Various apps such as Window Seat can access the flight plans that aircraft -- specifically airline flights -- have filed with the FAA. This is NOT about delays, nor about flight tracking, but things like the proposed departure time, the filed route of flight (shown in the Window Seat front page, starting with "SFO8 SFO SAC J32 BAM..."), and the filed altitude.
While the FAA has various API's and sources of data, I haven't been able to locate a way to access this information. I realize that various flight tracking services such as FlightAware offer this and a great deal of other data by subscription, but it would be nice if I could get it from the source rather than paying a third party for it, and since apps like WindowSeat seem to get access, it looks like it should be available somewhere.
Many thanks for any leads!