Investigators from the U.S. National Transportation Safety Board have found evidence of fan-blade fatigue cracking in the CFM56 engine that failed on a Southwest Airlines Boeing 737-700 en route from New Orleans to Orlando, Florida, on August 27. According to an investigative update issued Monday, the initial engine examination revealed that one fan blade separated from the fan disk during the flight and the root of the separated fan blade remained in its hub; however, the NTSB has not recovered the remainder of the blade. Metallurgical examiners found that the fracture surface of the missing titanium-alloy blade showed curving crack arrest lines consistent with fatigue crack growth.
During the flight, the left engine inlet separated from the engine, forcing the crew to divert to Pensacola, Florida, and land without further incident. 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. However, the airplane did experience cabin depressurization during the accident sequence. Pieces of the engine also damaged the wing and the empennage, but examiners found no fan blade or inlet material in the hole.
The NTSB said that future work will include 3-D measurements of the contact areas of all the blades, a non-destructive examination of the blade surfaces for cracks and a review of the engine maintenance records.
It said it would open the accident docket, containing factual group reports and other investigation-related material, “at a future date.”