Position error

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Position error is one of the errors affecting the systems in an aircraft for measuring airspeed and altitude. [1] [2] 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. It is caused by the location of the static vent that supplies air pressure to the airspeed indicator and altimeter.

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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 conduit 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. [3] 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. [4]

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.

Pitot system

Bernoulli's principle states that total pressure (or stagnation pressure) is constant along a streamline. [5] 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, [6] §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:

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:

See also

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Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator (ASI) on an aircraft, driven by the pitot-static system. It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure dynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed. Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from true airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass. Calibrated airspeed (CAS) is the IAS corrected for instrument and position error.

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error.

Pitot-static system system of pressure-sensitive instruments that is most often used in aviation

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. Other instruments that might be connected are air data computers, flight data recorders, altitude encoders, cabin pressurization controllers, and various airspeed switches. Errors in pitot-static system readings can be extremely dangerous as the information obtained from the pitot static system, such as altitude, is potentially safety-critical. Several commercial airline disasters have been traced to a failure of the pitot-static system.

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Primary flight display

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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. In an incompressible flow, stagnation pressure is equal to the sum of the free-stream static pressure and the free-stream dynamic pressure.

In fluid mechanics the term static pressure has several uses:

Machmeter

A Machmeter is an aircraft pitot-static system flight instrument that shows the ratio of the true airspeed to the speed of sound, a dimensionless quantity called Mach number. This is shown on a Machmeter as a decimal fraction. An aircraft flying at the speed of sound is flying at a Mach number of one, expressed as Mach 1.

References

  1. ↑ Kermode, A.C.,Mechanics of Flight, 10th Edition – page 65
  2. ↑ ”Of these errors the error in detection of static pressure is generally the most serious and has the special name, position error.” Dommasch, D.O., Sherby, S.S., and Connolly, T.F. (1967) Airplane Aerodynamics, 4th edition – page 51, Pitman Publishing Corp., New York
  3. ↑ ”It is virtually impossible to find a position where the static pressure is always exactly the same as the pressure in the free airstream away from the aircraft”. Kermode, A.C., Mechanics of Flight, 10th edition – page 65
  4. ↑ ”The amount by which the local static pressure at a given point in the flow field differs from free-stream static pressure is called the position error of the installation.” Measurement of Static Pressure on Aircraft NACA, TR-1364, p.646
  5. ↑ Providing the streamline does not enter a boundary layer, pass through a shock wave or undergo a major change in height so that gravitational potential energy changes are significant. These three factors are absent when considering the Pitot-static system in a subsonic aircraft.
  6. ↑ Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes

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