Hi Hardy,

I must apologize for my first post in this thread, which may have stirred up things. But for the high speed buffet onset value, expressed as a CAS number, PSX is modelled correctly indeed. Here is why I messed up:

In my Excel-sheet, I was using two different formulas to convert TAS into CAS, assuming they would both produce similar results (as they did with ISA conditions). One formula was used to calculate current CAS from current TAS, temperature and Mach-number, the second one was used to calculate the M0.9 number for CAS (from the value for TAS at M0.9). Based on this second one, I concluded that VMO (shown on the speed tape where the red blocks start) should increase or decrease with temperature changes. I was assuming that a change in TAS would also result in a change in CAS. Hence, my forum post. Wrong assumption, because *not* at a fixed Mach-number.

After some forum discussion, I started using the first formula *as well *to check calculated speeds at M0.9. In this way, a discrepancy showed up between the results of the first formula and the second one. Using the first formula, CAS at MMO remained a fixed number; using the second one, CAS would change with temperature.

The problem with the incorrect results from the second formula turned out to be the usage of ISA values for pressure *and density *at a specific flight level. But with temperatures deviating from ISA, so will density (ρ=P/RT). Using a temperature dependant density, both formulas now produce the same result: CAS at M0.9 remains fixed as temperature - and hence TAS for M0.9 - change. The dynamic pressure remains the same. Therefore, PSX is modelled correctly for the presentation of the high speed buffet onset CAS number. I am sorry to have created significant buffet onset here myself.

Humble regards,

Simon