UPS 2976 Engine Mount Animation • NTSB Hearing

Why It Matters

A single bearing failure can detach an entire engine, highlighting the critical need for rigorous inspection and design safeguards to maintain commercial aircraft safety.

Key Takeaways

• MD-11F left engine separated due to pylon aft‑mount fatigue failure.
• Lubrication groove cracks initiated fatigue in outer race of spherical bearing.
• Cracked outer race caused uneven load, leading to lug fracture and detachment.
• Failure propagated from outer race to lugs, then to wing clevis attachment.
• NTSB animation highlights design vulnerability and need for improved inspection.

Summary

The NTSB released an animation detailing the MD‑11F’s left‑engine pylon failure during take‑off, illustrating how the aft‑mount structure fractured and caused the engine to separate.

The aft mount consists of two triangular plates bolted together, a spherical bearing with a mono‑ball and outer race, and a lubrication groove. Fatigue cracks originated in the inner surface of the outer race at the groove’s forward and aft corners, grew outward, and eventually split the race into two halves, disrupting load distribution.

With the outer race divided, the forward and aft lugs experienced abnormal stresses, leading to bore‑originating cracks that propagated outward. During the accident flight the lugs fractured inboard, bent outboard, and the pylon detached from the wing clevis. Airport surveillance video captured the moment the left pylon and engine ripped away.

The analysis underscores a latent design weakness in the MD‑11F’s pylon lubrication system, prompting regulators and operators to reassess inspection intervals, lubrication practices, and possibly redesign the bearing assembly to prevent similar catastrophic failures.

Original Description

Credit: NTSB

NTSB Investigation Page: https://www.ntsb.gov/investigations/Pages/DCA26MA024.aspx

On November 4, 2025, about 1714 eastern standard time, United Parcel Service (UPS) flight 2976, a Boeing (McDonnell-Douglas) MD 11F airplane, N259UP, was destroyed after it impacted buildings and the ground shortly after takeoff from runway 17R at Louisville Muhammad Ali International Airport, Louisville, Kentucky. The 3 crewmembers aboard the airplane and 11 people on the ground were fatally injured. There were 23 others on the ground who were injured.

This animation illustrates the structure of the MD-11F aircraft engine-to-wing attachment and presents investigative findings related to the separation of the left engine.

The animation begins with an external view of a UPS MD-11 aircraft, showing two engines mounted under the wings and one engine mounted on the tail. The left engine-to-wing attachment pylon is highlighted in blue.

The view transitions to the internal structure of the pylon assembly. The pylon frame is attached to the wing at three locations:

• the aft mount, located at the rear of the structure,

• the forward mount, located near the center, and

• the thrust fitting, positioned behind the forward mount.

A three-dimensional model of the aft mount is then presented to illustrate its geometry, which is a triangular structure with forward and aft mounting lugs at the top. The animation shows how the aft mount connects to the wing clevis on the underside of the wing. The forward and aft lugs are shown fitting into the clevis.

The aft mount assembly hardware is then illustrated. This includes a bolt, a nut, a bushing, and a retaining collar. An external grease fitting on the bushing connects to internal lubrication passages.

The view transitions to show the spherical bearing that is housed within the forward and aft lugs. The bearing consists of a spherical monoball seated within a one-piece outer race that conforms to the ball’s shape. A cross-sectional view demonstrates how the monoball rotates within the outer race as the structure flexes during normal operation.

Arrows highlight the grooves on the inner and outer surfaces and three holes that distribute grease within the bearing.

The view shifts to a cross-sectional view of the outer race, which has a ¼-inch-wide groove around its circumference to aid in grease distribution. The image shows lines depicting the fatigue cracks that grew outward from the forward and aft corners of the groove; the lines are color-coded: red at the forward corner and yellow at the aft corner. A zoomed inset view on left shows the cracks propagating outward over time through the thickness of the outer race.

The animation illustrates the outer race separating into two pieces along the plane of the fatigue cracks from the forward corner of the grease-distribution groove. The separation of the outer race is shown again in a perspective view and once more with the bearing housed in the lugs to demonstrate how the pieces of the outer race could migrate within the lugs.

The separation of the outer race led to abnormal loading on the lugs, and the next sequence shows lines depicting fatigue cracks that initiated in the bores of the forward and aft lugs. Red lines indicate crack growth on the inboard side, and yellow lines indicate crack growth on the outboard side. The cracks propagated outward through the lugs, with complete fracture occurring at the inboard side of the lugs, followed by bending of the lugs and complete fracture at the outboard side of the lugs.

A two-dimensional graphic then illustrates how the separation of the left pylon aft mount allowed the engine to rotate up around the forward mount. The rotation led to failure of the forward mount and departure of the engine from the aircraft.

Finally, a surveillance video from the airport is shown at half of the actual speed. The footage captures the aircraft during the takeoff rotation and shows the left pylon and engine separating as a unit shortly after liftoff.

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