When a pitot tube becomes blocked but the drain remains clear, what will happen to the airspeed indicator?

When a pitot tube becomes blocked while the drain remains clear, the airspeed indicator is affected in a specific way. The total pressure collected by the pitot tube, which is essential for determining the indicated airspeed, can no longer enter due to the blockage. However, air can still exit through the clear drain, leading to changes in the pressure readings that the airspeed indicator uses.

Under these circumstances, as the aircraft ascends, the static pressure decreases while the blockage prevents the pitot tube from accurately measuring the total air pressure. Since the air pressure within the blocked pitot tube does not change but the static pressure does, the indicator will read a lower airspeed than normal. This results in the airspeed indicator showing a gradual decrease in indicated airspeed as altitude increases. Hence, the accurate response reflects this relationship between altitude, static pressure, and total pressure within the system.