The purpose of this study was to mathematically define a distance or travel-time interval in which air medical evacuation would benefit the patient more than ground transport. The authors derived mathematical formulas from known variables (ground travel, extrication and rendezvous times) and fixed averages (on-scene time, lift-off time, and speeds) and used those formulas to test actual flights for appropriateness. The formulas were: [formula: see text] where Y = ground travel time; R = rendezvous time; Z = extrication time; D = distance to scene (km); and X = air travel time. The formulas provide a guide to prospectively determine the legitimacy of air medical transport. They can also be used retrospectively as a guide for quality assurance purposes. During this study of 123 consecutive scene flights, helicopter benefitted all the entrapped patients but only one-third of non-entrapped patients. Of 44 flights from areas with known ground times, helicopter transport benefitted 14 of 16 entrapped, five of 16 non-entrapped, but only three of 17 rendezvous.