Muir Army Air Field, designated KMUI, is a military airfield located at Fort Indiantown Gap in Lebanon County, Pennsylvania, near the community of Annville. It serves as a key aviation asset for the Pennsylvania Army National Guard and supports training and operational missions for military rotary-wing and fixed-wing aircraft. The airfield is named in honor of Charles H. Muir, a U.S. Army general with ties to the region. Fort Indiantown Gap itself is one of the largest National Guard training installations in the eastern United States, and the airfield plays an integral role in supporting those training activities. While the airport holds both IATA code MUI and ICAO code KMUI, it does not operate as a commercial passenger airport. Its primary mission remains military aviation support, including helicopter operations, medevac training, and airlift exercises.