Which source supplies bleed air to the PACKS?

The source that supplies bleed air to the PACKS is external bleed air. In the context of aircraft systems, external bleed air typically refers to the pressurized air tapped from the engines or APU (Auxiliary Power Unit) when the aircraft is on the ground, or from one of the engines during flight. The bleed air is used to operate the environmental control systems, including the PACKs, which are responsible for providing conditioned air to the cabin and ensuring proper environmental conditions for passengers and crew.

Other sources, such as aerial bleed air, do not function in the same manner because they typically refer to bleed air sourced from aircraft engines during flight. Ground bleed air generally signifies air sourced at the airport, which is not the standard terminology used for bleed air during normal operations. Fuel bleed air is not a valid term in the context of bleed air systems; fuel components do not provide bleed air for PACK operation. Understanding these distinctions helps clarify why external bleed air is the correct answer in relation to how PACK systems are supplied with bleed air.