Pitot-Static Systems

Three basic pressure-operated instruments are found in most aircraft instrument panels. These are the sensitive altimeter, airspeed indicator (ASI), and vertical speed indicator (VSI). All three receive the pressures they measure from the aircraft Pitot-static system.
 
Flight instruments depend upon accurate sampling of the ambient atmospheric pressure to determine the height and speed of movement of the aircraft through the air, both horizontally and vertically. This pressure is sampled at two or more locations outside the aircraft by the Pitot-static system.
 
The pressure of the static, or still air, is measured at a flush port where the air is not disturbed. On some aircraft, this air is sampled by static ports on the side of the electrically heated Pitot-static head, such as the one in figure 3-1: "A typical electrically heated pitot static head". Other aircraft pick up the static pressure through flush ports on the side of the fuselage or the vertical fin. These ports are in locations proven by flight tests to be in undisturbed air, and they are normally paired, one on either side of the aircraft. This dual location prevents lateral movement of the aircraft from giving erroneous static pressure indications. The areas around the static ports may be heated with electric heater elements to prevent ice forming over the port and blocking the entry of the static air.
 
Pitot pressure, or impact air pressure, is taken in through an open-end tube pointed directly into the relative wind flowing around the aircraft. The pitot tube connects to the airspeed indicator, and the static ports deliver their pressure to the airspeed indicator, altimeter, and VSI. If the static ports should ice over, or in any other way become obstructed, the pilot is able to open a static-system alternate source valve to provide a static air pressure source from a location inside the aircraft. [Figure 3-2: A typical pitot static-system] This may cause an inaccurate indication on the pitot-static instrument. Consult the Pilot's Operating Handbook/Airplane Flight Manual (POH/AFM) to determine the amount of error.
 
Position Error
The static ports are located in a position where the air at their surface is as undisturbed as possible. But under some flight conditions, particularly at a high angle of attack with the landing gear and flaps down, the air around the static port may be disturbed to the extent that it can cause an error in the indication of the altimeter and airspeed indicator. Because of the importance of accuracy in these instruments, part of the certification tests for an aircraft is a check of position error in the static system.
 
The POH/AFM contains any corrections that must be applied to the airspeed for the various configurations of flaps and landing gear.
 
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Pitot-static head: A combination pickup used to sample Pitot pressure and static air pressure.
 
Static pressure: Pressure of the air that is still, or not moving, measured perpendicular to the surface of the aircraft.