If the airspeed indicator (ASI) bleeds to zero mid-flight, what is happening to the pitot tube?

When the airspeed indicator reads zero mid-flight, the most likely scenario involves the pitot tube becoming obstructed in some way that affects the pressure readings required for proper airspeed calculations. The correct answer indicates that the ram air opening of the pitot tube is frozen, yet the drain hole remains unobstructed.

In this condition, while the drain hole allows for some air pressure communication, the blockage at the ram air opening prevents ambient air from entering the tube as the aircraft moves through the atmosphere. Since the pitot tube must measure the dynamic air pressure to provide an accurate airspeed reading, a blockage at the ram air opening would cause the airspeed indicator to drop to zero, as there would be no pressure change information reaching the ASI. This is a common occurrence in freezing conditions where moisture can freeze in the pitot tube, leading to insufficient or incorrect pressure readings.

Understanding this scenario is crucial for pilots, as recognizing the symptoms of a malfunctioning airspeed indicator can lead to necessary corrective actions, such as using other flight instruments to gauge airspeed or considering a change in flight conditions to avoid ice buildup.