If you do the math right and use proper flying technique during test runs, you can arrive at an accurate number.

By Kevin Horton

How fast does your aircraft go? You could grab your trusty E-6B and use your airspeed indicator (ASI) reading, pressure altitude and outside air temperature (OAT) to calculate true airspeed (TAS). But if you are flying an Experimental/Amateur-Built aircraft, you may not know what the static source position error is. And your ASI probably has instrument error. What is a guy to do? The GPS is the answer. We can use the groundspeed information from the GPS to calculate TAS, as long as we are careful to use a method that is mathematically sound.

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