APU Bleed Air Valve The APU bleed air valve permits APU bleed air to flow to the bleed air duct. The valve closes automatically when the APU is shut down. The APU bleed air valve is DC controlled and pressure operated. With both the APU and engine bleed air valves open, and the engines operating at idle thrust, there is a possibility of APU bleed air back pressuring the 9th stage modulating and shutoff valve. This would cause the 9th stage valve to close.