Nondestructive deflection testing is used to assess the adequacy of airfield pavements and strengthening design for future operations of heavier aircraft. Deflection testing by a falling weight deflectometer (FWD) device is widely used for pavement nondestructive evaluation. The Pavement Evaluation Based on Dynamic Deflections (PEDD) microcomputer program is used to backcalculate reasonably accurate modulus values in the case studies presented in this paper. Nonlinear moduli of granular layers and soils associated with main gear load configurations used in large modern aircraft can reduce to one half of the backcalculated moduli. It is shown that the use of one set of design moduli and simplified load equivalency concepts, such as the ESWL concept for airfields, may be inadequate to assess pavement damage due to mixed aircraft traffic. Effects of temperature and moisture on seasonal nonlinear moduli and fatigue/rutting damage can be significant. This is shown by the results of the newly developed Pavement Design System for New and Existing Asphalt Pavements (PADAP) program for mechanistic pavement designs and simulations. This paper also demonstrates the use of advanced three dimensional-finite element (3D-FE) pavement analysis procedures for correctly simulating pavements subjected to dynamic loads. Good agreement is shown between simulated and measured FWD deflection time histories for asphalt pavements using the moduli backcalculated from the PEDD program. © 2004 ASCE.