Which term is defined as cabin pressure expressed as an equivalent altitude above sea level?

• A. Bernoulli's Principle
• B. Cage
• C. Cabin altitude
• D. Block altitude

Answer: (C)

Cabin altitude is cabin pressure expressed as an equivalent altitude above sea level. In a pressurized aircraft, the air inside the cabin is kept at a higher pressure than the outside air at cruising altitude. That inside pressure is described as an altitude—the height in the standard atmosphere at which you would experience the same pressure if you were on the ground. So, even though the aircraft might be cruising well above 30,000 feet, the cabin is typically maintained at a cabin altitude of around 6,000 to 8,000 feet. This makes the air feel “sea-level-ish” enough for comfortable breathing and reduces the effects of low outside pressure.

This is different from outside altitude or from other terms like pressure altitude, which are about the atmosphere outside the aircraft or the pressure corresponding to a given altitude in standard conditions. The options listed that refer to principles of fluid dynamics (like Bernoulli’s Principle) or non-aviation terms don’t describe the inside-air pressure in terms of an equivalent altitude, which is why cabin altitude is the correct concept.