Question about Autopilot

Why does the airspeed change from IAS to MACH above 26,000ft?

As the aircraft gets higher and higher, the airspeed starts to drop but the actual speed of the aircraft keep increasing. Since the speed is so high, it is more efficient to measure in Mach speed

At flight level 280 or 28,000 ft that happens

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Ok I understand now

At higher altitudes , the air density is lower than at sea level. Because the speed of sound increases with air temperature, and air temperature generally decreases with altitude , the true airspeed for a given Mach number generally decreases with altitude .

Credit: https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics)

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By request