Boeing Co. engineers were trying to determine why an aging United Airlines 727 developed a 20-inch crack in a part of its fuselage. The split was in the same general area as the rupture in an Aloha Airlines 737 that lost part of its roof over Hawaii last year. But the cracking in the United Airlines plane is not typical of the type found in older aircraft, in which tiny cracks form along lines of rivet holes where sections of the plane’s skin overlap. After the crack was detected last month, the FAA ordered inspections of the 47 oldest 727s. Since then, similar cracks have been found in three more planes, bringing the total to four, the FAA said. The new cracks, however, were said to be “minor,” compared to the 20-inch crack.