40 50 60 70 10 0 90 80 Atmospheric pressure 1,013 mb Atmospheric pressure 1,013 mb 40 50 60 70 10 0 90 80 Ram air pressure 1,013 mb Ram air pressure 1,019 mb Speed: 0 knot Speed: 60 knots In a stationary aircraft, the air throughout  the airspeed indicator has equalized with  local atmospheric pressure. The diaphragm  is collapsed like a deflated balloon,  and the needle is at zero. 1 As airspeed increases, the pressure  inside the diaphragm rises above local atmospheric pressure due to the force exerted down the pitot tube by the oncoming airflow. The diaphragm inflates like a balloon, moving the linkages and rotating the needle. 2 60 70 30 80 90 20 10 0 50 40 60 70 30 80 90 20 10 0 50 40 60 70 30 80 90 20 10 0 50 40 Airspeed indicator of an  aircraft flying at 60 knots  at 10,000 ft Airspeed indicator of an  aircraft flying at 60 knots  at sea level Airspeed indicator of an  aircraft flying at 60 knots  at 20,000 ft 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  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.  Figure 4-8.  Effects of altitude on the airspeed indicator. Figure 4-7.  Airspeed indicator operation.  True Airspeed (TAS) 4-4