How does the ASI/Mach meter behave during descent with a blocked static probe?

When a static probe becomes blocked during descent, the impact on the ASI (Airspeed Indicator) and Mach meter is significant due to the nature of how these instruments function. The ASI relies on the difference between static pressure (from the blocked probe) and total pressure (from the pitot tube) to indicate airspeed.

In the context of a blocked static port, the static pressure reading becomes erroneous. During descent, as altitude decreases, the static pressure naturally increases. However, a blocked static port means that the ASI is not accurately receiving the true static pressure, and it ends up calculating an airspeed based on incorrect static data.

As a result, the ASI will indicate a higher airspeed than the aircraft is actually experiencing. This over-reading occurs because the instrument is interpreting the blocked static pressure as if the aircraft were at a higher altitude than it actually is, leading to erroneous speed indications. Therefore, under these conditions, the ASI and Mach meter will indeed over-read.

This understanding is crucial for pilots and technicians, as relying on these inaccurate readings during descent could lead to unsafe operational decisions.