Pitot-static system

The pitot -static system supplies the barometric flight instruments in the cockpit of aircraft with print information about the static pressure and the dynamic pressure. Thus, airspeed, altitude and climb or descend to the appropriate instruments can be displayed. It is also an outdoor temperature sensor must be installed. The principle of operation of the system corresponds to that of Prandtl pitot tube.

Principle of measurement

When aircraft go up, go down, speed up or slow the rate of air pressure acting on it changed. The air pressure in the environment of an aircraft is composed of the static pressure (pressure at rest of ambient air based on the temperature and the existing altitude of the aircraft ) and the back pressure is experienced in the movement resistance.

The static pressure and the dynamic pressure to be used in the barometric flight control instruments for determining the altitude of the horizontal speed and the vertical speed.

Construction

The pitot -static system is composed of the following parts:

• Pitot tube (named after Henri Pitot, 1695-1771 )
• Static ports
• Line for the static pressure
• Total pressure line
• Barometric indicator in the cockpit instruments ( altimeter, airspeed indicator and variometer )

The pitot tube extends into the unaffected air stream and is, for example, attached to the tip of the nose below a surface with the opening outside the Flächenluftstömung or jets. Static ports can be located according to aircraft at various locations. There may be, all of which are not blown directly from the air flow is also more ports.

All three barometric flight instruments are connected via the line for the static pressure with the static ports. The airspeed indicator is also connected nor the total pressure line to the pitot tube.

For commercial aircraft ( airliner ) the variometer is now often supplied by the IRS or GPS. These aircraft have a number of pitot tubes, which are mounted at various locations on the aircraft. The pressure of this Pitot tubes are averaged and subtracted separately measured the static pressure of it. The determined dynamic pressure is proportional to the square of the flow rate and can be used for the determination of the IAS ( Indicated Air Speed ​​, German: indicated airspeed ) can be used.

Operation

The pitot -static system in an aircraft is to be understood as an integral unit. The air pressure in the form of static pressure and dynamic pressure is the medium to be measured. This is measured and on the three instruments, barometric altimeter, airspeed indicator, and vertical speed in different units ( feet, knots, miles per hour, m / s ) is displayed. In case of failure of the pitot -static system under instrument flight conditions ( IFR), a controlled flight is hardly possible, which is why the devices in airplanes that fly under these conditions are redundant. The conventional pitot -static measuring systems can not provide a reliable measurement when the airspeed is below 40 knots. Therefore, a different measuring method is used for helicopters.

Possibilities of error

For airplanes without pressurized cabin, another alternative static port is located in the cabin. This port may cause a faulty instrument display, as the cabin pressure is low because of the air flow above the car than the external pressure. Speed ​​and altitude appear to be higher than real. The variometer shows a temporary rise when the alternate static port is opened, followed by a stabilization phase and subsequent normal display.

A closure in the pitot -static system (freezing, insects, contaminated water spray, or similar) leads, depending on location at a different None at altimeter, variometer and airspeed indicator. This can then in misinterpretation even lead to a crash as the Birgenair accident in the Caribbean in 1996 or the Air France flight 447 over the Atlantic in 2009.

The following is a table of the possible errors of the pitot -static system, and the effect is illustrated that has to know at any IFR pilot.

Shows the descent too much speed

Shows the descent too little speed