Sunday, June 07, 2009

Ice in the pitot tube

Years ago, while translating Japanese accounts of the air war over Burma in the winter of 1941-1942, I was astonished to learn that the Japanese term for 'pitot tube' was--pitot tube! It seems that in the early 1700s, Henri Pitot found that one could measure the speed of an airstream by pointing a narrow tube into the flow. The tube was bent at 90 degrees. 'As this tube contains air,' explains Wikipedia, 'a pressure can be measured as the moving air is brought to rest.' What worked for the French 300 years ago still worked for the Japanese in 1942, and for the Americans as well. There's a pitot tube on every aircraft I've ever seen up close, including the 1946 Piper Cub I sometimes fly.

And also on Air France 447, lost with all souls over the South Atlantic last week. According to the NYT, the latest suspect in the crash is the homely pitot tube, which Airbus has recommended being replaced with one less prone to icing. Gravely explains the Gray Lady: 'Air-speed sensors can be crucial to the pilots’ ability to control an aircraft. A plane that flies too slowly can lose lift and crash, while one that is moving too fast can break up in the air.'

A Piper Cub, of course, is flown largely by the seat of one's pants. But when I venture more than a couple dozen miles from Hampton Airfield, I bring a portable Garmin GPS unit that I stick to the side window with a suction cup. Among other things, the Garmin measures--airspeed! And does it quite independently of icing conditions. The mind boggles to think that I have more reliable instruments in the Cub than did the captain of Air France 447. Blue skies! -- Dan Ford

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