If the pitot tube and its drain becomes blocked, what would the airspeed indicator do?

When the pitot tube becomes blocked, the airspeed indicator is unable to receive the dynamic pressure that allows it to measure the aircraft’s speed through the air. In this situation, the airspeed indicator starts to behave like an altimeter or vertical speed indicator (VSI), depending on the altitude changes the aircraft experiences.

If the aircraft is in level flight and the air pressure remains constant, the airspeed indicator will freeze at the last indicated speed. However, if the aircraft climbs or descends, the static pressure will change, and the airspeed indicator will respond to these changes based on the variations in the static pressure, similar to how an altimeter or VSI would reflect altitude changes.

This unique situation arises because the airspeed indicator relies on both static and dynamic pressures. When only static pressure is available due to a blocked pitot tube, its behavior mimics that of altimeters or VSIs, which respond to changes in atmospheric pressure.