I'm a bit late on this thread. I'm glad BIO 161 has got this sorted. He's not the first and won't be the last to have found this confusing.
I think this is an issue which the flight schools don't teach well. They teach the relationship between TAS and mach number (which depends on temperature) but they dont teach the analogous relationship between EAS and mach number (which depends on static pressure). The equations are simple (one was quoted earlier in this thread)
TAS = M x 661.48 x sqrt(T/T0)
; T0 = 288.15 kelvins
EAS = M x 661.48 x sqrt(p/p0)
; p0 = 1013.25 hpa
661.48 is speed of sound at ISA sea level.
Perhaps a reason for this is that our speed tapes display CAS not EAS. The equation relating CAS and Mach number is too complex for the ATPL courses so they don't teach it. This does not alter the fact CAS and Mach number are related by static pressure. (temperature is irrelevant)
CAS exists only because it is the best you can do with a mechanical ASI. With EFIS, CAS should eventually become obsolete. I think the Eclipse buisness jet displays EAS on the speed tape and so did the space shuttle. Perhaps Boeing and Airbus will eventually ditch CAS. Then compressibility error will be just a historical curiosity.
Last edited by Rivet gun; 17th September 2010 at 14:04.