If the drain of a blocked pitot tube remains clear, what effect does it have on airspeed readings?

When a pitot tube is blocked, but the drain remains clear, the impact on airspeed readings can be understood in the context of how a pitot-static system operates. A pitot tube measures dynamic pressure (the pressure caused by the aircraft's motion through the air), which is used to calculate airspeed. If the tube itself becomes blocked, the air pressure inside it cannot change in response to the external air pressure as the aircraft’s altitude changes or as the speed of the aircraft changes.

However, because the drain is clear, the static pressure is still able to escape, allowing for some pressure changes in the system. As the aircraft ascends or descends or accelerates or decelerates, the blockage means that the dynamic pressure indicated does not alter correctly, leading to an inaccurate indication of airspeed.

As altitude increases, for example, the total pressure in a blocked pitot tube will not adjust appropriately since that pressure cannot be equalized with the ambient pressure outside the system. Therefore, the airspeed readings will appear to decline gradually as the aircraft continues to climb, resulting in a misleading sense of airspeed that under-represents the actual performance of the aircraft.

Understanding this concept highlights that the gradual decrease in readings does not reflect the