What is the primary source of pressurization air in most jet aircraft?

The primary source of pressurization air in most jet aircraft is engine bleed air. In jet engines, some of the high-pressure air is extracted from the compressor section of the engine, often referred to as bleed air. This air is at a higher pressure and temperature, making it suitable for use in the pressurization system. After being extracted, the bleed air is conditioned—cooled and dehumidified—before being sent into the aircraft's cabin to maintain a comfortable environment for passengers and crew.

Cabin air recirculation, while it plays a role in maintaining cabin comfort by mixing fresh air with the air already inside the cabin, is not the primary source of pressurization air. Fuel system vapor is not used for pressurization purposes, as it is primarily a byproduct of fuel management and storage within the aircraft. Ambient air, while it can be utilized in some aircraft designs, particularly in lower altitude situations, is not the main source of pressurized air in jet aircraft where high altitude travel is standard. Therefore, engine bleed air serves as the essential method for providing the necessary cabin pressurization in most jet aircraft.