Which situation would result in the airspeed indicator acting as an altimeter?

When both the ram air input and the drain hole are blocked, the airspeed indicator can start to function like an altimeter due to the principles of how these instruments operate. The airspeed indicator measures dynamic pressure, which is the difference between the static pressure (ambient air pressure) and the total pressure (which includes the impact of the ram air).

If the ram air input is blocked, the instrument is unable to sense the dynamic pressure properly. Simultaneously, if the drain hole is blocked, any moisture or pressure changes inside the instrument could lead to erroneous readings. In this scenario, the airspeed indicator may begin to reflect changes in altitude instead of airspeed because it is no longer responding to dynamic pressure changes due to the blockage. Essentially, trapped static pressure could cause it to indicate a decrease in airspeed as the aircraft ascends or an increase as it descends, mimicking altimeter behavior.

In the other situations, while one blockage might influence the airspeed indicator, it would not create the same definitive altimeter-like performance as the combination of both blockages.