# An interesting technicality ... (a quiz)

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say, while flying at some altitude (cruising) I changed may indicated (CAS) airspeed by 1 knot (either increased or decreased). I am looking for a quick, practical formula that would tell me by how much my True Air Speed changed. I assume this formula should work quite well in "normal" range of speeds and altitudes but may produce some extra error above say 35,000 ft (in math that would be called a first order approximation). This is actually a quiz because I just came up with this simple formula ... Michael J.[link:jdtllc.com]http://jdtllc.com/images/RCsupporter.jpg

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A generally accepted way to estimate TAS is to add 2% of IAS for each 1000' of altitude. So if my IAS is reading 280kts at FL350, my formula would be 280+(280*(35*.02))=476kts. But the true TAS has to include temp and pressure factors, so one can get more complex in calculations--it's a handy formula regardless.I should add that the same formula can be applied to calc the change.... A 5 kt change can be calculated as 5+(5*(35*.02))=8.5 kts.-John

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Actually this "2%" formula is too crude to get a real sense what is going on. And will get completely wrong results for "delta CAS versus delta TAS". In fact the formula is this:delta TAS = delta CAS * 1.25This works for most altitudes in standard atmosphere.This relatively low 1.25 factor gets progressively larger when you go to slower than jet speeds (say below 200 CAS).Michael J.[link:jdtllc.com]http://jdtllc.com/images/RCsupporter.jpg

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The question is more of what formula can be considered simple enough to apply under most circumstances.Definitions:Indicated Air Speed (IAS) is read directly off the face of the ASI,Calibrated Air Speed, or IAS corrected for sensor and indicatorplacement/error is based on engineering data and mathematical modelsconcerning the operation of the pitot/static system. True airspeed(TAS) is CAS corrected for temperature & pressure.Your formula could throw people off if they don't have all relevant data. For the casual simmer, the 2 pct. formula works fairly well. Given my FL350 scenario, some might read your formula as TAS=CAS*1.25, which would be wrong (and not your formula's intent).

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>Your formula could throw people off if they don't have all >relevant data. Notice the word "delta" in my equation - it is the key. This is NOT the formula to convert CAS to TAS or vice versa.Michael J.

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I know that--the average simmer might not. Point is, depending on the level of accuracy required in the stated problem, multiple solutions may suffice. The 2 pct. formula is simple, and widely accepted. By no means is it accurate, but again for the casual simmer who knows nothing of vectors, trig, calculus, etc....the formula works.Your original post is probably where I went "astray" in my solution from your POV:"say, while flying at some altitude (cruising) I changed may indicated (CAS) airspeed by 1 knot (either increased or decreased). I am looking for a quick, practical formula that would tell me by how much my True Air Speed changed."

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