Pitot–static system

A pitot–static system generally consists of a pitot tube, a static port, and the pitot–static instruments.

[2] The Code of Federal Regulations (CFRs) require pitot–static systems installed in US-registered aircraft to be tested and inspected every 24 calendar months.

The pitot tube is most often located on the wing or front section of an aircraft, facing forward, where its opening is exposed to the relative wind.

[1] By situating the pitot tube in such a location, the ram air pressure is more accurately measured since it will be less distorted by the aircraft's structure.

The static port is most often a flush-mounted hole on the fuselage of an aircraft, and is located where it can access the air flow in a relatively undisturbed area.

While the explanations below explain traditional, mechanical instruments, many modern aircraft use an air data computer (ADC) to calculate airspeed, rate of climb, altitude and Mach number.

In some aircraft, two ADCs receive total and static pressure from independent pitot tubes and static ports, and the aircraft's flight data computer compares the information from both computers and checks one against the other.

A traditional mechanical airspeed indicator contains a pressure diaphragm that is connected to the pitot tube.

As pressure in the case decreases, the internal barometer expands, which is mechanically translated into a determination of altitude.

[5] The vertical speed is measured through a mechanical linkage to a diaphragm located within the instrument.

The area surrounding the diaphragm is vented to the static port through a calibrated leak (which also may be known as a "restricted diffuser").

At higher altitudes the compression is not correctly accounted for and will cause the instrument to read greater than equivalent airspeed.

Hysteresis is an error that is caused by mechanical properties of the aneroid capsules located within the instruments.

This false reading may be caused by abnormally large changes in an aircraft's pitch.

A large change in pitch will cause a momentary showing of movement in the opposite direction.

This error is caused by the air flowing past the static port at a speed different from the aircraft's true airspeed.

Variable errors are caused by external factors such as deformed panels obstructing the flow of air, or particular situations which may overstress the aircraft.

[6] Lag errors are caused by the fact that any changes in the static or dynamic pressure outside the aircraft require a finite amount of time to make their way down the tubing and affect the gauges.

This type of error depends on the length and diameter of the tubing as well as the volume inside the gauges.