What occurs to the Airspeed Indicator (ASI) when a pitot tube is blocked but the drain hole is open?

When a pitot tube becomes blocked and the drain hole remains open, the airspeed indicator is unable to accurately measure the dynamic pressure from the airflow. In this scenario, the ASI will still read the static pressure but will not register any change in dynamic pressure due to the blockage of the pitot tube. Normally, during flight, the ASI measures the difference between static pressure and dynamic pressure to determine airspeed.

Since the static pressure is not affected and does not take into account the airspeed, the ASI would read zero when the aircraft is in different phases of flight (such as climb, descent, or level flight), leading to the reading of 0. This can be particularly dangerous as pilots rely on the ASI for critical speed information necessary for safe flight operations.

In situations where a pitot tube is blocked but the drain hole is clear, it is essential for pilots to recognize that the ASI has lost its ability to provide accurate information and to address the situation accordingly.