In the early 18th century, the concept of measuring liquid flow speed is credited to French mathematician, Henri Pitot. Pitot observed, measured and recorded water flow rates of the River Seine in northern France using a simple tube formed from copper. Later in the 19th century, French hydraulic engineer, Henry Darcy, enhanced the original pitot concept while researching fluid dynamics. Today, pitot-static systems are utilized in aircraft, wind tunnel testing and for numerous industrial applications.
Three aircraft instruments requiring pitot-static input are the airspeed indicator, altimeter and the vertical speed indicator. The small opening at the front of the pitot tube allows total pressure to enter the pressure chamber. Total pressure consists of dynamic pressure plus static pres- sure. Essentially, there are two pitot-static configurations used on aircraft today. Many high-wing airplanes have a separate pitot tube and a separate static port. The standard pi- tot tube typically hangs under a wing and has a separate static port located at some point along the fuselage and is flush mounted. By contrast, many low-wing airplanes sport a pitot mast also called a pitot blade. The mast de- sign integrates both the pitot and static port within a single module. These components require minimal maintenance; however, it is crucial to keep the pitot tube and static port(s) clear of all debris. Before each flight, be certain to drain aircraft static systems on those aircraft equipped with static drains. This line item is typically not included on most aircraft checklists. So, refer to the aircraft manufacturers’ static line diagram and recommendations for your specific airplane make and model.
The pitot-static system provides important data critical to safe aircraft operations. When parking an airplane, it is essential to consistently protect the pitot from all possible contamination. A pitot tube cover should fit securely over the pitot to prevent dirt, debris and nesting insects from obstructing the pitot tube and static chamber. Covering the pitot tube is an essential post flight activity whether an aircraft is tied down on the ramp or stored safety inside a hangar. University of Florida entomologists observed and documented mud daubers constructing a nest in less than three hours. So while you’re enjoying lunch at the airport cafe, mud daubers may be taking up residence inside the pitot tube chamber. During winter months, neglecting to properly install a pitot tube cover can result in frost, ice or snow entering and blocking the static chamber. Even a partially blocked pitot system in flight can cause unreliable and erratic instrument readings resulting in a tragic accident.
Over the years, we’ve all seen a variety of homemade pitot tube covers on various aircraft ranging from yellow tennis balls to smiling rubber ducks and various plastic farm animals. One pilot used synthetic material leftover from his wife’s most recent craft project to make a pitot tube cover for his classic high wing Cessna. The airplane wings were bright white like his new pitot cover. During his preflight inspection, the pilot failed to remove the white pitot cover. This was like looking for a polar bear in a snowstorm. No color contrast. On takeoff, the pilot also neglected to observe the airspeed indicator as the airplane accelerated and rotated. Only then did he discover his error. Looking out the left window, the pilot saw the white pitot tube cover still in place underneath the white Cessna wing.
The pilot remained in the traffic pattern and returned to make an embarrassing landing. Another aircraft owner fabricated a temporary pitot cover but learned a hard and expensive lesson. The thin plastic material he selected to use was incompatible with heat. Pitot tubes get very hot. The clear wrapping completely melted over the pitot tube like glaze on a donut. So, please consider standardizing all control locks, gust locks, cowl covers and pitot tube covers with highly conspicuous “Remove Before Flight” red streamers. Some years ago, I recall seeing an old Beechcraft Barron with a makeshift control lock. The pilot inserted an oil-soaked, wooden handle screwdriver in the control column. Imagine the consequences of forgetting to remove this during preflight, especially on a dark night.
- During preflight, take care when installing and removing pitot tube covers.
- Avoid disturbing the critical alignment of the pitot tube with the airplane’s wing.
- Store the pitot tube cover so that it is not exposed to dirt, oil, lint, chemicals or other contaminants. If the inside of the pitot tube cover is compromised, then the entire pitot-static system will also be compromised.
- When washing and waxing the airplane, prevent static port clogging by temporarily covering the static ports with a long strip of brightly colored painters’ tape. Avoid spraying water directly into the pitot tube and static ports. Be certain to remove the painters’ tape upon completion.
- Standardize all control locks, gust locks, cowl covers and pitot tube covers with highly conspicuous and durable “Remove Before Flight” RED streamers.
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This story was related to new student pilots at Laredo AFB in the early 70’s — I was one of them! At a base open house, a man was taking his children around to look at the numerous Air Force aircraft on static display. He stopped and pointed at the pitot tube on a T-38, which had a red canvas cover and ‘Remove Before Flight’ streamer on it. Pointing at the pitot tube, he asked his kids, “You know what that is?” They replied, “No, Papa, what is it?” The dad proudly announced, “That’s the machine gun!” The kids all “Oooohed! and Aaaahed!” Whether this ‘tale’ is true or not, we students made sure we properly pre-flighted our ‘machine gun’ on every flight!
Great story! I can identify with that because I was an Air Force brat at Biggs AFB, El Paso in the early 1950’s. My dad was a Crew Chief on a Douglas C-124 GlobeMaster. At 5 years old, I too was convinced this stately cargo aircraft had a machine gun. Dale, thank you for your service!
Leo, On our solo out and backs from Laredo, we students flew our T-38s into El Paso International (ELP — where the strawberry pie in the terminal was delicious!) and there was always one student (maybe more) who had to be ‘reminded’ that they were NOT supposed to land at Biggs (BFI), which is just a little bit north of and parallel to ELP!
It was my honor to serve, and I got paid to fly jets!