Safety and efficiency are two of the most important goals of the national airspace system (NAS). However, these goals are typically viewed as competing or independent, rather than complementary. The complementary aspect of safety and efficiency is illustrated from the point of view of human factors and ergonomics. Statistical methods were used to analyze safety and efficiency performance measurements, operational errors, and daily flight time index (DFTI). Factors related to weather and complexity were found to be more likely on days with poor operational performance as measured by DFTI. The results indicate a positive relationship between NAS safety and efficiency and imply that NAS safety performance may be improved by increasing efficiency, and vice versa.