Rutting is a major distress in airport flexible pavements. Subgrade vertical strain criteria are used in many airport flexible pavement design procedures to consider the development of rutting. Several research studies have identified the limitations associated with these criteria. Design criteria based on subgrade stress ratios (SSRs) are being considered for evaluating subgrade rutting in airport flexible pavements. In this paper, the SSRs based on measured vertical subgrade stresses are related to surface rutting in flexible pavements subjected to repeated trafficking of Boeing 777 and 747 simulated test gears at the US Federal Aviation Administration (FAA) National Airport Pavement Test Facility (NAPTF). The results indicated overstressing of the subgrade in two test sections, and this was confirmed by trench study results. A good correlation was obtained between the SSRs and the surface rut depths, supporting the validity of developing SSR-based rutting transfer functions for airport flexible pavements serving the next generation of aircraft.Key words: rutting, subgrade stress ratio (SSR), National Airport Pavement Test Facility (NAPTF), pressure cell (PC), new generation aircraft, airport flexible pavement.