How does the airspeed indicator function?

The airspeed indicator functions by comparing two different air pressures: the ram air pressure captured by the pitot tube and the static air pressure measured from the static port. As the aircraft moves through the atmosphere, air enters the pitot tube, which creates a pressure known as ram air pressure that increases with speed. This pressure is then compared to the static pressure, which remains fairly constant and reflects ambient atmospheric conditions around the aircraft.

By calculating the difference between these two air pressures, the airspeed indicator provides the pilot with an accurate indication of the aircraft's speed relative to the surrounding air, known as indicated airspeed (IAS). This information is crucial for safe flying, enabling pilots to make informed decisions regarding takeoff, landing, and maneuvering in flight.

The other options fail to accurately describe the airspeed indicator's functionality. For instance, measuring only static air pressure would not provide the necessary information about airspeed, and calculations based on engine RPM do not relate to airspeed measurements directly. Additionally, measuring altitude pressure fluctuations pertains more to an altimeter rather than the airspeed indicator.