Aircraft pilots rely upon a number of accurate and constantly updating metrics to safely operate aircraft. Of the several systems that work together to provide this relevant information to the cockpit, one of the most important is the pitot-static system. In this blog, we will highlight everything you need to know about the pitot-static system, including its constituent parts, function, and some common issues that may arise.
A pitot-static system is a group of various connected components that work together to bring real-time data about an aircraft's airspeed, Mach number, and altitude. This system collects data through a pitot tube, which is a component placed on the aircraft's wing in such a way that it is exposed to the relative wind. When a plane is at cruising altitude, icing may pose a small risk to the pitot tube, although it is typically mitigated by the electric heating system. Pitot tubes collect information about a particular type of air pressure, called dynamic pressure, determined by the simple formula (air density x airspeed squared, divided by 2).
The pitot-static system also contains one or more static pressure ports found at areas of the fuselage that receive a steady and undisturbed airflow. Similar to the pitot tube, these ports are also at risk for icing, which is why most aircraft have two ports or an alternative port inside the aircraft's cabin. These ports take samples of the static air from around the inlet and measure it, which helps to determine altitude. Information from the pitot tube and static ports make it to the plane's air data computer, if equipped with one, or directly to the three associated instruments.
The altimeter measures altitude using the static air pressure measurements collected from the ports. To do this, altimeters are equipped with an aneroid barometer that expands as air pressure decreases, as is the case when the aircraft gains altitude. Conversely, the barometer inches back to its original size as the plane descends.
Also connected to the static pressure port is the vertical speed indicator (VSI). This instrument is used to measure the aircraft's ascending or descending velocity in feet per minute. It calculates this by measuring the air pressure in a sealed diaphragm and then comparing it to a metered exit port located at the back of the instrument case. The pressure differential between these two ports is then measured and calculated to reflect current vertical velocity. Finally, the airspeed indicators use information from both the static pressure ports and pitot tube. Using the aforementioned formula, the instrument measures dynamic pressure and relates it directly to airspeed.
While pitot-static system disturbances and failures are exceedingly rare, it is still important to be aware of the few problems they may face during normal operation. The most common problem occurs when ice or some other debris blocks the inlet of the static vent. Since all three instruments rely on the readings from this component, there will be immediate misreadings on the various displays. For example, the vertical speed indicator will quickly revert to zero, even if the aircraft is in ascent or descent. Thankfully, most aircraft have a secondary and tertiary port to prevent such occurrences from disrupting real-time data, and the pilot can easily switch between the various inputs. The other common disruption is a blockage in the pitot tube. This problem only affects the airspeed indicator, and the changes are usually very subtle. If the blockage occurs due to icing, the pilot may ignite the electric heater to regain accurate readings.
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