In perpetual pavements, damage from bottom-up cracking can be limited to the top surface lift through using a very thick surface layer or a binder-rich intermediate layer. This can be attained by maintaining tensile strains at the bottom of the hot-mix asphalt (HMA) layer below a certain value known as the fatigue endurance limit (FEL). This paper presents a method for estimating a strain-based FEL for flexible airfield pavements. The proposed method is based on the concept that a 50% reduction in HMA layer modulus would indicate initiation of fatigue cracking. Falling weight deflectometer (FWD) testing results, collected from National Airport Pavement Test Facility (NAPTF) flexible pavement sections, were analyzed to determine at which loading pass each section had a 50% reduction in HMA layer modulus (Nf50). NAPTF tensile strain data were also used to determine the tensile strain at Nf50 for each pavement section by averaging the peak tensile strains. The proposed approach was validated by comparing its results to those obtained using a common FEL estimation model known as the rate of dissipated energy change (RDEC) model. To further verify the results of the proposed approach, peak tensile strain was plotted vs. number of loading cycles for all sensors. Using these plots, the peak tensile strain at which the variability in the strain gauge data increased was used as an estimate of a possible FEL. The Nf50 tensile strains estimated using the proposed method were comparable to the values determined from RDEC and variability approaches.