open and the APU air is supplied to the pneumatic system.

• A. Open
• B. Remain unchanged
• C. Close
• D. Delay opening

Answer: (C)

In AUTO, the bleed system coordinates sources to avoid cross-flow. When the APU bleed valve is open and supplying air to the pneumatic system, the engine bleed valves are closed to isolate the engine bleed ducts from the APU bleed path. This prevents APU bleed air from feeding back into an engine’s bleed line and ensures the pneumatic system is supplied solely by the APU bleed. If the engine bleed valves remained open, there could be unwanted cross-flow or backflow, complicating system pressure and engine starting logic.

In AUTO, the bleed system coordinates sources to avoid cross-flow. When the APU bleed valve is open and supplying air to the pneumatic system, the engine bleed valves are closed to isolate the engine bleed ducts from the APU bleed path. This prevents APU bleed air from feeding back into an engine’s bleed line and ensures the pneumatic system is supplied solely by the APU bleed. If the engine bleed valves remained open, there could be unwanted cross-flow or backflow, complicating system pressure and engine starting logic.