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A static pitot tube, or Prandtl tube as it is also known, is the primary component of a measurement system that determines air speed and altitude in airplanes. Pitot tube systems achieve this measurement with a tube facing into an airflow to compare the relative pressure of the air which 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 static pitot tube is one of 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 the speed of the air flow can be achieved by comparing the difference between the total pressure and the ambient or static pressure. Static pitot tube systems achieve these comparisons by placing an open tube facing into the airflow to measure total pressure and a static port in an area of low turbulence to measure ambient pressure. Both inputs may then be used to extrapolate various readings such as air speed, altitude, and rate of climb.
Pitot tube systems either rely on separate ram air tubes and static ports or combination tubes which measure both total and static pressure. These are known as static pitot 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 perforated with a row of equidistant holes. These holes allow the system to read the ambient or static air pressure. Both inputs are then channeled to the measurement instruments where they are translated into graphic readouts.
Most static pitot tube assemblies are mounted on the outside of an aircraft's fuselage. This requires the tube to be bent at right angles to present the opening in the tube straight into the airflow; it also gives the pitot tube its characteristic L shape. The static pitot tube normally features internal heater elements which prevent ice build up and blockage. A blocked pitot tube creates false airspeed, altitude, and rate of climb readings. This has been the cause of several catastrophic aircraft accidents in the past and bears testimony to the importance of the correct functioning of pitot systems.