NTSB: UPS MD-11 Crash Linked to Design Flaw Known to Boeing Since 2011

WASHINGTON, D.C. – The National Transportation Safety Board (NTSB) has linked the fatal crash of United Parcel Service (UPS) flight 2976 to the failure of a specific engine mounting component that Boeing knew was prone to fatigue cracking nearly 15 years prior to the accident.

In an investigative update issued Jan. 14, 2026, the NTSB disclosed that the left engine and pylon separated from the MD-11F shortly after takeoff on Nov. 4, 2025, at Louisville Muhammad Ali International Airport (SDF). The catastrophic separation resulted in the deaths of the three crewmembers and 11 people on the ground, with another victim on the ground succumbing to injuries 51 days later.

Fatigue in Critical Bearing Assembly

Investigators focused on the "pylon aft mount bulkhead," specifically the spherical bearing assembly which attaches the engine pylon to the wing. According to the update, the spherical bearing race fractured into two pieces due to "fatigue cracking originating around the entire circumference at the edge of the design recess groove".

This fatigue cracking extended through approximately 75% of the bearing race's thickness, causing an overstress failure of the remaining material. Once the bearing failed, the engine pylon separated from the wing, severing fuel lines and igniting a fire.

A Known Issue

The NTSB report highlights that the design of the failed part (part number S00399-1) was the subject of a Boeing Service Letter (MD-11-SL-54-104-A) issued on Feb. 7, 2011.

That 2011 letter informed operators of four previous failures of the same bearing assembly on three different aircraft. In those cases, the failure mechanism was identical to the UPS accident: fatigue initiating at the design recess groove.

However, the 2011 Service Letter stated that Boeing had reviewed the failures and determined they "would not result in a safety of flight condition". In the prior incidents, the fractured bearing race pieces migrated but did not result in engine separation.

Flawed Parts Remained "Permissible"

While the 2011 Service Letter introduced a new spherical bearing configuration (part number S00399-523) that eliminated the design recess groove, it did not mandate the replacement of the older parts.

Crucially, the Service Letter did not prohibit the installation of the older, fatigue-prone part number (S00399-1) as a replacement for unserviceable bearings. This allowed the older design to remain in the global supply chain and on active aircraft.

Operators were instructed to inspect the assembly during routine maintenance—typically every 60 months—by visually checking if the bearing race was flush with the mounting lugs. The failure on UPS flight 2976, however, resulted in a catastrophic separation shortly after rotation.

Investigation Continues

The NTSB is currently reviewing how UPS incorporated the 2011 Service Letter into its maintenance program. Investigators are also scrutinizing the correspondence between Boeing and the Federal Aviation Administration (FAA) that led to the issuance of the Service Letter and the determination that the issue did not pose a safety flight risk.

Preliminary Flight Data Recorder (FDR) analysis shows the aircraft performed nominally until approximately 1713:11 EST, when parameters for the No. 1 engine became unreliable and the fire warning activated.