If the engine is at idle and insufficient air is drawn from P2.5, which source is used instead?

In the context of the Pilatus PC-12 NG's engine system, when the engine is at idle and insufficient air is drawn from the P2.5 port, the alternate source for engine performance and operation is the P3 air.

P2.5 air is primarily used for engine control and operation during normal operations. However, when the engine is at idle, the demand for air may drop, and if the P2.5 port does not provide enough air for the engine’s requirements, the system automatically shifts to utilize P3 air instead. The reason P3 air is used is that it is compressed air sourced from the engine's high-pressure compressor, thus ensuring that even at low power settings, the engine receives adequate air to maintain proper combustion and performance characteristics.

This design allows for an effective backup system that ensures engine performance remains stable under various operating conditions, particularly when primary air sources might be insufficient.