•  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.  •  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.  Figure 4-12.  Speed limitation placards placed in the glider and in view of the pilot.  OR OR Valid when  lower or side hook  is installed: Maximum winch-launching speed Maximum aerotowing speed Maximum maneuvering speed Valid when  front hook only  is installed:  Maximum aerotowing speed Maximum maneuvering speed 65 KIAS 81 KIAS 81 KIAS 81 KIAS 81 KIAS Maximum winch-launching speed Maximum aerotowing speed Maximum maneuvering speed Maximum aerotowing speed Maximum maneuvering speed  120 km/hr IAS 150 km/hr IAS 150 km/hr IAS 150 km/hr IAS 10 km/hr IAS 4-6 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  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.  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.  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] the flight route or area in order to make flight decisions  concerning soaring or landing options. 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. Static vents Altimeter Figure 4-13.  Static vents and altimeter plumbing. Atmospheric Pressure and Altitude Other Airspeed Limitations Principles of Operation Altimeter