The Federal Aviation Administration will issue a directive that will require inspections of certain CFM56-7B engines, the type involved in a deadly incident Tuesday on a Southwest Airlines flight, according to a statement from the FAA.

The directive will require ultrasonic inspections of fan blades when they reach a certain number of takeoffs and landings, the statement said. Any blades that fail the inspection will have to be replaced.

NTSB investigators found evidence of "metal fatigue" in the fan blades of the engine that blew apart from Southwest Flight 1380 Tuesday, which resulted in the death of passenger Jennifer Riordan, who was partially sucked out of the airplane.

The had to make an emergency landing in Philadelphia. Seven other passengers on the flight suffered minor injuries.

The NTSB will be sending a crew to Southwest's headquarters in Dallas to go over the airline's maintenance records, the NTSB Chairman Robert Sumwalt said.