A U.S. safety investigator said United Parcel Service Inc. decided not to take action after 2008 and 2011 service letters from Boeing flagged cracking in a key structural component in the type of plane that crashed in Louisville, Kentucky, last year. 

“Investigators found that UPS had reviewed the service letters and determined no further action was required,” said Chihoon Shin, the investigator in charge for the UPS accident. The comments came on the first day of a two-day investigative hearing into the UPS tragedy, which killed 15 people on the aircraft and on the ground. 

According to Shin, Boeing in 2008 issued a service letter informing operators of McDonnell Douglas MD-11F cargo jets of fractures in a component of the plane’s engine mount called the spherical bearing that helps uniformly distribute loads across the structure. At the time, however, the planemaker determined the problem wouldn’t result in a safety of flight issue. It released a revision of the letter in 2011 that added information about a redesigned spherical bearing. 

Transcripts released by the NTSB also show that mechanics told investigators they weren’t aware of the 2011 Boeing service letter. 

“I have never heard of that until now,” one mechanic with ST Engineering San Antonio Aerospace, which handled maintenance for UPS’s MD-11s, said when asked about it. The revelation came in a tranche of documents released at the start of the NTSB hearing. 

The UPS McDonnell Douglas MD-11F cargo jet involved in the November 2025 crash lost its left engine during takeoff. It cleared a fence at the end of the runway at Louisville Muhammad Ali International Airport but then plunged into terrain and buildings outside of the facility’s perimeter.

Safety investigators this week are working to uncover additional clues as to what caused the deadly accident. 

A preliminary report found fatigue cracks in that key structural component that helped secure the turbine to the aircraft. In an update in January, investigators disclosed that Boeing, which took over McDonnell Douglas in 1997, warned operators in the 2011 service letter about past failures of the spherical bearing race on three different planes. 

The UPS accident in 2025 added to a deadly year for global aviation. In June of that year, 241 passengers and crew aboard an Air India flight died when the aircraft crashed shortly after takeoff. Months earlier, a U.S. Army helicopter collided with an American Airlines Group Inc. regional jet on approach to Ronald Reagan Washington National Airport, resulting in the deaths of 67 people. 

Following the UPS crash, the U.S. Federal Aviation Administration grounded all McDonnell Douglas MD-11 and MD-11F cargo aircraft. The agency prohibited further flight until operators completed inspections and repairs using safety protocols established by Boeing and approved by the regulator. 

FedEx recently began flying the aircraft again after the FAA gave the green light. UPS has retired all of its MD-11 planes.