The U.S. Federal Aviation Administration has expanded requirements published in an emergency advisory directive issued in response to the catastrophic failure of a CFM CFM56-7B turbofan on a Southwest Airlines Boeing 737. The new AD, published in the Federal Register on May 2, broadens the requirement for performing initial ultrasound inspection on each fan blade from before the fan blade accumulates 30,000 cycles to before it reaches 20,000 cycles since new, or within 113 days from the May 14 effective date of the directive. It also requires repetitive inspections every 3,000 cycles thereafter.
The new AD resembles an April 20 service bulletin issued by CFM. That service bulletin recommended ultrasound inspections within 20 days of CFM56-7B fan blades that have run more than 30,000 cycles since new, regardless of the timing of the last shop visit. It also recommended inspection of fan blades with 20,000 cycles by the end of August. The SB also called for operators to repeat the inspection every 3,000 cycles, which amounts to roughly two years of service. The emergency AD, issued at virtually the same time, didn’t include the requirement for inspection of blades with 20,000 cycles or the repetitive inspection every 3,000 cycles.
The FAA estimates the amended order covers 3,176 engines under U.S. registry.
The regulatory actions stem from discoveries of cracks in one of the fan blades of the left engine that failed during an April 17 flight of a Southwest Airlines Boeing 737-700 from New York La Guardia Airport to Dallas Love Field. The airplane diverted to Philadelphia International Airport, where U.S. National Transportation Safety Board investigators found evidence that one of the fan blades in the left engine separated at its hub attachment point. During the flight, pieces of the engine penetrated a cabin window above the left wing, killing one of the passengers.
The incident appeared reminiscent of a 2016 case of uncontained engine failure in another Southwest Airlines 737-700, one of whose CFM56s exploded during a flight from New Orleans to Orlando, Florida. In that case, the NTSB determined that a fan blade separated from the fan disk during the flight due to metal fatigue. Although engine debris created a 5-inch by 16-inch hole in the fuselage just above the left wing, none of the debris penetrated the passenger compartment.