What is the effect of a blocked pitot and drain?

In the context of aircraft instrumentation, a blocked pitot tube affects the measurement of dynamic pressure, which is what the airspeed indicator relies on to provide accurate speed readings. When the pitot tube is blocked, it cannot sense the dynamic pressure from the airflow, which typically gives an indication of the aircraft's speed through the air.

If both the pitot tube and the drain are blocked, the airspeed indicator becomes inoperative in the normal sense and starts behaving like an altimeter because it relies on static pressure. When the aircraft climbs, the static air pressure decreases, similar to how an altimeter operates, resulting in a higher reading on the airspeed indicator. Conversely, while descending, the reading will decrease. Therefore, with a blocked pitot and drain, the airspeed indicator would reflect changes in altitude rather than true airspeed, mimicking the behavior of an altimeter as the static pressure alters with altitude changes.

This understanding highlights the critical importance of instrumentation in aviation and the necessity of ensuring that all inputs to instruments like the airspeed indicator function as intended for safe flight operations.