I have noticed that when calculating True Air Speed for any given altitude, the faster the Indicated Air Speed, there seems to be a larger increase in TAS? My thinking was that at a constant altitude the relationship between IAS/TAS would be linear, e.g you double IAS, and TAS would double.. But it seems to increase more? Not sure if I'm missing something here?
For example: 5000ft at 100knts IAS = 114knts TAS , 5000ft at 110knts IAS = 126knts TAS
Only a couple Knots difference, and it seems to make sense but I can't find any answers on it to confirm it? Would appreciate anybody to shed light on it, thanks!