Fly the airplane to a cruise altitude and maintain level flight. Read from the air speed indicator the indicated air speed. This air speed is not corrected for instrument or position error.
Refer to the air speed calibration data for that air speed indicator; note the deviation, and apply that difference to the indicated air speed to see the calibrated air speed. The air speed calibration for the aircraft can be found in the aircraft manual or the pilot's operating handbook for the airplane.
Note the outside air temperature on the OAT gauge. Temperature that is not standard will have an affect on the true air speed calculation. Standard temperature at sea level is 15 degrees Celsius or 59 degrees Fahrenheit. As you increase in altitude, the standard lapse rate is 2 degrees C or 3.6 degrees F for every thousand feet of altitude.
Derive the pressure altitude by setting the pressure in the Kohlsman window to 29.92 inches. This will yield the pressure altitude. Pressure altitude is necessary because the calculations for true air speed are based on standard pressure. Standard pressure is 29.92 inches.
Calculate the true air speed by using the air speed computer to correct the calibrated air speed for non-standard temperature and pressure. You will need to input the pressure altitude, outside air temperature and the calibrated air speed. The result will yield the true air speed.