Which instrument uses ambient atmospheric pressure to indicate altitude?

• A. Altimeter
• B. Altitude (AGL)
• C. Airspeed Indicator
• D. Attitude Indicator

Answer: (A)

Altitude is shown by measuring the ambient pressure of the air around the aircraft. The instrument that does this is essentially a sealed barometer with internal diaphragms that respond to the outside static pressure through a static port. Since atmospheric pressure decreases with altitude, the altimeter converts that pressure into an altitude reading. You adjust the altimeter using the local pressure setting so the display corresponds to height above mean sea level (or above the airfield, depending on the setting). This makes the altimeter the only instrument among the choices that directly indicates altitude from ambient atmospheric pressure. The other instruments relate to speed, attitude, or ground reference rather than altitude.