If the pitot tube becomes completely blocked by ice during flight and the aircraft has no pitot heat, what will happen to the airspeed indicator?

When the pitot tube becomes completely blocked by ice and the aircraft lacks pitot heat, it results in a failure of the airspeed measurement system because the pitot tube is responsible for measuring the dynamic air pressure that correlates with airspeed.

In a climb, as the aircraft ascends, it encounters a decrease in the atmospheric pressure outside of the aircraft while the static pressure remains trapped within the blocked pitot tube. This pressure differential creates a scenario where the airspeed indicator will not register the true airspeed; instead, it will perceive an artificially high airspeed because it is unable to measure the lower pressure properly.

Conversely, during a descent, the aircraft experiences an increase in atmospheric pressure while the static pressure remains unchanged within the blocked pitot tube. This change results in the airspeed indicator reading lower than the actual airspeed because the trapped static pressure causes the indicator to misinterpret the pressure differential, leading to an under-read condition.

Therefore, the block in the pitot tube results in the airspeed indicator over-reading during climbs and under-reading during descents, which aligns with the assertion provided in the chosen answer.