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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] 

## The Effects of Altitude on the Airspeed Indicator 

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] 

## Types of Airspeed 

There are three kinds of airspeed that the pilot should understand: IAS, calibrated airspeed (CAS), and true airspeed (TAS). [Figure 4-9] 

## Indicated Airspeed (IAS) 

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 

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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 . 

## Calibrated Airspeed (CAS) 

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. 

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 

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