At what speed does the Mach number window on the ASI become inactive?

The Mach number window on the Airspeed Indicator (ASI) becomes inactive at a specific speed due to the limitations of the equipment and the nature of the measurement. In this case, the Mach number window is designed to provide accurate information about the aircraft's speed relative to the speed of sound up until the threshold of 0.40 Mach.

Above this threshold, the ASI can still provide airspeed readings, but the accuracy or relevance of the Mach number indication may not be reliable. This is because as the aircraft approaches transonic speeds, various aerodynamic effects come into play that can cause discrepancies in the indicated Mach number. Therefore, limiting the Mach number window to 0.40 ensures that pilots receive accurate and relevant information during flight operations, particularly at lower speeds where the relationship between indicated airspeed and true airspeed remains consistent and reliable.