The Pentagon on Friday grounded its fleet of F-35 fighter jets after discovering a cracked engine blade in one plane. The problem was discovered during what the Pentagon called a routine inspection at Edwards Air Force Base, Calif., of an F- 35A, the Air Force version of the sleek new plane. The Navy and the Marine Corps are buying other versions of the F-35, which is intended to replace older fighters like the Air Force F-16 and the Navy F/A-18.