Preliminary aircraft design codes requIre a more complete and integrated geometry definition than that used by conceptual design codes. This thesis documents the design and creation of an interactive CAD system which converts the geometry descriptions commonly used in conceptual aircraft design codes to descriptions that meet the requirements of preliminary design systems. In particular, the conversion of ACSYNT Hermite surface data of aircraft models to the non-uniform hi-cubic B-Spline surface representation is addressed. The topics discussed in this thesis include the design and development of an interactive graphics user interface, the design and coding of an intersection method for non-uniform hi-cubic B-Spline surfaces utilizing subdivision techniques and the development of a one-dimensional filleting algorithm for blending surfaces along iso-parametric curves.