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• The yellow arc—caution range. The pilot should avoid this area unless in smooth air. 

• The red line—never-exceed speed. This is the maximum speed at which the glider can be operated in smooth air. This speed should never be exceeded intentionally. 

## Other Airspeed Limitations 

There are other important airspeed limitations not marked on the face of the airspeed indicator. These speeds are generally found on placards  [Figure 4-12]  in the view of the pilot and in the GFM/POH. 

• Maneuvering speed (Va)—maximum speed at which the limit load can be imposed (either by gusts or full defection of the control surfaces for one cycle) without causing structural damage. If rough air or severe turbulence is encountered during flight, the airspeed should be reduced to maneuvering speed or less to minimize the stress on the glider structure. Maneuvering speed is not marked on the airspeed indicator. For gliders, if there is a rough airspeed (V$_{b}$) limitation published the pilot should be below that speed for maximum gust intensity. 

• Landing gear operating speed—maximum speed for extending or retracting the landing gear if using glider equipped with retractable landing gear. 

• Minimum sink speed—important when thermalling. 

• Best glide speed—airspeed that results in the least amount of altitude loss over a given distance, not considering the effects of wind. 

• Maximum aerotow or ground launch speed— maximum airspeed that the glider may safely be towed without causing structural damage. 

## Altimeter 

The altimeter measures the static air pressure of the surrounding air mass. A flexible plastic tube connects the altimeters static pressure inlet to the static port holes located on the side of the glider. [Figure 4-13]  If set to the proper local pressure, the altimeter needles and dial indicate heights above mean sea level (MSL). [Figures 4-14 and  4-17] Glider pilots must be fully aware of the ground elevations on 

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the flight route or area in order to make flight decisions concerning soaring or landing options. 

## Atmospheric Pressure and Altitude 

Atmospheric pressure is caused by the weight of the column of air above a given location. At sea level, the overlying column of air exerts a force equivalent to 14.7 pounds per square inch, 1013.2 mb, or 29.92 inches of mercury. The higher the altitude is, the shorter the overlying column of air is and the lower the weight of that column is. Therefore, atmospheric pressure decreases with altitude. At 18,000 feet, atmospheric pressure is approximately half that at sea level. [Figure 4-16] 

## Principles of Operation 

The pressure altimeter is simply an aneroid barometer that measures the pressure of the atmosphere at the level at which the altimeter is located and presents an altitude indication in feet. The altimeter uses static pressure as its source of operation. Air is denser at the surface of the earth than aloft; as altitude increases, atmospheric pressure decreases. This difference in pressure at various levels causes the altimeter to indicate changes in altitude. Figures 4-15 and 4-17  illustrate how the altimeter functions. The presentation of altitude varies considerably between different types of altimeters. Some have one pointer while others have more. 

The dial of a typical altimeter is graduated with numerals arranged clockwise from 0 to 9 inclusive, as shown in Figure 4-14 . Movement of the aneroid element is transmitted through a gear train to the three hands, which sweep the calibrated dial to indicate altitude. The shortest hand indicates altitude in tens of thousands of feet; the intermediate hand in thousands of feet; and the longest hand in hundreds of feet, subdivided into 20-foot increments. 

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