upper (top) and lower (bottom) limits of the arcs correspond  to airspeed limitations for specific gliders configurations  which are discussed later in this chapter. These speed  limitations are set by the manufacturer.  Figure 4-5  shows  the anatomy of the airspeed indicator and where the pitot  and static pressure inlets are located.  The airspeed indicator depends on both pitot pressure and  static pressure.  [Figure 4-6]  When pitot pressure and static  pressure are the same, zero airspeed is indicated. As pitot  pressure becomes progressively greater than static pressure,  airspeed is indicated by the needle pointing to the speed  scale. The airspeed instrument contains a diaphragm that  senses differential pitot and static pressure. The diaphragm  expands or contracts according to the difference of static and  pitot pressures; this movement drives the needle (airspeed  needle pointer) on the face of the instrument.  [Figure 4-7] shows that the IAS at which a glider stalls in steady wings- level flight does not vary with altitude. However, pilots must  remember that different gliders stall at different speeds. The  IAS shown in  Figure 4-10  may not be the stall speed for each  particular glider. The IAS at which never exceed speed (V$_{NE}$) is reached  decreases with altitude. The glider flight manual (GFM)  should include a table, such as the one shown in  Figure 4-11, that details how V$_{NE}$  should be reduced with altitude. These  figures vary from glider to glider; therefore, pilots should  always refer to the manual specific to the glider they are flying,  which should show a chart similar to the one in  Figure 4-11 . Like pressure, air density also decreases with altitude. The  airspeed indicator’s diaphragm is calibrated to correctly  display airspeed when the air through which the aircraft  is moving is of average sea level density. Above sea level,  due to the lower air density, the buildup of pressure in  the diaphragm is lower, and the airspeed indicator reads  artificially low. The higher the altitude above sea level, the  more erroneous the airspeed indicator value.  [Figure 4-8] CAS is IAS corrected for installation and instrument error.  Although manufacturers attempt to keep airspeed errors to a  minimum, it is impossible to eliminate all errors throughout  the airspeed operating range. At certain airspeeds and with  certain flap/spoiler settings, the installation and instrument  error may be significant. The error is generally greatest at  low airspeeds. In the cruising and higher airspeed ranges,  IAS and CAS are approximately the same.  There are three kinds of airspeed that the pilot should  understand: IAS, calibrated airspeed (CAS), and true airspeed  (TAS).  [Figure 4-9] IAS is the direct instrument reading obtained from the  airspeed indicator, uncorrected for variations in atmospheric  density, installation error, or instrument error.  Figure 4-10 It is important to refer to the airspeed calibration chart  to correct for possible airspeed errors because airspeed  limitations, such as those found on the color-coded face  of the airspeed indicator, on placards in the cockpit, or in  Figure 4-5.  Anatomy of the airspeed indicator. 4-3 Types of Airspeed Indicated Airspeed (IAS) Calibrated Airspeed (CAS) The Effects of Altitude on the Airspeed Indicator Figure 4-6.  The functionality of the airspeed indicator depends on  both pitot and static pressure.  40 50 60 70 10 0 90 80 Total pressure inlet Static vents Static pressure inlet Pitot tube Airspeed indicator Pitot tube 40 50 60 70 10 0 90 80 Static pressure inlet Total (pitot) pressure inlet Display needle Linkages and gearing: Connect the diaphragm capsule to the display needle. Enclosure: Airtight except for the static pressure inlet. It has  a glass front through which the display needle  can be viewed. Diaphragm capsule: A capsule with elastic properties that is airtight, except for its connection to the  total pressure inlet. Like a balloon, it  expands as the pressure inside it  increases above the pressure outside it.