In many studies of flows it is fundamental to determine the modulus and direction of fluid velocity at some points in the region studied.
There are several methods for the determination of fluid velocity, among them what identifies the difference between total and static pressure introduced by Henri Pitot, which is one of the most used.
Henri Pitot was born in France in 1695 and was an important engineer specializing in hydraulics. Pitot began to become interested in fluids and tested several of his designs and theories in the Seine, until he invented in 1732 a device to measure the speed with which the fluid moved, known today as a Pitot tube, that allows the operation of one of the most important aircraft instruments, the speedometer.
Basically, it is a tube installed parallel to the relative wind and with an orifice facing directly to the airflow resulting from the aerodynamic speed of the aircraft. This orifice communicates with the interior of an aneroid capsule, installed in the speedometer of the aircraft. The instrument box receives the static air pressure from a static source, which is not affected by the aircraft’s speed variation.
When the aircraft is stationary and there is no relative or real wind, the pressure entering the pitot hole is only static atmospheric pressure. The aneroid capsule then remains in a neutral position and the speed indicated is zero.
When the aircraft moves in the air mass, the relative wind causes an increase in the air pressure admitted by the pitot tube orifice, relative to the static pressure, and this “impact pressure”, added to the static pressure, causes the capsule aneroid expand. The expansion movement of the capsule is transmitted to the hands of the speedometer by rods and gears, which causes the hand to move, indicating to the pilot the speed of the aircraft.
Theoretically, then, the process is very simple. In practice, it is much more complicated. To begin with, an aircraft does not fly in environments of constant pressure and, consequently, constant air density. And in airplanes flying at high speeds above 250 Knots, we will have to consider the effects of compressibility due to this high speed.
The tubes are very simple components, with no moving parts, but they can still suffer problems, almost all related to their obstruction. The major obstruction problems are caused by ice, which can form rapidly, especially in cumuliform cloud formations.
To avoid ice, pitot tubes are usually equipped with a heating system, of the electric resistance type.
The obstruction of the tubes can have much more serious effects than the simple lack of indication of speed. The aircraft’s automation and alerting systems rely on correct speed parameters to operate. If speed parameters are no longer valid, electronic flight management systems will provide disparate information, and the autopilot will no longer function properly.
One of the most known, and most tragic, was the crash with the Boeing 757-225 that fulfilled the Birgenair 301 flight. The Boeing 757 in question had been stopped for 25 days in Puerto Plata, Dominican Republic. When it finally took off, on February 6, 1996, heading to Frankfurt, the commander’s speedometer began to give invalid indications, affecting the autopilot and the auto-throttle system. Although the fault only affected the commander’s instrument, the crew made a series of coordination errors and eventually fell into the Caribbean Sea, killing all 13 crew and 176 passengers on board.
In the investigation, it was found that one type of Caribbean wasp nestled in the pitot tube, while the aircraft was standing at the airport, and the crew had not put the protective covers on the tubes.
Patricia Rodrigues Gonring
Chemical Engineer Student of Federal University of Espírito Santo – Alegre – ES