Among the systems that power the basic flight instruments, the pitot-static system is the most important. It is responsible for three of the six basic instruments, two of which are required for even the most basic VFR airplane.
The pitot-static system includes one or more static ports, a pitot tube, and all the associated plumbing. Although it’s described as a full system regardless of the associated instruments, only the static port is used for the altimeter and vertical speed indicator, while both the pitot tube and static port are used with the airspeed indicator.
The static port is nothing more than a very small hole somewhere on the airplane that measures the outside air pressure. They are typically found on the side of the fuselage, and can be on one side or both. Another popular location is on the back of the pitot mast, as is the case with the Piper Cherokee line. The pressure is fed through the airplane’s internal tubing and up to the pitot-static instruments, where bellows expand and contract to indicate climbs and descents.
The airspeed indicator’s pitot tube is a bit more complicated. The open hole in the front of the instrument measures ram air pressure to provide the speed indication. This shouldn’t be confused with velocity of the air moving into the tube. The air doesn’t flow past the airspeed indicator. Instead, it pushes on a diaphragm, and the difference between the static pressure from the static port, and the ram air pressure from the pitot tube results in the airspeed indication.
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