Wait, what? I do understand and I honestly don't know what all this is, but nasa's technical documents all seem to state this.
Here are some more:
NASA Technical Note; Calculation of Wind Compensation for Launching of Unguided Rockets (Page 8 Trajectory Simulation, 2nd Paragraph) ..."this simulation assumes ... the missile position in space is computed relative to
a flat nonrotating Earth"
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040008097.pdf
NASA Technical Paper 2768; User's Manual for LINEAR, a FORTRAN Program to Derive Linear Aircraft Models (Page 12, Program Overview) ... “Within the program, the nonlinear equations of motion include 12 states representing a rigid aircraft flying in
a stationary atmosphere over a flat nonrotating Earth”
https://www.nasa.gov/centers/dryden/pdf/88072main_H-1259.pdf
NASA Technical Paper 2835; "User's Manual for LINEAR, a FORTRAN Program to Derive Linear Aircraft Models" (Page 1, Summary) AND (Page 126 , Report Documentation Page, Section 16) "The nonlinear equations of motion used are six-degree-of-freedom equations with
stationary atmosphere and flat, nonrotating earth assumptions."
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890007066.pdf
NASA Technical Memorandum; Determination of Angles of Attack and Sideslip from Radar Data and a Roll Stabilized Platform (Page 2, Section 16.) “The method is limited, however, to application where
a flat, nonrotating earth may be assumed.”
NASA Contractor Report 186019; An Aircraft Model for the AIAA Controls Design Challenge (Page 11, Equation of Motion and Atmospheric Model) ... “The nonlinear equations of motion used in this model are general six-degree-of-freedom equations representing the flight dynamics of a rigid aircraft flying in
a stationary atmosphere over a flat nonrotating Earth.”
https://www.nasa.gov/centers/dryden/pdf/88248main_H-1777.pdf
NASA Contractor Report 3073; Investigation of Aircraft Landing in Variable Wind Fields (Page 6, Chapter II - Aircraft Landing Model) ... "The Aircraft trajectory model employed in this study was derived based on the following assumptions: a)
The Earth is flat and non-rotating. "
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790005472.pdf
NASA Technical Memorandum 81238; A Mathematical Model of the CH-53 Helicopter (Page 17, Equations of Motion) .. "The helicopter equations of motion are given in body axes with respect to
a flat, nonrotating Earth."
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810003557.pdf
Engineering Experiment Station, Georgia Institute of Technology, Prepared for NASA; Atmospheric Oscillations (Page 10) ... "A model frequently used is that of a flat, nonrotating earth." ... (next paragraph) .. "The most one can profitably simplify the problem is to consider an isothermal atmosphere,
plane level surface, and a nonrotating Earth."
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19650015408.pdf
NASA Tecnical Paper 2002-210718; Stability and Control Estimation Flight Test Results for the SR-71 Aircraft With Externally Mounted Experiments (Pages 10-11 Equations of Motion) ... "These equations assume a rigid vehicle and
a flat, nonrotating Earth."
https://www.nasa.gov/centers/dryden/pdf/88733main_H-2465.pdf
NASA Technical Memorandum 100996; Flight Testing a VSTOL Aircraft to Identify a Full-Envelope Aerodynamic Model (Pages 4-5, State Estimation) ... “For aircraft problems, the state and measurement models together represent the kinematics of a rigid body for describing motion
over a flat, nonrotating Earth…”
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880014378.pdf
NASA Ames Research Center; Singular Arc Time-Optimal Climb Trajectory of Aircraft in a Two-Dimensional Wind Field (Page 2, Section II. Singular Arc Optimal Control) ... “In our minimum time-to-climb problem, the aircraft is modeled as a point mass and the flight trajectory is strictly confined in
a vertical plane on a non-rotating, flat Earth."
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060053337.pdf
C'mon, these are their own TECHNICAL documents.