What is the purpose of the pack cooling fans in the air conditioning system?

The role of pack cooling fans is to ensure that the air conditioning system operates effectively during conditions that may restrict air circulation, such as slow flight and when on the ground. In these scenarios, the aircraft might not be moving fast enough to allow the natural airflow over the heat exchangers to provide adequate cooling. As such, the fans actively circulate air to maintain appropriate temperature levels within the cabin and other systems, preventing overheating and ensuring passenger comfort.

In contrast, while supplying cooling air to heat exchangers during cruise flight is essential, this function primarily relies on the natural airflow due to the aircraft’s speed rather than fan assistance. The PSU (Passenger Service Unit) receives air from the air conditioning system but is not directly related to the function of the pack cooling fans. Cooling the APU compartment is an important task, but it is typically managed through separate systems and does not specifically utilize the pack cooling fans dedicated to the cabin environment. Thus, the fans' primary purpose is indeed to provide cooling air during phases like slow flight and ground operations where passive cooling may fall short.