On 6 August the atomic attack began with a flight of three special reconnaissance F-13As (RB-29s) which took off to report the weather over the primary and secondary targets. Col. Tibbets followed in his B-29 aircraft, "Enola Gay", an hour later, accompanied by two other B-29s which would observe the drop. While on the way to Japan, Major Claude Eatherly, flying "Straight Flush", radioed that Hiroshima was clear for a visual bomb drop. Navy weapons expert Captain William Parsons armed the bomb while in flight, as it was deemed too dangerous to do this on the ground at North Field, lest an accident happen and the bomb go off, wiping out the entire base. At 8:15 am, the "Enola Gay" released Little Boy from an altitude of . The radar fuse on the bomb had been preset to go off at an altitude of above the ground. In the ensuing explosion, yielding about 12 kilotons of TNT in explosive power, about 75,000 people were killed and 48,000 buildings were destroyed.