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A pitot static tube measures air speed and altitude in airplanes by comparing the gauge pressure of air flowing into the tube with that flowing around it. It consists of two tubes, one for total pressure and the other for ambient pressure. The tube is mounted outside the aircraft and equipped with heating elements to prevent blockage.
A pitot static tube, or Prandtl tube as it is also known, is the main component of a measurement system that determines air speed and altitude in airplanes. Pitot tube systems achieve this measurement with a tube facing a stream of air to compare the gauge pressure of the air that has flowed into the tube with that flowing around it. By comparing these two variables, a set of calibrated instruments can accurately measure a range of relevant environmental elements. The pitot-static tube is one of the three basic types of pitot tube and combines both captive and bypass airflow.
A flow of air creates a pressure value, known as total pressure, which is a combination of the pressure created by the speed of the airflow and the surrounding ambient air pressure. Accurate calculation of airflow velocity can be achieved by comparing the difference between total pressure and ambient or static pressure. Static pitot tube systems achieve these comparisons by placing an open tube facing the airflow to measure total pressure and a static port in a low turbulence area to measure ambient pressure. Both inputs can then be used to extrapolate various readings such as airspeed, altitude and rate of climb.
Pitot tube systems are based on separate piston air tubes and static ports or combined tubes that measure both total and static pressure. These are known as pitot static tubes and consist of two tubes, one inside the other. The inner tube is open at the end and collects and measures the total pressure. The outer tube is separated from the inner by a baffle and is punched with a row of evenly spaced holes. These holes allow the system to read ambient or static air pressure. Both inputs are then conveyed to the measuring instruments where they are translated into graphical readings.
Most pitot static tubes are mounted outside an aircraft’s fuselage. This requires the tube to be bent at right angles to present the opening in the tube directly into the air stream; it also gives the pitot tube its characteristic L-shape. The static pitot tube is normally equipped with internal heating elements which prevent ice formation and blockage. A blocked pitot tube creates false readings of airspeed, altitude, and rate of climb. This has been the cause of many catastrophic aviation accidents in the past and is testament to the importance of properly functioning pitot systems.
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