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Mechanics had performed visual and fluorescent penetrant inspections of the fan blades in the CFM56-7B turbofan that failed during an April 17 flight of a Southwest Airlines Boeing 737-700 within the period required by Federal Aviation Administration rules, the U.S. National Transportation Safety Board revealed in a May 3 investigative update. The engine in question had accumulated 32,000 cycles since new and 10,712 cycles since a November 2012 overhaul when it failed. At the time of the overhaul, maintenance protocols did not include requirements for eddy-current inspections prompted by the August 27, 2016 accident in Pensacola, Florida, in which a fan blade fractured on a CFM56-7B on another Southwest Airlines 737.
Now, the NTSB materials group is working to estimate the number of cycles associated with fatigue crack initiation and propagation in the fan blade that cracked and separated from its hub and to evaluate the effectiveness of inspection methods used to detect such cracks.
An emergency airworthiness directive issued by the Federal Aviation Administration on April 20 required within 20 days “a one-time ultrasonic inspection of all 24 fan blade dovetail concave and convex sides to detect cracking” as instructed by a service bulletin issued by CFM on the same day. The AD applied to various models of the CFM56-7B that have accumulated 30,000 or more total flight cycles. A new AD published in the Federal Register on May 2 broadened 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 May 3 NTSB update confirmed the fan blade at the center of the investigation into the April 17 accident had separated at the root; the dovetail remained installed in the fan disk. Investigators recovered two pieces of the blade within the engine between the fan blades and the outlet guide vanes and examination of the fan blade dovetail exhibited features consistent with metal fatigue initiating at the convex side near the leading edge.