Pressurization — Maintaining cabin air pressure above ambient to allow safe flight at high altitude.

Pressurization is the process of pumping compressed air (bleed air from engines or electric compressors) into the aircraft cabin to maintain a safe and comfortable pressure altitude inside, while the aircraft flies at high altitude where ambient pressure is dangerously low.

Typical cabin pressure altitude is maintained at 6,000-8,000 ft equivalent, even when the aircraft is cruising at FL350-FL410. The pressure differential between inside and outside can exceed 8 PSI, placing significant structural loads on the fuselage. The B787 and A350 maintain lower cabin altitudes (6,000 ft) thanks to composite fuselage construction.