A UPS cargo plane crashed in Kentucky in November after one of its engines fell off during takeoff, killing 15 people, including the crew and people on the ground. The plane was an MD-11F freighter. The US National Transportation Safety Board (NTSB) found cracks in the engine mounting part that Boeing had noticed on other planes 15 years ago. Boeing said back then the issue "would not result in a safety of flight condition." The MD-11 is an older plane design made by McDonnell Douglas, which Boeing bought in 1997. Though no new MD-11s have been built since 2001, Boeing still supports parts and service. The NTSB said the cracks showed "fatigue" or repeated stress, causing fractures in a key bearing and mount. Boeing had found the same problem on four occasions affecting three planes and sent a "service letter" in 2011 telling operators to inspect the part every five years. This letter is not legally binding. Tim Atkinson, an aviation safety expert, called the NTSB update "disturbing" and said the engine mount is vital. He said, "It's extraordinary that Boeing concluded that a failure of this part would not have safety consequences." Boeing has faced criticism before for safety issues, including the 737 Max crashes that killed 346 people and recent quality problems. Boeing said, "We continue to support the investigation led by the NTSB. Our deepest condolences go out to the families who lost loved ones." The NTSB continues its investigation and has not yet issued final conclusions on the crash.