Two days of public hearings in Washington last week revealed that Boeing and the Federal Aviation Administration had been aware of a critical cracking risk in the aft pylon of the Boeing MD-11 freighter for more than two decades before the component failed during the takeoff of UPS Flight 2976 in Louisville last November, killing all 15 people aboard. The National Transportation Safety Board’s investigators presented evidence of at least ten prior incidents involving the same spherical bearing — the part believed to have fractured and initiated the catastrophic separation of the left engine and pylon — dating back to 2002. For the aviation industry, the hearings revived questions about whether the mechanisms that translate known risk into mandatory corrective action are fit for purpose.
A bearing that should have been replaced
The spherical bearing in question sits within the aft pylon bulkhead, a structural junction point that connects the engine to the wing. Its function is load distribution; its failure mode, according to NTSB investigators, involves a fracture of its outer race — a crack that progressively alters the distribution of stress on the surrounding structure.
Boeing’s maintenance planning documentation called for general visual and detailed visual inspections of the pylon area at 72-month intervals. On a component whose failure mode is microscopic cracking, a visual inspection every six years offers limited assurance. What the hearings established was that the inadequacy of this approach was not a new discovery: investigators presented testimony showing that outer race fractures or migration involving this bearing had been recorded at least ten times since 2002, across multiple aircraft and operators. Of those ten incidents, only four were formally submitted to the FAA as required service difficulty reports.
The aircraft involved in the Louisville crash was itself subject to a last-minute substitution: the originally assigned aircraft developed a fuel leak before departure, and the crew boarded a second MD-11F that had recently passed inspection. Within minutes of takeoff, the left engine pylon detached, taking the engine with it. The NTSB has indicated that the spherical bearing in this aircraft showed evidence of prior fracture that was not detected at its last scheduled inspection.
Regulation by recommendation
Perhaps the most damaging testimony to emerge from the hearing concerned the threshold at which known risk becomes regulatory obligation. Boeing witnesses acknowledged that the company had recommended replacement of the component as part of its service guidance, but replacement was framed as advisable rather than mandatory. In the absence of an airworthiness directive — a formal FAA instrument — operators could and did continue flying aircraft with the component in place.
This distinction between recommendation and mandate sits at the heart of a wider regulatory debate. In the years since the Boeing 737 MAX accidents of 2018 and 2019, the FAA has faced persistent criticism over its reliance on manufacturer self-certification and its preference for guidance over enforcement. The Louisville hearing brought that debate into sharp focus once more. The NTSB has noted that had all ten prior bearing failures been properly reported, a pattern would have been visible to the FAA decades before November 2025.
Boeing stated that the bearing’s fracture changed the load distribution on the dual-lug attachment design in ways the original engineers had not anticipated, effectively defeating the redundancy built into the structure. From a systems perspective, this constitutes a common-cause failure — a single event that simultaneously compromises two components intended to fail independently. The NTSB has indicated that the full investigative report, including formal findings and safety recommendations, will be released later this year.
Whether those recommendations prompt the FAA to issue mandatory replacement orders for the remaining fleet of MD-11 freighters — operated primarily by FedEx, as UPS’s remaining aircraft have since been grounded — or to re-examine inspection protocols more broadly, remains to be seen. What the Louisville hearings have demonstrated, with uncomfortable clarity, is that institutional knowledge of a safety risk is not, by itself, sufficient to prevent a fatal accident.
Key vocabulary:
- pylon – the structural assembly that connects an aircraft engine to the wing or fuselage; it must bear enormous forces in normal flight and is subject to strict certification requirements.
- spherical bearing – a mechanical joint that allows movement in multiple directions; in an engine pylon, it distributes load and accommodates slight flex during flight.
- airworthiness directive (AD) – a mandatory order issued by an aviation authority requiring operators to inspect, modify, or replace a component; unlike service recommendations, ADs have legal force and must be complied with.
- common-cause failure – a failure in which a single event simultaneously disables two or more components that were designed to be independent, thereby defeating the redundancy built into the system.
- service difficulty report – a report that operators in the United States are required to file with the FAA when a failure, malfunction, or defect occurs on an aircraft; these reports form the evidence base for regulatory decisions.
CEFR Level C1 / ICAO Level 5-6