Tuesday, September 29, 2009

When in doubt, blame the pitot tube

They still don't know what caused the loss of an Air France jetliner over the south Atlantic this summer. The plane simply failed to arrive in Paris. The crew did not send out distress calls although the aircraft's computers did. The crash recorder sank to the bottom of the Atlantic and was not found. Intensive sea searches turned up some debris, but offered no clues.
A review of the aircraft's maintainance records revealed the aircraft was equipped with an older style of pitot tube. Lacking anything thing else to blame, the pitot tube became the culprit. Last week the European Aviation Safety Agency issued an emergency air worthiness directive to check the torque on the pneumatic disconnect union going to the pitot tube.
Trouble is, all the pitot tube does is make the airspeed indicator work. The plane will fly just fine without an airspeed indicator at all. The pitot tube is just a micely made bit of pipe facing forward into the airstream. Air is rammed into the opening of the pitot tube by the plane's motion thru the air and this pressure is measured by a sensitive gauge calibrated to read in knots instead of pounds per square inch. Since the tube is merely a piece of pipe, under very low pressure, it's failure modes are limited. About all it can do is ice up which makes the airspeed indicator stop working. To prevent icing, pitot tubes have electric heaters built into them. The one on the old F106 would heat the tube hot enough to burn your hand.
But the real issue is this, loss of airspeed indication is not going to crash a plane. It might possibly confuse an autopilot, but that's why the airplane carries a pilot and a co pilot.

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