See this thread: in Windows, the numpad is not usable in Flightgear, except if you disable numlock prior to startup and enable it once the program is launched. I 've experienced this both on windows vista, 7 and 10 (actually never knew this hack was possible until now).
Can confirm with current Next.
I've pushed fixes for this to a merge request. Please test!
https://sourceforge.net/p/flightgear/flightgear/merge-requests/353/