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 of their airspeed, and also measuring altitude and altitude trends. Pitot tubes are usually found along the front fuselage or along the wing of an aircraft.

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, and simple pitot tubes typically have just the one hole at the front. However, commercial aircraft will often use pitot-static tubes with two openings rather than separate pitot tubes and static ports.

A simple pitot tube can only measure stagnation pressure, 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. But how exactly do these components work?

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Pitot tubes
Pitot tubes can clearly be seen here on a Boeing 737NG. Photo: Simple Flying/Dylan Ashe

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 various applications in other equipment. The tubes are usually beneath the cockpit on the front fuselage. Alternatively, they can also be found along the wing. The pitot-static tube has two openings: one in front, and one on the side.

These are 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 reading is ultimately what is used to calculate a given aircraft's airspeed.

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 readings for indicated airspeed (IAS), based on measuring the dynamic pressure. This is different from groundspeed, as planes can encounter resistance during flight, such as headwind and air density.

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Pitot Tubes
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, as inaccurate estimations may cause pilots to run out of fuel if the flight goes on longer than expected.

Various safety-critical incidents

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, with one example being an Aeroflot flight that was forced to land after its pitot tubes were not heating up adequately.

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.

Lufthansa, Parked Aircraft, Stored Aircraft
Testing the pitot-static system is vital for ensuring safety. Photo: Lufthansa

The crash of Birgenair flight 301

Perhaps the most notable instance where wasps nesting inside a pitot tube later led to a fatal crash involved Birgenair flight 301. The aircraft had been grounded for 20 days, and its pitot tubes were left uncovered, allowing wasps to burrow inside. This prompted an instrument malfunction, which resulted in the flight stalling, before crashing into the ocean near Puerto Plata, Dominican Republic.

According to the Aviation Safety Network, all 176 passengers and 13 crew members onboard the Boeing 757-200 involved in the crash died due to its violent impact with the sea. The accident was the deadliest ever to have taken place in the Dominican Republic, and played a role in Birgenair's subsequent bankruptcy.

Source: Aviation Safety Network