Commercial Pilot License (CPL) Practice Exam

When both the pitot tube and the drain hole of an aircraft's static system become blocked, the airspeed indicator can produce misleading readings. The pitot tube measures the dynamic air pressure and is essential for the airspeed indicator to function accurately. If the pitot tube is blocked, it can lead to incorrect airspeed readings.

However, if the drain hole is also blocked, the static pressure trapped inside the system can no longer equalize, particularly as the aircraft climbs or descends. Under these conditions, the airspeed indicator may start to give readings that correspond to the altimeter's function. More specifically, if the aircraft climbs, it will indicate a decrease in airspeed as if the aircraft were descending. Conversely, if the aircraft descends, it may indicate an increase in airspeed, similar to an altimeter response.

This cross-functioning occurs because the airspeed indicator depends on both the dynamic pressure from the pitot tube and the static pressure from the static system. When both systems are compromised, the airspeed indicator can, therefore, behave like an altimeter, potentially leading to confusion for the pilot and affecting flight safety.