shown as the difference between indicated airspeed and equivalent airspeed. Pitot system Bernoulli's principle states that total pressure (or stagnation pressure) is constant along a streamline. There is no variation in stagnation pressure, regardless of the position on the streamline where it is measured. There is no position error associated with stagnation pressure. The pitot tube supplies pressure to the airspeed indicator. Pitot pressure is equal to stagnation pressure providing the pitot tube is aligned with the local airflow, it is located outside the boundary layer, and outside the wash from the propeller. Pitot pressure can suffer alignment error but it is not vulnerable to position error. Aircraft design standards Aircraft design standards specify a maximum amount of Pitot-static system error. The error in indicated altitude must not be excessive because it is important for pilots to know their altitude with reasonable accuracy for the purpose of traffic separation. US Federal Aviation Regulations, Part 23, §23.1325(e) includes the following requirement for the static pressure system: The system error, in indicated pressure altitude, ..., may not exceed ± per speed for the [operating speed range for the aircraft]. The error in indicated airspeed must also not be excessive. Part 23, §23.1323(b) includes the following requirement for the airspeed indicating system: The system error, including position error, ..., may not exceed three percent of the calibrated airspeed or , whichever is greater, throughout the [operating speed range for the aircraft]. Measuring position error For the purpose of complying with an aircraft design standard that specifies a maximum permissible error in the airspeed indicating system it is necessary to measure the position error in a representative aircraft. There are many different methods for measuring position error. Some of the more common methods are: use of a GNSS receiver while flying a triangular course trailing conduit with static source, stabilized by a plastic cone tower fly-by with photographs of the passing aircraft taken from the tower to accurately show the height of the aircraft above or below the tower trailing bob with both Pitot and static sources See also Airspeed Altitude Global Positioning System Pitot-static system Reduced vertical separation minima References Bibliography Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London . Kermode, A.C. (1972) Mechanics of Flight, Longman Group Limited, London External links Determining static system error Aircraft instruments Air navigation Airspeed