At Reading, by 1956 the 148th Fighter Squadron F-51Ds were reaching the end of their service life and the Air Force was phasing the Mustang out of the inventory. In an effort to upgrade to an all jet fighter force, the Air Force required Air National Guard Air Defense Command units to upgrade to jet-powered aircraft. Tests of operating F-84Fs at Spaatz Field had shown that the runways at the airport were too short to operate jet fighters safely from the runways, and the Reading Airport Commission and Air National Guard authorities found themselves in a conflict over the use of Reading Municipal Airport for tactical jet operations.