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the GFM or Pilot’s Operating Handbook (GFM/POH), are usually CAS. Some manufacturers use IAS rather than CAS to denote the airspeed limitations mentioned. The airspeed indicator should be calibrated periodically. 

Dirt, dust, ice, or snow collecting at the mouth of the pitot tube may obstruct air passage and prevent correct indications. Vibrations may also destroy the sensitivity of the diaphragm. 

## True Airspeed (TAS) 

TAS is the true speed at which the aircraft is moving through the air. The airspeed indicator is calibrated to indicate TAS only under standard atmospheric conditions at sea level (29.92 inches of mercury ("Hg) and 15 °C or 59 °F). Because air density decreases with an increase in altitude, the glider must be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure. Therefore, for a given TAS, IAS decreases as altitude increases; for a given IAS, TAS increases with an increase in altitude. 

Fortunately for the pilot, the amount by which the airspeed indicator underreads approximately cancels out the air density related changes to the glider’s flight dynamics. This means that if the IAS at the point of stall at 1,000 feet in steady wings-level flight is 40 knots, then the IAS at 20,000 feet at the point of the stall is also 40 knots, despite the fact that the stall is actually occurring at a TAS that is 14 knots higher. Therefore, the pilot needs to remember only one set of numbers that work at all altitudes. A decrease in air density with altitude also affects a glider’s flight dynamics. For example, a glider that stalls at a TAS of 40 knots in steady wings-level flight at 1,000 feet stalls at a TAS of 54 knots at 20,000 feet. Consider the inconvenience that this would cause if the airspeed indicator did actually display TAS. The pilot would need to use quick reference cards continuously to look up the stall speed, best L/D speed, and minimum sink speed for the current altitude—not a 

