# True Airspeed & Ground Speed

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Hi.Hope this dosen't sound a dumb question,but could someone please explain what the difference between TAS and GS are.I believe its is something to do with the wind effecting the speed of the aircraft,and is the speed on the Navigation Display shown in Knots or MPH.Also another question.On the PROGRESS page of the FMC the DISTANCE To Go to your destination, is this shown in Nautical Miles or Statute miles.Any help much aprreciated.Tony

Your thinking it has something to do with wind is right on the mark. Ground speed is the combination of true air speed (TAS) and wind effects. If you've had vectors in math, it is just the sum of the TAS and wind vectors; otherwise, it gets a little more involved unless the wind is exactly a headwind or tailwind. If TAS is 450kts and wind is exactly on the tail at 50kts then GS is 500kts. This is because the aircraft is traveling through the air, which itself is moving with respect to the ground. The interesting notion is that there is no experiment that can be done inside the aircraft to prove that it is moving at all as long as it is moving at a uniform speed (no acceleration in any direction). This is one reason it feels like you're just hanging in the air with no discernable motion (if the air is smooth) except the slow progress of the ground below.All speeds in aviation are assumed to be in nauticle miles, due to convention and that it is practical. By definition, there are 60nm to one degree latitude, which aids navigation.

Dan Downs KCRP

>The interesting notion is that there is no>experiment that can be done inside the aircraft to prove that>it is moving at all as long as it is moving at a uniform speed>(no acceleration in any direction). This is one reason it>feels like you're just hanging in the air with no discernable>motion (if the air is smooth) except the slow progress of the>ground below.And we can thank Mr. Einstien for that principle of nature... ;)

Ryan Maziarz

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Einstien used the priciple, but Newton used it in "Principia" but it was sailing ships instead of airplanes, of course. Albert liked trains.

Dan Downs KCRP

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