The speed of sound (Mach 1.0) varies with temperature; lower temp equals a lower TAS value for Mach 1.0. Tony's using standard atmosphere, which assumes a linear temperature lapse rate of 2 deg. C/1,000 feet from sea level (+15 deg. C) up to the tropopause (nominally at 11,000m/36,089 ft.), where the temperature stops falling (in the stratosphere it starts increasing again). At sea level, Mach 1 is around 661 kts TAS, at the tropopause it's about 573 kts TAS. (First person to bring up KEAS vs. KTAS gets a head slap from Leroy Jethro Gibbs!). Google will provide you with more details, should you wish.palmermd wrote:as long as we are off topic...why is it that the mach number is changing when the TAS is not?