In the ACS movement process, where does the air come from when the engine is idle?

In the context of the ACS (Air Control System) movement process, when the engine is idle, the air comes from P3. This is because P3 represents the high-pressure air produced by the engine that is used for various pneumatic systems within the aircraft, including starting the engine and pressurizing the cabin.

When the engine is idling, it is not generating as much thrust, which results in a decrease in airflow through the engine's core. However, P3 continues to supply the necessary air for various systems. This is essential for maintaining adequate performance in components that rely on high-pressure air, ensuring that systems such as de-icing and environmental controls function properly.

In contrast, the other components mentioned, such as P2.0 and P2.5, refer to different stages of air pressurization within the engine, primarily associated with the higher operational ranges and functions that are not typically utilized at idle. P4 also corresponds to a specific section in the air distribution system or engine, not directly delivering air during idle operations. Understanding this flow of air helps in appreciating the overall operation and efficiency of the engine's environmental control and pneumatic systems during various phases of operation.