Which sources are capable of providing air to the aircraft's packs?

The sources capable of providing air to the aircraft's packs include engines, the Auxiliary Power Unit (APU), and a ground air cart. Each of these plays a significant role in supplying the necessary air for the aircraft's environmental control system.

Engines provide bleed air from the compression stages, which is essential during flight and when the aircraft is running to maintain cabin pressure and temperature. The APU serves as a reliable alternative source of bleed air, especially when the aircraft is on the ground and the engines are not operating. Additionally, a ground air cart can be connected to the aircraft while it is stationary on the ground; it supplies conditioned air to the packs, allowing for cabin climate control without the need to start the engines or APU.

This comprehensive capability ensures that the aircraft can maintain comfortable conditions for passengers and crew, regardless of whether it is airborne or on the ground.