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How to Adjust Calibrated Airspeed for True Airspeed

Avoid flying with any pilot who tries to figure this out in his head. The sophisticated computations to convert calculated airspeed (CAS) -- also known as indicated airspeed (IAS) -- into true airspeed (TAS) take up a small portion of the average blackboard and are beyond the range of armchair math. Instead, experienced pilots concentrate on flying the aircraft safely while quickly making the conversion from CAS to TAS the way they've done for more than 70 years: Using the E6-B flight computer. Not a computer in the digital sense, this is actually a circular slide rule that calculates flight factors such as fuel burn, time en route and wind corrections. Calculated airspeed is simply the airspeed indicated by a properly calibrated airspeed indicator in the cockpit. True airspeed is the calculated airspeed with such influencing variables as air density, altitude and outside air temperature crunched into a corrected number. Since 1940, the venerable E6-B does it all for you.

Things You'll Need

E6-B model flight computer

Instructions

- 1
  Note the CAS as displayed on the airspeed indicator in the cockpit.
- 2
  Change the altimeter barometric pressure setting to 29.92 and note the new altitude displayed. This is the pressure altitude. Return the altimeter barometric pressure to its original setting.
- 3
  Observe the outside air temperature shown on the OAT indicator in the cockpit. If it is displayed in Fahrenheit, use the conversion table at the bottom of the E6-B computer to convert it to Celsius.
- 4
  Turn the inner disk on the E6-B computer until the large numbers on the inner disk match their corresponding numbers on the outer disk.
- 5
  Rotate the inner disk so the aircraft's pressure altitude is displayed in the lower part of the right window of the inner disk and aligns with the aircraft's outside air temperature in Celsius on the scale just above it.
- 6
  Do not rotate the disks further. Find the calculated airspeed on the airspeed scale of the inside disk. Read the figure on the outside disk immediately opposite the calculated airspeed. This is the true airspeed.

Aviation