What Are Pitot Tubes And How Do They Work?

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Pitot tubes are crucial components onboard aircraft used to measure a range of important data. The tubes are popularly known as speedometers, giving pilots a gauge on their airspeed, and also measure altitude and altitude trend. Pitot tubes are usually found along the front fuselage or along the wing of an aircraft.

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Pitot tubes are commonly placed beneath the front fuselage. Photo: Airbus

What exactly are pitot tubes?

Pitot tubes aren’t just inventions innovated for aviation; they’re also commonly found in industrial machinery, boats and even Formula 1 cars. A pitot tube is essentially a flow sensor instrument. Simple pitot tubes have one hole at the front; however, planes will often use pitot-static tubes with two openings rather than separate pitot tubes and static ports.

Pitot-static instrumentation on an aircraft. Photo: Getty Images

A simple pitot tube can measure stagnation pressure only due to its single opening. The advantage of a pitot-static tube is its ability to measure static pressure in addition to stagnation pressure. Static ports along the side of the pitot tube give it this all-in-one capability, meaning there is no need to install separate static ports elsewhere.

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How pitot tubes work

Pitot tubes work as flow sensors to measure the speed and pressure of air, liquid or gas. These readings allow pilots to gauge airspeed and altitude and have a wide variety of applications in other equipment. The tubes can usually be spotted beneath the cockpit on the front fuselage or along the wing.

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Pitot tubes clearly visible on a 737NG. Photo: Simple Flying/Dylan Ashe

The pitot-static tube has two openings, one in the front and one along the sides, used to measure air, gas or liquid flow. This system involves placing a pitot tube inside another tube with static ports. The front hole measures the stagnation pressure, while the side openings (static ports) gauge static pressure. The difference between these two measurements is called dynamic pressure – this is what is used to calculate airspeed.

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Pitot tubes are critical instruments

Measuring airspeed is an indispensable part of flying and essential for all pilots to keep a gauge of. Pitot tubes provide indicated airspeed (IAS), based upon measuring the dynamic pressure. This is different from groundspeed, as planes can encounter resistance during flight, such as headwind and air density.

Pitot tubes provide pilots with vital data on airspeed and altitude. Photo: Getty Images

Planes flying too slowly won’t generate enough lift and risk spiraling into a stall, while aircraft flying too quickly risk damage as they aren’t built to withstand certain speeds. Airspeed is also vital in estimating flight length – inaccurate estimations may cause pilots to run out of fuel mid-journey if the flight goes on longer than expected.

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Incidents involving pitot tubes

As a critical part of an aircraft’s instrumentation, pitot tubes have been at the center of several accidents. As the tubes are exposed to the elements, they are in danger of icing over, which has led to several fatal accidents. Pitot tube failures will always force a plane to divert – in October, an Airbus flight over Russia was forced to land after its pitot tubes were not heating up adequately.

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Testing the pitot-static system is important for ensuring safety. Photo: Lufthansa

In one case, wasps nested inside a pitot tube, which later led to a fatal crash onboard Birgenair Flight 301. The aircraft had been grounded for 20 days and its pitot tubes were left uncovered, allowing wasps to burrow inside. Australia is to this day combating nuisance wasps that are nesting inside pitot tubes. Another high-profile case involved a disgruntled American Airlines mechanic tampering with a pitot tube, which led to an emergency landing.

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