It is probably normal that a plane isn't perfect during the development phase, and that the JSBSim xml file contains wrong or unrealistic numbers or errors like unknown unit specifications. But if FG simply crashes to desktop during startup when it disagrees with something in my plane model, it isn't really helpful, especially because it crashes without any hint in the log file. The generated crashrpt.xml doesn't contain anything helpful either (helpful for me, that is). When I have changed more than one file, I don't even know which file is causing the crash. At the moment, there are only about five of them, and there isn't any level of complexity yet, no animations, no systems, no instruments, no sound. What will happen when the model contains several 100 files?
The cause of the crash simply was that I had used "N/M/S" instead of "N/M/SEC". That seems like an idiotic beginners mistake, and it probably was, but in my defense: "M/S" is correct, and S (s to be exact) is also correct in the SI system. I don't dare to ask why SEC was chosen in this case, and if a dice was involved...
Even if an experienced plane maker knows the units, it still could happen that he simply mistypes them. Does this really have to cause a crash? Much worse errors like missing files and xml syntax errors do leave pop-up messages or at least clues in the log file, and in these cases FG just stops in a controlled manner, without triggering the crash reporter.
I write this as a warning for other plane makers, because I spent about 90 minutes finding this little mistake. And yes, I was frustrated, so excuse my perhaps a little harsh tone, but it's ok now!