Which statement accurately describes an aircraft's speed?

Airspeed refers to the speed of an aircraft relative to the surrounding air and is crucial for understanding aircraft performance, particularly during flight operations. The correct statement indicates that airspeed can differ from speed over ground, which is influenced by factors such as wind.

When flying, if an aircraft is flying into a headwind, its ground speed—the speed at which it is moving over the ground—will be less than its airspeed. Conversely, if the aircraft is in a tailwind, the ground speed will exceed the airspeed. This distinction is fundamental because pilots must consider both airspeed and ground speed during flight planning, manoeuvring, and when calculating fuel efficiency and estimated time of arrival.

The other statements do not accurately capture the relationship between airspeed and ground speed, or they misrepresent the nuances of how airspeed is measured under different conditions, making them less applicable. Understanding this difference helps in recognizing how environmental factors can impact flight dynamics and the importance of measuring airspeed for safe and efficient operation.