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

When the pitot tube is blocked but the drain hole remains open, the Airspeed Indicator (ASI) is unable to receive new pitot pressure information. The ASI relies on the dynamic pressure created by the airflow entering the pitot tube to determine the aircraft's speed. If the pitot tube is blocked, the pressure inside the tube does not change with aircraft speed, but the open drain hole allows any static pressure changes to escape.

As the aircraft's speed increases, the ASI does not register changes in dynamic pressure and cannot display the correct speed. Instead, since static pressure can escape, the ASI will read as if the aircraft is stationary or may drop to zero, reflecting the lack of correct input from the pitot tube.

In essence, with a blocked pitot tube and an open drain hole, the ASI cannot accumulate the necessary pressure to portray the aircraft's speed accurately, typically resulting in the reading dropping to zero. This is why the answer indicating that the ASI drops to zero is correct in this situation.