When both the pitot and static ports are blocked without a clear drain hole, what happens to the airspeed indicator?

When both the pitot and static ports are blocked without a clear drain hole, the airspeed indicator will behave like an altimeter. This situation occurs because the pitot tube, which measures dynamic pressure (the pressure of air impacting the aircraft), and the static port, which measures ambient air pressure, are both unable to receive new airflow data.

In this condition, the airspeed indicator will "freeze" at the last indicated airspeed before the blockage occurred. As the aircraft climbs or descends, the ambient conditions change, altering the pressure. The airspeed indicator is designed to read the difference between the static pressure and the dynamic pressure, and with no new input from either port, it will not show a change in airspeed. Instead, it will reflect changes in pressure much like an altimeter, which measures changes in altitude based on static pressure changes without receiving any new dynamic pressure information. Therefore, it indicates a constant airspeed regardless of the actual airspeed of the aircraft, mimicking the behavior of an altimeter.