I posted this to the Apple // enthusiasts Facebook page. In one of the comments someone (Paul Schlyter) pointed out: "Also note that the Fortran-2015, Fortran-2008 or Fortran-2003 of today is quite different from the FORTRAN-77 at the time of Apple Fortran." So What are we dealing with here exactly?
@Roger Levy Money & Time. The same thing driving these competitions. Millions of lines of code, hundreds of thousands of man-hours. This is why the IRS ran code written for IBM machines which became obsolete in the 1950's right up until the end of the century.
@Roger Levy A tremendously massive existing number of existing software packages and analysis that rely on FORTRAN, which would also require being rewritten. Think of the Boeings with decades of CFD simulations correlated to wind-tunnel data and flight testing. You can't just use another language without proving that you would get the same answers. <-- This is the biggest part of the problem.
@Matt Craw Not just Boeing. I used to work for Pratt & Whitney Aircraft simulating jet engines starting with Fortran IV, let alone 66 or 77 on an IBM 360. I last did that back in the late '70's. At this point, I have no idea what "modern" means in this context.
Thanks for the lively conversation and patience in waiting for access to the FUN3D software. If you are interested in the specifics of the code you can check out the software manual at: https://fun3d.larc.nasa.gov/index.html