:::::::Well I don't mean to quibble but Jane's and Aviation get their facts from primary sources, which is exactly where the other 'non-secondary' sources get their data from.  The only actual primary source is sitting on the Warton Proquis Information Management System and has a classification of 'SECRET'.  In fact, only the header sheet is actually sitting on the system, the document itself is locked away in a fire-proof safe with back-up copies at Iron Mountain.  One of the two sources does specify an altitude, and then there's just the simple fact that the Vmax altitude given by the Austrian Airforce is roughly true for most fighter jets.  This is a primary source backed up by common knowledge, more than enough to base a calculation on.  There is a reason why fighters achieve top speed at this altitude: https://en.wikipedia.org/wiki/File:Comparison_US_standard_atmosphere_1962.svg and https://en.wikipedia.org/wiki/File:AltitudeEnvelopeText.GIF.  Furthermore using a = sqrt(Gamma*R*T) to calculate the speed of sound is very much not 'original research'.  It's a very well established formula that's expressed on your own pages. https://en.wikipedia.org/wiki/Speed_of_sound#Speed_in_ideal_gases_and_in_air  Here R is expressed in J/molK which divided by the molar mass gives J/kgK, i.e. (8.31451J/molK)/(0.0289645kg/mol) = 287J/kgK.  This is kind of why I get annoyed discussing things here.  I'm stating things that are indisputable facts as an engineer in conjunction with a primary source, and being told that they are merely 'opinions' or 'original research'.  The link I've posted also specifies the speed of sound at 11km altitude (295m/s or 294.6m/s exactly using formula) and the temperature there (273-57) = 216K.