We present a multiyear evaluation of a regional aircraft observation system (TAMDAR). We compare TAMDAR observation errors with errors in traditional reports from commercial aircraft (AMDAR), and evaluate the impact of TAMDAR observations on forecasts from the Rapid Update Cycle (RUC) over a three-year period. Because of the high vertical resolution of TAMDAR observations near the surface, we developed and employed a novel verification system that compares RUC forecasts against RAOBs every 10; this revealed TAMDAR-related positive impact on RUC forecasts—particularly for relative humidity forecasts—that were not evident when only RAOB mandatory levels were considered. In addition, we performed multiple retrospective experiments over two 10-day periods, one in winter and one in summer; these allowed us to assess the impact of various data assimilation strategies, and varying data resolution. TAMDAR’s impact on 3-h RUC forecasts of temperature, relative humidity, and wind is found to be positive and, for temperature and relative humidity, substantial in the region, altitude, and time range over which TAMDAR-equipped aircraft operate.