What is the function of an 'airspeed indicator'?

The function of an airspeed indicator is to measure the aircraft’s speed relative to the surrounding air, which is essential for safe and effective flight operations. This device utilizes the principle of differential pressure; it compares the pressure from the air entering a pitot tube (dynamic pressure) with the ambient pressure. The result is a reading that indicates how fast the aircraft is moving through the air, which is crucial for optimizing performance during various phases of flight, such as takeoff, climb, and landing.

Understanding an aircraft's speed relative to the air around it allows pilots to make critical decisions regarding maneuvering, stall awareness, and overall aircraft control. Maintaining an appropriate speed is vital for ensuring that the wings generate sufficient lift to sustain flight and to avoid stalling. The other options do not pertain directly to the function of the airspeed indicator, as altitude measurement, fuel efficiency monitoring, and engine power settings are indicated by different instruments.