Boeing Knew of Faulty MD-11 Engine Mount Part Years Before UPS Crash

New findings from the U.S. National Transportation Safety Board indicate Boeing had previously identified and flagged a structural issue in a component tied to the fatal November 2025 UPS MD-11 freighter crash, prompting renewed scrutiny over safety practices for ageing aircraft.

The crash of UPS Flight 2976 in Louisville, Kentucky, on November 4, 2025 resulted in 15 fatalities, including all three flight crew and 12 people on the ground. Shortly after takeoff, the left engine and its mounting assembly separated from the McDonnell Douglas MD-11F, triggering an immediate loss of control and impact in an industrial area.

Investigators determined that a key bearing race within the left engine pylon’s mounting structure exhibited fatigue cracking, a failure that compromised the engine’s attachment to the wing. Such cracking ultimately allowed the engine and pylon assembly to detach during rotation, at approximately 30 feet above ground, before the aircraft stalled and crashed.

Federal investigators now note that Boeing, which acquired McDonnell Douglas, issued a service letter in 2011 alerting operators to failures of the same bearing race on four separate MD-11s. The 2011 guidance acknowledged previous issues but concluded the part would not create a “safety of flight condition” and recommended periodic visual inspections rather than mandatory replacement. Maintenance documentation from the UPS aircraft shows the last detailed inspection of the mount parts occurred in October 2021, with no immediate requirement for further checks until thousands of cycles later.

The distinction between a service letter and an airworthiness directive proved critical. The former is advisory in nature and does not compel operators to take corrective action. The Federal Aviation Administration did not issue a follow-on mandate requiring replacement or enhanced inspections at the time, leaving operators to interpret and implement the recommendations at their discretion.

The NTSB’s evolving probe raises questions about how earlier advisories were integrated into maintenance programmes and whether the inspection recommendations sufficiently addressed the long-term fatigue risk. Preliminary findings suggest the cracks in the engine mount may have progressed over many flight cycles, particularly on an MD-11 with a service life of more than three decades.

The MD-11 model, which evolved from the earlier DC-10 design, has been largely retired from passenger service but continued to serve in cargo operations with carriers such as UPS and FedEx. Following the Louisville accident, the entire MD-11 fleet, along with related DC-10 variants, was grounded globally under emergency airworthiness directives to permit inspections and potential repairs.

Industry analysts note that structural fatigue in high-stress components like engine pylons becomes more pressing as airframes age. The incident has prompted discussion over inspection thresholds, risk assessments for legacy aircraft, and the adequacy of manufacturer advisories versus regulatory mandates.

Beyond the technical dimensions, the case underscores broader regulatory and accountability issues for aircraft manufacturers and operators alike, particularly as aviation stakeholders confront ageing fleets and emerging safety data. The NTSB continues to analyse flight recorder data, maintenance records, and design history to formulate a probable cause, which will inform industry recommendations and potential regulatory action.