Investigators say that was caused by metal fatigue. They discovered a telltale crack, which then caused another crack to form.
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"It was on the interior part of the fan blade so not more than likely, certainly not detectable from looking at it from the outside," said Robert Sumwalt, NTSB chairman during a news conference.
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That means the crack probably was not visible during a visual inspection. The engine is made by CFM, a joint venture of General Electric and a French company.
ABC News is reporting that CFM had recommended last year that airlines do an ultrasound test to look for cracks, but the test was not mandated by the Federal Aviation Administration.
Dr. David Niebuhr is a professional engineer specializing in metallurgy. He told ABC7 News that no metal or material is perfect.
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"Once that crack starts with repeated loading and unloading, the crack eventually grows and grows and then it reaches a critical point where the remaining metal connected can no longer support the load, and you have catastrophic failure," said Dr. Niebuhr from his office in San Luis Obispo.
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Sections of the engine are expected to be tested in a lab, such as Anamet in Hayward. A scanning electron microscope can magnify the surface by a factor of five to 10 thousand times.
"The stresses are generated by the rotational movement and the gas forces that occur within the turbine blade during operation," said Ken Pytleewski, director of materials engineering and laboratories at Anamet, "And those forces determine the fatigue life of the material."
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