Definition
An aircraft whose cabin is not sealed and actively maintained at a higher air pressure than the surrounding atmosphere. The cabin pressure inside the aircraft is the same as the outside air pressure at whatever altitude the aircraft is flying.
Plain English
An aircraft where the air inside the cabin is the same thinness as the air outside. As the aircraft climbs, the air inside gets thinner along with the air outside.
Context Anchor
Seen in discussions of high-altitude flying, oxygen use, and pitot/static system problems where cabin air pressure may affect instrument indications.
Derivation
Pressurize' comes from the Latin 'pressura' (a pressing). The prefix 'un-' means 'not.' So an unpressurized aircraft is one that does not press extra air into the cabin to keep it denser than the outside air.
Why Pilots Care
Determines when supplemental oxygen must be used to avoid hypoxia.
Grounding Statement
As an unpressurized aircraft climbs, the cabin air pressure drops along with the surrounding outside air pressure.
Intuition Check
Unpressurized does not mean there is no air pressure inside the aircraft. It means the aircraft is not actively maintaining a higher, controlled cabin pressure as it climbs.
Example Sentence 1
Because the trainer was an unpressurized aircraft, the instructor leveled off at 10,000 feet to stay within comfortable oxygen limits.
Example Sentence 2
Many training aircraft are unpressurized, limiting their practical altitude without oxygen.