In the early 1700s there was not an accepted method for measuring streamflow until a French hydraulic engineer had an idea. His idea started with a glass tube bent at 90°. By inserting this tube in a stream flow, with its opening pointed upstream, water entered the tube and rose a distance, h, above the surface as shown on the figure. So,if friction losses are negligible, the velocity of the stream, V, is approximately 2gh, where g is the acceleration of gravity.
Henri Pitot had just invented the pitot tube which is very similar to the modern day pitot tube protruding from an airplane. Instead of a stream flow, a plane’s pitot tube measures air flow which for an aircraft is its airspeed. Pitots are used is every type of aircraft from a single engine airplane to the most advanced bomber in the world, the B-2 Bomber.
Yes the B-2 bomber. You know this bomber with its distinctive flying wing design. On February 23, 2008, a 1.2 billon dollar B-2 stealth bomber plunged to the ground shortly after taking off from an air base in Guam on February 23, 2008. Both pilots amazingly ejected safely. This was the first time one had crashed.
Several months after the accident the Air Force said “distorted data introduced by a B-2 Spirit’s air data system skewed information entering the bomber’s flight control computers ultimately causing the crash of the aircraft on takeoff.” To put this simply, moisture in the plane’s pitot tube caused the wrong airspeed to be reported. This caused control computers to calculate an inaccurate airspeed and a negative angle of attack upon takeoff.
This accident is not without precedent. This type of accident has happened in military and civilian planes in the past. In the case of the B-2, earlier crews had learned of the vulnerability and had developed a simple fix – turn on the pitot tube heater to evaporate the water. Unfortunately, the procedure wasn’t documented and not everyone knew of it.
Still Interested: Check out an impressive early all wing aircraft from the 1940s