The airspeed indicator's pressure measurement design is constructed for relative (or gauge) pressure measurements as opposed to measuring absolute pressure. The static port measures the outside ambient pressure or static pressure (P). In addition, the ASI is also connected to the static air line which is connected to the static port. The airspeed indicator (ASI) is connected to the ram air line (see fig 2-1), which measures total pressure (P T). As previously mentioned, the sequence as well as the combination in which the blockages occur is also important. Understanding how the instruments that rely on the Pitot-static system are connected helps understand what can happen during a blockage. Note there also may be an alternate static air source that can be activated (usually by the pilot) if the static port is blocked and it provides another source for the static air. Examples include water that may freeze, insects that make a nest while the aircraft is parked, other debris that enter the ports, or the Pitot tube and/or static port covers not having been removed before flight. Blockages may occur in these three openings in any combination. Occasionally the static air port may be integrated as part of the Pitot tube itself making it a Prandtl tube instead of just a plain Pitot tube. The third opening, which is perpendicular to the airstream, is from the static port. In the Pitot tube there is a ram air opening that faces the airstream directly and a relatively much smaller drain hole that allows moisture to drain out of the Pitot tube. The Pitot-static system (Figure 2-1) usually uses three basic openings for its probes which can get blocked. Most radars are not able to determine the aircraft's altitude with precision and this is why the transponder transmitted pressure altitude information is so important to the controller.įig 2-1 The Pitot-static system uses a Pitot tube and static air probe. As a result, the air traffic controller may receive incorrect altitude information regarding the affected aircraft on their radar screens. Blockages can affect the pressure altitude information that the transponder transmits (when iterrogated by the radar) back to the radar facility and air traffic control (ATC). Transponders with mode C and S are also connected to the Pitot-static system. One of the main contributors to the malfunction of these instruments are blockages by ice or foreign objects in the Pitot-static system. Because of this, when they do, the results can be devastating. The airspeed indicator (ASI), the altimeter, and vertical speed indicator (VSI) are instruments that pilots learn to rely on, and they rarely fail or malfunction. Online Aviation Instrument Simulators + E6b, CR3 and other Flight Computer Calculators This lower pressure of the air provides the driving force for sucking in fuel.Article: Pitot-Static System Blockages and Failures The larger diameter is necessary to accommodate the much higher coolant vapor volume after the valve.Ĭarburator application: The narrower tube is accompanied by a decrease in air pressure in the throat of the carburetor. So the viscous pressure decrease dominates. This pressure change is much larger than the static pressure change associated with the decrease in kinetic energy accompanying the increased diameter. In the compressor/evaporator application, there is a valve between the compressor and evaporator that features a very large viscous pressure drop. The dynamic pressure is related to the kinetic energy of the fluid which, when it changes, causes a corresponding change in the static pressure.Ĭondenser/evaporator application: The basic Bernoulli equation applies to situations in which viscous pressure decreases are negligible. Static and dynamic pressure: It is the static pressure that really matters in practical situations.
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