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A sometimes overlooked part of the flight test program is calibration of the airspeed indicator (ASI). The airspeed indicator is (arguably) the single most important instrument we have on board our airplane, and it's important that owners/pilots make sure that the ASI has been calibrated (that it has been corrected for errors).
Indicated Airspeed (IAS) is the speed of the aircraft as shown on the airspeed indicator.
Calibrated Airspeed (CAS) is the indicated speed of the aircraft after it is corrected for position and instrument error. CAS is equal to true airspeed in standard atmospheric conditions at sea level. It is important that builders/owners calibrate their airspeed, both with flaps up and down, and mark their ASI accordingly.
An excellent source for information and details on how to do this is the National Test Pilot School website (www.ntps.edu), and specifically, their download page on Using GPS to Determine Pitot-Static Errors.
According to the worksheet, we're going to note GPS speeds as we maintain specified IAS while maintaining specific headings at a given altitude. We'll do this for different given headings to account for prevailing winds.
Additional equipment required: A handheld GPS and a portable outside air temperature (OAT) gauge (shown above).
Roger is ready to fly with clipboard and GPS in hand:
Filling in the worksheet as we fly:
After flying, Roger inputs the data in the worksheet:
...and then transfers the info to the ASI:
It's important that the builder/owner properly document the CAS in the pilot operating handbook (POH).
For a good example of this, see page 5-4 in the AMD Zodiac series POH (11/09).