During descent, what does a blocked pitot probe cause the ASI/Mach meter to do?

When the pitot probe is blocked, it cannot accurately measure the dynamic pressure of the air flowing into it. The airspeed indicator (ASI), which relies on this dynamic pressure to provide readings, will therefore fail to interpret the actual airspeed correctly. Specifically, a blocked pitot probe results in a situation where the ASI will under-read airspeed during a descent.

As altitude decreases and an aircraft descends, the static pressure decreases. However, if the pitot tube is blocked, it cannot detect this change appropriately because it is unable to receive new air pressure data. Consequently, the ASI interprets that the dynamic pressure is lower than what it actually is, leading to an underestimation of the true airspeed.

This phenomenon can lead pilots to believe they are flying slower than they are, which may impact their performance and control of the aircraft during critical phases of flight like descent.