The Pentagon has suspended all test flights of the new F-35 fighter jets after discovering a cracked blade in the engine of one of the planes.
The problem was discovered during an inspection at Edwards Air Force Base in California.
The engine is being shipped to manufacturer Pratt Whitney in Connecticut for evaluation.
The Pentagon has grounded all versions of the radar-evading warplane. The Marines and the Navy have also been testing the aircraft in the Pentagon's $396 billion fighter jet program.
The Pentagon said the grounding of the 51 planes was a precautionary measure and it was too early to know the full impact of the newly discovered problem.
Friday marked the second grounding of the warplane in two months. The F-35 program has been marked by cost overruns and technical problems.