To convert IAS to TAS, first determine Calibrated Airspeed (CAS). CAS is the result of IAS
corrected for instrument error. This instrument error is recorded in the cockpit in the form of
an Airspeed Calibration Card which gives what the airspeed indicator reads (IAS) and what
it should read (CAS).
For a given IAS ("X"), TAS will generally increase with an increase in altitude.
Pressure Altitude (PA) is the measurement of atmospheric pressure from the "Standard
Datum Plane." To find Pressure Altitude, first determine the Calibrated Altitude. Calibrated
Altitude is Indicated Altitude PLUS or MINUS instrument error. Example follows:
= 10,000 feet
= 10,000 feet
Next, find the difference between the given altimeter setting and the Standard Datum
Plane. If the local altimeter setting was 31.12" and the Standard Datum Plane is 29.92", the
problem would look like this:
Standard Datum Plane
1.20" (pressure difference)
Then, change the pressure difference (1.20") to altitude (feet) using the standard lapse rate
of 1" Hg (mercury) = 1,000 feet. A difference of 1.20" Hg would equal 1,200 feet.
Finally, you must either ADD or SUBTRACT the pressure difference (1,200 feet) from the
Calibrated Altitude (10,000 feet). If the given altimeter setting is less than 29.92", you ADD.
If the given altimeter is greater than 29.92", (like the above example) you SUBTRACT:
- 1,200 feet
To assist in determining whether to add or subtract the pressure difference, apply
the term "LAGS" which stands for:
- If the given altimeter setting is Less (than 29.92"), then ADD
- If the given altimeter setting is Greater (than 29.92"), then SUBTRACT