Approximations made in aviation noise prediction models, including the Federal Aviation Administration's current standard approach, often simplify the physics of noise production and propagation, and use methods not designed to calculate low frequency noise behavior. With the growing demand for air transportation, it is increasingly important to compute the impact of noise on communities and facilitate compatible land use management. This research, intended to provide options on enhancing the noise prediction capabilities for the Federal Aviation Administration's Aviation Environmental Design Tool in support of their Next Generation Air Transportation System (NextGen), explores the advantages of using a hybrid Parabolic Equation-Fast Field Program model for computing aviation noise contours. The hybrid model overcomes the elevation angle limitations associated with using a pure parabolic equation approach. Several numerical examples of the effects of a refractive atmosphere, range-dependent ground properties, atmospheric turbulence, and irregular terrain, accurately incorporated by the hybrid model, are considered, with emphasis on low frequency noise. The mechanics and preliminary results of converting a two-dimensional vertical sound field calculation to two-dimensional horizontal contour maps are presented.