Which instrument is used to determine the aircraft's speed relative to the surrounding air?

The airspeed indicator is the instrument specifically designed to measure the aircraft's speed relative to the surrounding air. It functions by comparing the dynamic pressure of the air flowing into the pitot tube with the static pressure from the static port. The difference between these pressures gives the airspeed, which is vital for the pilot to ensure safe flight operations, such as maintaining appropriate speeds for takeoff, landing, and maneuvering.

The other instruments serve different purposes: the altimeter measures altitude based on atmospheric pressure, the vertical speed indicator shows the rate of climb or descent, and the heading indicator provides the direction in which the aircraft is pointed. Each of these instruments plays an essential role in flight, but only the airspeed indicator directly provides information about the aircraft's speed in relation to the air around it.