A linear aircraft model for a rigid aircraft of constant mass flying over a flat, nonrotating earth is derived and defined. The derivation makes no assumptions of reference trajectory or vehicle symmetry. The linear system equations are derived and evaluated along a general trajectory and include both aircraft dynamics and observation variables.
That link is right...https://ntrs.nasa.gov/citations/19890005752
Document Number (FOIA) /ESDN (CREST): CIA-RDP86-00513R001343720008-3
Search "flat" and "firmament"
That's right...the CIA...https://www.cia.gov/readingroom/document/cia-rdp86-00513r001343720008-3.
I could go on. 81 Government & Affiliate Documents that admit Flat Earth