Aircraft Pitot Static Systems
The pitot static system is critical to safe flight. This system feeds the altimeter, airspeed indicator, and vertical speed indicator with ram air (pitot input) and pressure (static) input to determine altitude, airspeed, and rate of vertical climb (the vertical speed indicator). Aircraft are designed with Pitot probes which face into the direction of travel. As speed increases, airflow into the pitot tube increases (ram pressure) and correlates to an increase in speed which feed the airspeed indicator. The static ports (often flush-mounted on the fuselage of the aircraft) measure the atmospheric pressure of the air to determine altitude. These input sources (pitot tubes and static ports) are connected to the airspeed indicator, VSI (vertical speed indicator), and altimeter directly via airlines. All 3 indicators receive a static input, however, the airspeed indicator is the only one that receives a pitot input.
Inside the altimeter, there are bellows of air that represent the "ground" atmosphere. As we increase altitude, the static pressure reduces around the bellows allowing them to expand and drives the indicator pointer to a higher displayed altitude.
The vertical speed indicator has a diaphragm inside that has a direct input from the static port. Inside, the diaphragm has a calibrated leak that which limits the rate at which the pressure can change. As the static pressure value increases or decreases with changes in altitude, this pressure change results in a display of the rate at which the aircraft is climbing.
The airspeed indicator receives it's input from both the static ports and pitot tubes. As the ram pressure enters the diaphragm, it will expand (displaying a faster airspeed), or constrict (lower airspeed). But why do we need a static input? Indicated aircraft speed is inversely relative to atmospheric pressure. As the pressure decreases with altitude, airspeed (indicated) will increase. This static input is needed to correct for the pressure effect on the speed of the aircraft.
One major issue that can be faced with the pitot static system is moisture entering the system. This is combated through holes in the back of the pitot tubes that drain as the air entering the tube passes through. There is always an element of water vapor in our atmosphere and as it becomes more saturated, or an aircraft is flying through rain, there is a risk of water making it into the instruments. Drains prevent this. Another risk associated with water and the pitot tubes is icing. If a pitot tube were to have ice build-up inside the tube, there would be an inaccurate reflection of aircraft speed. Most modern pitot static systems have a heater circuit that will remain on during flight to keep ice from accumulating.
REFERENCE
ERAU SpecialVFR. 2017 Aug 01. Pitot static instruments [Video]. Youtube. https://www.youtube.com/watch?v=kdFGbUouE_4&list=PLzW-Ub1FWeZzdOHQhNK0U0Ci1a-VRH8IO&index=49
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