In 1953, the 111th units at Philadelphia Airport were re-equipped with North American F-51D Mustangs and assumed an air defense mission. At Reading, the 148th Fighter-Interceptor Squadron (FIS) continued its air defense mission with the Mustang until 1956. With the end of the line for the Mustang in USAF service, the United States Air Force, in an effort to upgrade to an all jet fighter force, required Air National Guard Air Defense Command units to upgrade to jet-powered aircraft. The Reading Airport Commission and National Guard authorities found themselves in a conflict over the use of Reading Municipal Airport for tactical jet operations. Unable to resolve these differences, the USAF inactivated the 148th Fighter-Interceptor Squadron on 30 June 1956.