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What is a pitot tube?

Investigators are looking into the possibility that faulty airspeed indicators caused the crash of Air France Flight 447



NEEPSTER/FLICKR

As Brazilian authorities retrieved 16 more bodies from the Air France Flight 447 crash this morning, investigators were focusing on airspeed indicators called pitot tubes as the cause of the disaster.

On May 31, the Airbus A330 jet was flying from Rio de Janeiro to Paris when it vanished during a thunderstorm with 228 people on board. Initial speculation centered around the possibility of a lightning strike, but last week Airbus released a memo stating that "there was inconsistency between the different measured airspeeds" coming from different pitot tubes. The pitot tube is a classic fluid dynamic sensor named for its inventor, Henri Pitot, who in the 18th century developed it to measure the speed of rivers and canals in France.

Airbus's pitot tubes were known to have icing problems, and Air France had begun replacing them on April 27 when an improved version was released. Flight 447, however, had not received the new sensor. On Monday, Brazil revealed that it had found the plane's vertical stabilizer, suggesting that it been sheared off due to high speeds.

To find out more about pitot tubes, we spoke to Ken Powell, an aerospace engineer at the University of Michigan at Ann Arbor.

[The following is an edited transcript of the conversation.]

What is a pitot tube?
Basically, a pitot tube is used in wind tunnel experiments and on airplanes to measure flow speed. It's a slender tube that has two holes on it. The front hole is placed in the airstream to measure what's called the stagnation pressure. The side hole measures the static pressure. By measuring the difference between these pressures, you get the dynamic pressure, which can be used to calculate airspeed.

On an airplane, the pitot tube can be mounted in a number of ways, including jutting out from the edge of the wing or sticking up from the fuselage.

How can ice cause the pitot tube to malfunction?
Basically, if there's a blockage of that tube, then you will get an incorrect reading for the pressure difference and your airspeed. Ice accretion on wings is a big problem, and it can also build up on a pitot tube. Pitot tubes must be calibrated to work properly, and if ice changes the shape of airflow around the tube, then it will give an incorrect reading.

Why is measuring airspeed important?
Planes have a certain operating envelope, and it's dangerous to fly at too low a speed because you can stall, and it's also dangerous to fly at too high a speed because of structural reasons.

Can't pilots just use Global Positioning System (GPS) to measure their airspeed?

That's a very good point. I'm not a pilot so I don't to what extent commercial pilots double-check airspeed readings with ground speed readings from GPS. But you don't need something to give you a completely wrong reading to throw you off, it could be off by less than 25 percent and still create a dangerous situation.

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