What does it mean when the Mach number becomes blank on the ASI?

When the Mach number becomes blank on the Airspeed Indicator (ASI), it indicates that the aircraft is below a threshold where the Mach number can be accurately displayed, specifically below Mach 0.40. This happens because the ASI is designed to show Mach numbers at speeds where the measurement is relevant and meaningful. Below Mach 0.40, the aerodynamic effects and the behavior of the airspeed measurements may not correspond accurately to the critical subsonic speeds, leading the instrument to not display a value. Understanding this helps pilots interpret the instrument readings appropriately, ensuring they are aware of the flight conditions and limitations.