What occurs during a pitot blockage with a clear drain hole?

When a pitot blockage occurs with a clear drain hole, the airspeed indicator will read zero. This situation arises because the pitot tube, which is responsible for measuring dynamic pressure (the difference between static air pressure and total pressure), becomes blocked. In this case, as the airplane ascends, the air pressure inside the pitot tube cannot equalize, leading the instrument to register a static value.

Given that the drain hole is clear, there is no additional air pressure entering the tube, which means there’s no dynamic pressure detected. Therefore, the airspeed indicator shows zero. This scenario highlights the importance of maintaining the pitot system in good working order and also keeping in mind the impact of blockages, as well as understanding how various components of the airspeed indication system interact with one another.