The cabin altimeter indicates which two readings?

• A. Cabin altitude and cabin differential pressure
• B. Cabin altitude only
• C. Cabin differential pressure only
• D. Outside air temperature and humidity

Answer: (A)

The cabin altimeter is designed to display two pressure readings: the cabin altitude and the cabin differential pressure. Cabin altitude is the equivalent altitude inside the cabin based on the internal air pressure, and it tells you how “high” the cabin is pressurized relative to sea level. The cabin differential pressure is the difference between the inside cabin pressure and the outside ambient pressure; this tells you how much pressure the fuselage is must withstand.

Knowing both readings is essential: you monitor cabin altitude to ensure passengers and crew aren’t exposed to too-low pressure, and you monitor differential pressure to stay within structural limits of the aircraft. Outside air temperature and humidity are measured by different sensors and are not shown on the cabin altimeter.