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*Derivation and Definition of a Linear Aircraft Model*. After a whole bunch of formulas we have this conclusion stating:

*This report derives and defines a set of linearized system matrices for a***rigid aircraft of constant mass, flying in a stationary atmosphere over a flat, nonrotating earth**. Both generalized and standard linear system equations are derived from nonlinear six-degree-of-freedom equations of motion and a large collection of nonlinear observation (measurement) equations. This derivation of a linear model is general and makes no assumptions on either the reference (nominal) trajectory about which the model is linearized or the symmetry of the vehicle mass and aerodynamic properties.

I understand the nonrotating portion in ref to the atmosphere and the airplane being in the same system. But what happened to the curvature? Why are they talking about flat plane?