What happens to Indicated Airspeed (IAS) with a constant Mach number as altitude increases?

When an aircraft is flying at a constant Mach number and the altitude increases, the Indicated Airspeed (IAS) decreases. This is due to the relationship between Mach number, airspeed, and air density. As altitude increases, the air pressure and density decrease.

Mach number is the ratio of the true airspeed (TAS) to the speed of sound in the surrounding air, which also varies with temperature and pressure. While the Mach number is held constant, the true airspeed will increase to maintain that Mach number because the speed of sound decreases at higher altitudes due to lower temperatures. However, the Indicated Airspeed, which is derived from dynamic pressure and is affected by air density, will decrease. The altimeter and airspeed indicators work based on the denser air at lower altitudes, so as the density decreases, the IAS decreases even if the aircraft is maintaining a constant Mach number.

This phenomenon illustrates the difference between true airspeed, indicated airspeed, and groundspeed, reinforcing the importance of understanding how air density and pressure influence aircraft performance parameters at various altitudes.