As altitude increases, how does indicated airspeed behave relative to true airspeed?

As altitude increases, the relationship between indicated airspeed and true airspeed is such that indicated airspeed tends to decrease relative to true airspeed. This phenomenon is primarily due to the fact that indicated airspeed is derived from static air pressure and dynamic air pressure, which are affected by atmospheric conditions.

At higher altitudes, the air density decreases. Because indicated airspeed is impacted by the density of the air in which the aircraft is flying, a decrease in air density at higher altitudes results in a lower reading of indicated airspeed when compared to true airspeed. True airspeed, on the other hand, is the actual speed of the aircraft through the air and increases as altitude rises, due to the reduction in drag and less resistance from the thinner air.

This understanding highlights the importance of differentiating between indicated airspeed and true airspeed when analyzing aircraft performance across varying altitudes.