In a recent revelation, Boeing acknowledged it had cautioned aircraft owners as far back as 2011 about a faulty component which later played a role in a tragic UPS plane crash last year. Despite this warning, at the time, Boeing did not consider the issue to pose a significant safety risk, as per the National Transportation Safety Board’s (NTSB) statement on Wednesday.

The disaster in question occurred in November 2025, when a UPS aircraft met a catastrophic fate shortly after takeoff from Louisville, Kentucky. The left engine detached from the wing during the plane’s runway roll, leading to a crash that claimed 15 lives.

According to the NTSB, Boeing had previously identified four failures of a critical part designed to secure the MD-11’s engines to its wings. These failures were recorded on three different planes. Despite this, Boeing, which assumed responsibility for these models after acquiring McDonnell Douglas, concluded back then that the issue did not constitute a “safety of flight condition.”

Investigations following the crash unearthed troubling evidence of cracks in the components responsible for attaching the engine to the wing. These defects had gone undetected in the routine maintenance checks, raising serious concerns about the sufficiency of the aircraft’s maintenance schedule. Notably, the last thorough inspection of these essential engine mount parts occurred in October 2021, with the next comprehensive check scheduled after about 7,000 additional takeoffs and landings.

The timeline for when these cracks began to form remains uncertain. However, this incident draws parallels to a similar tragedy in 1979, when an American Airlines DC-10, the MD-11’s forerunner, suffered a deadly crash in Chicago. In that instance, the left engine also detached during takeoff, resulting in 273 fatalities.

That previous crash led to the worldwide grounding of 274 DC-10s. The airline workhorse was allowed to return to the skies because the NTSB determined that maintenance workers damaged the plane that crashed while improperly using a forklift to reattach the engine. That meant the crash wasn’t caused by a fatal design flaw even though there had already been a number of accidents involving DC-10s.

But former FAA and NTSB crash investigator Jeff Guzzetti said that a service bulletin McDonnell Douglas issued in 1980 did identify failures of the spherical bearing race as a “safety of flight condition” so it’s surprising that Boeing didn’t call it that in 2011. He said that American had removed the engine of that plane so it could inspect that bearing.

“I just think it raises questions regarding the adequacy of the severity of the 2011 service letter, and it also raises questions about how UPS incorporated that information and acted upon it,” Guzzetti said.

NTSB didn’t say whether there had been additional documented failures of the spherical bearing race since 2011. Investigators found that part broken into two pieces after the UPS crash, and the lugs that held that part were cracked.

Photos released by the NTSB of the Nov. 4 crash show flames erupting as the rear of the engine starting to detach before it flew up and over the wing. Then the wing was engulfed by fire as the burning engine flew above it.

Three pilots on the plane that was headed for Hawaii were killed along with 12 more people on the ground near Louisville’s Muhammad Ali International Airport.

The factual report released Wednesday doesn’t state what caused the engine to fly off, but it’s clear that investigators are focused on the failure of this bearing. The ultimate conclusion won’t come though until the NTSB’s final report, which usually doesn’t come until more than a year after a crash.

The report does make clear that neither of the plane’s two other engines were on fire before the crash. Some experts had previously speculated that debris flying off of the left engine might have damaged the engine on the tail.

Boeing, UPS and the Federal Aviation Administration are limited on what they can say while the NTSB investigation is ongoing, so they all declined to comment on Wednesday’s report. Boeing and UPS both expressed condolences to the families that lost loved ones in the crash.

“We remain profoundly saddened by the Flight 2976 accident,” UPS spokesperson Jim Mayer said. “Our thoughts continue to be with the families and Louisville community who are grieving, and we remain focused on the recovery effort,” Mayer said.

The 34-year-old MD-11 plane only got 30 feet (9.1 meters) off the ground before crashing into several industrial buildings just past the runway and generating a massive fireball that could be seen for miles. Dramatic videos of the crash showed the plane on fire as it plowed into buildings and released a massive plume of smoke.

Airlines quit flying this type of plane commercially years ago because it isn’t as efficient as newer models, but they had continued to fly for cargo carriers like UPS and FedEx and a few of these planes were also modified for use in firefighting. All the MD-11s that had been in use and 10 related DC-10s have been grounded since the crash.