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 and 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 Here R is expressed in J molK which divided by the molar mass gives J kgK i e 8 31451J molK 0 0289645 kg 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 11 km altitude 295 m s or 294 6 m s exactly using formula and the temperature there 273 57 216 K