Which atmospheric condition will lead to an over-reading airspeed indicator during descent?

An airspeed indicator can show an over-reading during a descent when the static port is blocked. The airspeed indicator operates by measuring the difference between the pressure from the pitot tube, which senses dynamic pressure (airspeed), and the static pressure from the static port. If the static port becomes blocked during descent, the static pressure reading remains constant while air pressure from the pitot tube still decreases as the aircraft descends.

As the aircraft descends, the dynamic pressure indicated by the pitot tube decreases, but since the static port pressure is erroneously stuck at a higher level due to the blockage, the result is a misleadingly high airspeed reading. In other words, because the static pressure doesn’t reflect the actual increase in altitude, the airspeed indicator will show a speed higher than what is actually true.

In contrast, a blocked pitot tube would typically result in an under-reading of airspeed, showing a lower speed, especially in a descent as the dynamic pressure is not able to equalize correctly. A clear static port would enable proper functioning of the airspeed indicator, and full pitot and drain function would also ensure accurate readings. These conditions would not lead to an over-reading of airspeed.