What does a blocked ram hole with an open drain cause the airspeed indicator to read?

When the ram hole of an airspeed indicator is blocked but the drain is left open, it results in a specific condition for the operation of the instrument. The ram hole is responsible for receiving dynamic pressure as the aircraft moves through the air, and blocking it means that the airspeed indicator cannot sense the changes in pressure that correspond to the aircraft’s speed.

In this scenario, since the ram air pressure cannot reach the indicator due to the blockage, the instrument will not reflect any changes in speed and ultimately will show a reading of zero. This occurs because the airspeed indicator relies on the difference between static and dynamic pressure for its readings; without the dynamic pressure from the ram air, there is nothing for the airspeed indicator to work with.

The other choices highlight alternate responses that would not manifest in this situation, as the particular condition of having a blocked ram hole specifically leads to the instrument showing zero airspeed.