Which flight instrument indicates altitude by sensing pressure changes?

• A. Altimeter
• B. Altitude (AGL)
• C. Altitude (MSL)
• D. Airworthiness

Answer: (A)

Altitude is determined by atmospheric pressure, and the instrument that uses that pressure change to show how high you are is the altimeter. It contains a sealed barometric chamber that senses ambient air pressure; as you climb and the pressure drops, the mechanism moves the hands on the dial to indicate a higher altitude. The readings are referenced to a pressure setting you select (like local QNH) so the display corresponds to altitude above mean sea level, or with other settings, height above the airport. The other options describe types of altitude references or completely different concepts, not an instrument that senses pressure to indicate altitude.