But understanding how the system works is just as critical.

The pitot tube measures an aircraft’s ram air pressure in flight, while the static port measures ambient (outside) air pressure present whether or not the aircraft is in flight. The airspeed indicator relies on information from the pitot tube and the static port, while the altimeter and the vertical speed indicator rely only on the static port.

The airspeed indicator measures the difference between dynamic and static air pressure. If the pitot tube opening becomes blocked by ice or insects and its drain hole is clear, ram air cannot enter the pitot system. The air already in the system vents through the drain hole, and the remaining pressure drops to ambient air pressure. The airspeed indicator quickly decreases to zero because the instrument senses no difference between ram and static air pressure.

If the pitot tube opening and the drain hole become clogged simultaneously, then the pressure in the pitot tube is trapped. There’s no change in airspeed indication whether the airspeed increases or decreases. If the static port is unblocked and the aircraft changes altitude, then the airspeed indicator notes a change—this change is not related to a change in airspeed but a change in static pressure. The airspeed indicator now works like an altimeter, indicating faster when you climb and slower when you descend. Ask your CFI to go over these scenarios—and how to react—in greater detail.

For the altimeter, a blocked static port means that the instrument is no longer receiving new information; therefore, it is indicating the altitude of the aircraft at the time the blockage occurred. The vertical speed indicator will remain at zero once the static port is blocked.

During the takeoff roll, visually confirm the airspeed indicator is operational when the needle or digital display shows movement. In visual meteorological conditions, verify the accuracy of other instruments once at a safe altitude to do so. You can overcome any misinformation caused by a pitot-static system blockage by simply looking outside and landing as soon as practicable.