Position error
Encyclopedia
Position error is one of the errors affecting the systems in an aircraft for measuring airspeed
and altitude. It is not practical or necessary for an aircraft to have an airspeed indicating system and an altitude indicating system that are exactly accurate. A small amount of error is tolerable.
to the altimeter
and the airspeed indicator
. This static port and the conduit constitute the aircraft's static system. The objective of the static system is to sense the pressure of the air at the altitude at which the aircraft is flying. In an ideal static system the air pressure fed to the altimeter and airspeed indicator is equal to the pressure of the air at the altitude at which the aircraft is flying.
As the air flows past an aircraft in flight, the streamlines are affected by the presence of the aircraft and the speed of the air relative to the aircraft is different at different positions on the aircraft's outer surface. In consequence of Bernoulli's principle
, the different speeds of the air result in different pressures at different positions on the aircraft's surface. The ideal position for a static port is a position where the local air pressure in flight is always equal to the pressure remote from the aircraft, however there is no position on an aircraft where this ideal situation exists for all angles of attack
. When deciding on a position for a static port aircraft designers attempt to find a position where the error between static pressure and free-stream pressure is a minimum across the operating range of angle of attack of the aircraft. The residual error at any given angle of attack is called the position error.
Position error affects the indicated airspeed
and the indicated altitude. Aircraft manufacturers use the aircraft flight manual to publish details of the error in indicated airspeed and indicated altitude across the operating range of speeds. In many aircraft, the effect of position error on airspeed is shown as the difference between indicated airspeed
and calibrated airspeed
. In some low-speed aircraft, the position error is shown as the difference between indicated airspeed
and equivalent airspeed
.
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.
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 error in indicated airspeed must also not be excessive. Part 23, §23.1323(b) includes the following requirement for the airspeed indicating system:
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
and altitude. It is not practical or necessary for an aircraft to have an airspeed indicating system and an altitude indicating system that are exactly accurate. A small amount of error is tolerable.
Static system
All aircraft are equipped with a small hole in the surface of the aircraft called the static port. The air pressure in the vicinity of the static port is conveyed by a conduitElectrical conduit
An electrical conduit is an electrical piping system used for protection and routing of electrical wiring. Electrical conduit may be made of metal, plastic, fiber, or fired clay. Flexible conduit is available for special purposes....
to the altimeter
Altimeter
An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...
and the airspeed indicator
Airspeed indicator
The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot.- Use :...
. This static port and the conduit constitute the aircraft's static system. The objective of the static system is to sense the pressure of the air at the altitude at which the aircraft is flying. In an ideal static system the air pressure fed to the altimeter and airspeed indicator is equal to the pressure of the air at the altitude at which the aircraft is flying.
As the air flows past an aircraft in flight, the streamlines are affected by the presence of the aircraft and the speed of the air relative to the aircraft is different at different positions on the aircraft's outer surface. In consequence of Bernoulli's principle
Bernoulli's principle
In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy...
, the different speeds of the air result in different pressures at different positions on the aircraft's surface. The ideal position for a static port is a position where the local air pressure in flight is always equal to the pressure remote from the aircraft, however there is no position on an aircraft where this ideal situation exists for all angles of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
. When deciding on a position for a static port aircraft designers attempt to find a position where the error between static pressure and free-stream pressure is a minimum across the operating range of angle of attack of the aircraft. The residual error at any given angle of attack is called the position error.
Indicated airspeed
Indicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....
and the indicated altitude. Aircraft manufacturers use the aircraft flight manual to publish details of the error in indicated airspeed and indicated altitude across the operating range of speeds. In many aircraft, the effect of position error on airspeed is shown as the difference between indicated airspeed
Indicated airspeed
Indicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....
and calibrated airspeed
Calibrated airspeed
Calibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
. In some low-speed aircraft, the position error is shown as the difference between indicated airspeed
Indicated airspeed
Indicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....
and equivalent airspeed
Equivalent airspeed
Equivalent airspeed is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an airspeed...
.
Pitot system
Bernoulli's principleBernoulli's principle
In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy...
states that total pressure (or stagnation pressure
Stagnation pressure
In fluid dynamics, stagnation pressure is the static pressure at a stagnation point in a fluid flow.At a stagnation point the fluid velocity is zero and all kinetic energy has been converted into pressure energy . Stagnation pressure is equal to the sum of the free-stream dynamic pressure and...
) 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
Pitot tube
A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...
supplies pressure to the airspeed indicator
Airspeed indicator
The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot.- Use :...
. 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 systemPitot-static system
A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments...
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
Federal Aviation Regulations
The Federal Aviation Regulations, or FARs, are rules prescribed by the Federal Aviation Administration governing all aviation activities in the United States. The FARs are part of Title 14 of the Code of Federal 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 ±30 feet per 100 knot 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 five knots, 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 GNSSGlobal Navigation Satellite SystemA satellite navigation or SAT NAV system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location to within a few metres using time signals transmitted along a line-of-sight by radio from...
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 bomb with both Pitot and static sources
See also
- Pitot-static systemPitot-static systemA pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments...
- AirspeedAirspeedAirspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
- Altitude
- Reduced Vertical Separation MinimaReduced Vertical Separation MinimaReduced Vertical Separation Minima or Minimum is an aviation term used to describe the reduction of the standard vertical separation required between aircraft flying above FL285 and up to FL410 from 2,000 feet to 1,000 feet...