Which instrument measures the aircraft's altitude?

• A. Variometer
• B. Altimeter
• C. Radar altimeter
• D. Airspeed indicator

Answer: (B)

The altimeter is the instrument specifically designed to measure an aircraft's altitude above a specified reference point, typically sea level. It does this by measuring the atmospheric pressure; as the aircraft ascends, the pressure decreases, and the altimeter translates this change into an altitude reading. This is essential for pilots to maintain proper altitude during flight and comply with air traffic control regulations.

While the variometer is useful for measuring the rate of climb or descent—often utilized in gliders—the primary function of altitude measurement belongs to the altimeter. The radar altimeter, on the other hand, provides an altitude reading above the terrain directly below the aircraft by using radar waves, making it ideal for lower altitude operations but not serving the wider purpose of measuring altitude in relation to sea level like the altimeter does. The airspeed indicator is focused on measuring the speed of the aircraft relative to the surrounding air and does not provide altitude information.