Question

Under International Standard Atmosphere conditions, why does a constant indicated airspeed result in a higher true airspeed as an aircraft climbs to a higher altitude?

Accepted Answer

Indicated airspeed is a measure of the dynamic pressure acting on the aircraft, which is determined by the density of the air and the speed of the aircraft through that air. Dynamic pressure is the force exerted by air moving over an object. As an aircraft climbs into the International Standard Atmosphere, the air density decreases because the air molecules are spread further apart. To maintain a constant indicated airspeed, the aircraft must maintain a constant dynamic pressure. Since the air is thinner at higher altitudes, the aircraft must travel at a higher velocity to encounter enough air molecules to maintain that same dynamic pressure. This higher velocity relative to the air mass is called true airspeed. Therefore, as air density decreases with altitude, the true airspeed must increase to keep the dynamic pressure, and thus the indicated airspeed, constant. For example, if an aircraft flies at a constant indicated airspeed at sea level where the air is dense, it requires a certain velocity to produce a specific amount of dynamic pressure. At a higher altitude where the air is half as dense, the aircraft must move twice as fast to strike the same number of air molecules per second, resulting in a significantly higher true airspeed.

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Under International Standard Atmosphere conditions, why does a constant indicated airspeed result in a higher true airspeed as an aircraft climbs to a higher altitude?