For a constant IAS, what happens to the Mach number as altitude rises?

As altitude increases, the Mach number tends to increase when considering constant Indicated Airspeed (IAS). This phenomenon occurs because, as altitude rises, air density decreases, leading to a reduction in the speed of sound in the atmosphere. Since Mach number is the ratio of true airspeed (TAS) to the speed of sound, even if the IAS remains the same, the TAS will actually increase with altitude due to the lower air density. Consequently, since the speed of sound decreases more significantly than the IAS remains fixed, the Mach number rises.

Understanding this relationship is crucial for pilots and aviation professionals, as managing Mach number is essential for ensuring safe and efficient flight, particularly at higher altitudes where aircraft may be transitioning into supersonic regimes.