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## Pitot-Static Instruments 

There are two major divisions in the pitot-static system: 

1. Impact air pressure due to forward motion (flight) captured in the pitot tube transferred to instruments by way of air pressure lines or tubing. 

2. Static or free air pressure sensed at static air ports designed to be free of motion induced pressure variations. This reference pressure is necessary because the free pressure of the atmosphere decreases as altitude increases and changes due to the current weather (barometric) pressure variations. This static or free air pressure is transferred to instruments by way of the static pressure lines or tubing. 

## Impact and Static Pressure Lines 

The impact air pressure (air striking the glider because of its forward motion) is taken from the pitot tube, which is mounted on either the nose or the vertical stabilizer. [Figure 4-1]  Pitot tubes are aligned with the relative wind. These locations minimize air disturbance or turbulence during glider flight through the air. 

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When a glider is in flight, the oncoming air tries to flow into the open end of the pitot tube. [Figure 4-2]  Connecting a diaphragm to the back end of the pitot tube means that the air flowing in has nowhere to go. The pressure in the diaphragm rises until it is high enough to prevent any further air from 

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entering. Increasing the airspeed of the glider causes the force exerted by the oncoming air to rise. More air is able to push its way into the diaphragm and the pressure within the diaphragm increases. The pressure inside the diaphragm to oncoming airflow increases as airspeed increases. 

The static pressure (pressure of the still air) is taken from the static line that is attached to a port, or ports, mounted flush with the side of the fuselage or tube mounted on the vertical stabilizer. [Figure 4-3]  Gliders using a fuselage flush mounted static source have two vents, one on each side of the fuselage. This compensates for variation of static pressure due to changes in glider attitude and air turbulence. 

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The openings of both the pitot tube and the static port(s) should be checked during the preflight inspection to ensure they are free from obstructions. Clogged, or partially clogged, openings should be cleaned by a certificated mechanic. Blowing into these openings is not recommended, because this could damage flight instruments. 

## Airspeed Indicator 

The airspeed indicator  displays the indicated airspeed (IAS) of the glider through the air. [Figure 4-4] Some airspeed indicator dials provide color-coded arcs that depict permissible airspeed ranges for different phases of flight. The 

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