“Pitot tube” is perhaps the most mispronounced item on the preflight checklist. It is also an important part of the airplane that provides essential airspeed information.  Confirming the tube is uncovered and clear of obstructions ensures the pitot-static instruments do not give erroneous readings in flight.

There are three instruments—the airspeed indicator, the vertical speed indicator, and the altimeter—that are part of the pitot-static system, yet the airspeed indicator is the one instrument in the cockpit that obtains information from both the pitot tube and the static air ports on the sides of the aircraft.

The pitot tube measures dynamic air pressure, which is only present when the aircraft is moving. There are two openings on the pitot tube—one in the front that measures dynamic pressure and another in the back that serves as a drain.

As part of every preflight, inspect the pitot tube to ensure neither opening is blocked. Insects, dirt, ice, and other obstructions can become lodged in the tube and prevent the airspeed indicator from providing accurate readings to the pilot. Some pilots purchase a pitot tube cover that protects the pitot tube whenever the aircraft isn’t flying. Even if the aircraft you fly has a pitot tube cover, the openings themselves must be visually inspected after the cover is removed.

If the main opening of the pitot tube (but not the drain) becomes blocked, the pitot tube can no longer measure dynamic air pressure. Therefore, the airspeed indicator doesn’t sense a difference between dynamic and static pressure. In that case, the airspeed indicator will show zero.

If both openings of the pitot tube are obstructed, the pressure that had already entered the tube at the time of the blockage becomes trapped, so no change is noted on the airspeed indicator when flying level. However, the static ports are still measuring static pressure, so with any change in altitude (and therefore a change in static pressure), the airspeed indicator will increase or decrease. The airspeed indicator turns into an altimeter in this scenario.

The best time to find out if there's a problem with your airspeed indicator is on the ground, not in the air. A quick check early in the takeoff roll to confirm the airspeed indicator has begun to move (commonly called out “airspeed alive”) provides some assurance that the pitot-static instruments are working properly.

Once airborne, ice can obstruct the pitot tube, which is one reason why many aircraft that do not have the capability to de-ice wings or propeller do have a heating element in the pitot tube. Every pilot should understand how to use this feature and consult the Pilot's Operating Handbook or Airplane Flying Manual for details.

Learn more about aircraft instruments in Chapter 8 of the FAA’s Pilot’s Handbook of Aeronautical Knowledge.