What happens to the airspeed indication when a pitot tube is blocked and the drain hole is also blocked?

When a pitot tube is blocked, and the drain hole is also blocked, the system experiences a situation where it cannot measure dynamic pressure accurately. In this scenario, the airspeed indicator cannot receive the necessary airflow information to function correctly, leading to an unreliable reading.

As the aircraft climbs, the atmospheric pressure decreases, and the airspeed indication will reflect a lower value than the actual speed. Conversely, if the aircraft descends, the indication may show an increased speed. This erratic behavior occurs because the pressure inside the pitot tube that feeds the airspeed indicator becomes trapped and does not represent the current external air pressure conditions. Thus, the airspeed readings can no longer be trusted.

This demonstrates the critical need for both the pitot tube and the drain hole to be functioning correctly for necessary and reliable airspeed indications in flight.