Stephen Cohen / Getty Images Boeing was aware of a critical problem in the UPS aircraft that went down last year, killing 15 people, officials said. The National Transportation Safety Board revealed on Wednesday that the UPS cargo plane that crashed near Louisville, Kentucky, last November, had a structural flaw Boeing had previously deemed non-hazardous. Investigators found cracks in the assembly securing the left engine, a part that had fractured in a similar fashion on at least four prior occasions across three other planes. Boeing had issued a 2011 service letter acknowledging the fractures but stating they “would not result in a safety of flight condition.” The aircraft, an MD-11F built by McDonnell Douglas (acquired by Boeing in the 1990s), caught fire shortly after takeoff from Louisville and crashed into multiple buildings, including a petroleum recycling facility. Three crew members and 12 people on the ground died. After the crash, Boeing urged all MD-11s be grounded during the NTSB probe, a step followed by UPS and temporarily enforced by the FAA. Boeing told the Daily Beast: “We continue to support the investigation led by the NTSB. Our deepest condolences go out to the families who lost loved ones and our thoughts remain with all those affected.” Read more at The Daily Beast.