During normal operation, what is the primary source of air for the air-conditioning PACK?

The primary source of air for the air-conditioning PACK during normal operation is indeed both the APU (Auxiliary Power Unit) and aircraft engines. When the aircraft is in flight or during certain operational configurations, the air-conditioning system can utilize bleed air from the engines, which is a common practice as it provides a substantial and continuous source of high-pressure air required for the air-conditioning system to function effectively.

Additionally, when the aircraft is on the ground and not using engines for power, the APU serves as an auxiliary power source that can also supply bleed air for the air-conditioning system. This dual capability ensures that the air-conditioning PACK receives an adequate supply of air regardless of the aircraft's operational status, thereby maintaining a comfortable and regulated cabin environment for passengers and crew.

This dual sourcing from both the aircraft engines and the APU is critical for maintaining proper air-conditioning functionality and is an essential aspect of aircraft systems design. Ground carts, while they can provide air and power when needed, are not the primary source during normal operations, which is why they are not part of the correct answer. The reliance on engines and the APU ensures optimal performance and reliability in the air-conditioning systems aboard the aircraft.