AS9120B, ISO 9001:2015, AND FAA AC 0056B ACCREDITED

What Is the Difference Between Airspeed and Ground Speed?

Pilots rely on four types of speed measurements when operating an aircraft: ground speed, true airspeed, indicated airspeed, and calibrated airspeed. The difference between airspeed and ground speed when discussing the speed of an aircraft is how the speed is measured. Ground speed is measured relative to the Earth below the plane while airspeed is measured relative to the air through which the plane is flying. Ground speed defines the measurement for how quickly a plane will reach its destination. Airspeed, on the other hand, provides a measurement for the effect of airflow on the plane’s ability to fly.

While meter ground speed relies on one factor, horizontal distance, for measurement, airspeed is measured in different ways for a multitude of purposes. True airspeed measures only the flow of air around the plane, but pilots factor in additional variables to the indicated airspeed shown by the airspeed indicator in order to calculate calibrated airspeed. Indicated airspeed is calculated from the pitot-static system and measures the difference between the aircraft’s total pressure and its static pressure in order to find its dynamic pressure. Indicated airspeed relies on the height of the plane in the air because air density lessens as the plane climbs higher. Lower density creates a greater difference between true airspeed and indicated airspeed. The airspeed indicator is a pilot’s instrument which measures airspeed using only the differential between static pressure of the air and the dynamic pressure, or ram pressure, of the aircraft’s forward motion.

The second way of measuring airspeed is calibrated airspeed. Calibrated airspeed is more accurate than indicated airspeed because it includes additional variables that affect the true airspeed of the aircraft. Calibrated airspeed is specific to the conditions in which the plane is flying, including altitude and temperature, as well as the shape of the aircraft wings. Since the airspeed indicator only measures pressure, not speed, it will report inaccurate readings when there are changes in pressure, altitude, and other related factors. As the flaps and other moving parts of the wing adjust during the flight, the air flows differently around the plane and this also must be accounted for. Pilots must understand the multiple types of airspeed in order to make correct calls when operating the vehicle. True airspeed is found when calibrated airspeed accurately accounts for all variables affecting the conditions the plane is flying in.

Ground speed is important for passengers and crew alike because it provides an indication of how long a flight will take to reach its destination. Airspeed indicator is mainly significant to the crew as it informs pilots on how to fly. After being measured through the pitot-tube and then calibrated in regards to environment, airspeed is frequently reported in knots or miles per hour. Knowing the conditions of the surrounding air, as well as how the airflow relates to pressure and other factors lets the pilot know when to speed up or how to avoid dipping below stall speed. Additionally, this information informs the pilot on how to bank or turn. For example, if the pilot should deploy the flaps to decrease airspeed in response to a change in airflow.

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