The speed tapes on current Airbus and Boeing aircraft are programmed to display CAS (instrument error is negligable and static source position error is automatically corrected for). However the compressibility error is deliberatly left in in order to make the speed tape read the same as would a traditional ASI. This is one of the reasons why indicated stall speed increases with altitude.
CAS exists only because of the limitations inherent in the mechanical ASI. An EFIS could just as easilly be programmed to display EAS which would be theoretically preferable.
If you want to know your EAS in flight, here's how. Print out the table below. The column labelled ESS is what might be called the "equivalent speed of sound" i.e the speed of sound expressed as EAS. Look up the ESS value for your flight level and then multiply this by your Mach number. This gives your EAS (in knots).
You will find that the EAS is always less than the CAS displayed on your speed tape. The difference is compressibility error. I have not bothered with the table below FL 100 because the compressibility error is small at lower altitudes.
Flight ESS
level
410 277.8
400 284.6
390 291.5
380 298.6
370 305.9
360 313.3
350 320.9
340 328.6
330 336.4
320 344.3
310 352.3
300 360.5
290 368.7
280 377.1
270 385.6
260 394.2
250 403.0
240 411.8
230 420.8
220 429.9
210 439.1
200 448.4
190 457.9
180 467.5
170 477.2
160 487.0
150 496.9
140 507.0
130 517.2
120 527.5
110 538.0
100 548.6
Last edited by Rivet gun; 4th Oct 2010 at 18:53.