Summary of "NTSB Animation - Engine-to-Wing Attachment Design Overview and Findings"

Scientific concepts / nature phenomena presented

This video is an aircraft accident-investigation animation focused on mechanical failure, fatigue crack growth, load distribution, and lubrication-related bearing geometry in an engine-to-wing attachment.

Engine-to-wing attachment system (MD-11F)

The MD-11F has three engines: two wing-mounted and one tail-mounted.
The wing-mounted engines attach to the wings via a pylon (highlighted in blue).
The pylon attaches to the wing at three attachment points:
- Pylon aft mount
- Pylon forward mount
- Thrust fitting (behind the pylon forward mount)

Pylon aft mount design

The pylon aft mount contains two triangular-shaped plates bolted together.
It interfaces with a wing structure called a wing clevis via:
- a forward lug
- an aft lug
The connection uses:
- a bolt
- a bushing (with a collar securing it)
- a nut securing the bolt

Bearing / lubrication system (critical to load distribution)

A spherical bearing sits within the forward and aft lugs.
Bearing geometry:
- Mono ball inside a one-piece outer race
- The ball can rotate and move slightly within the race
- Purpose of slight movement: reduce stress as the structure flexes under:
  - engine weight
  - aerodynamic forces
  - thrust loads
Lubrication feature:
- The mono ball and outer race contain shallow grease grooves and holes designed to distribute grease.
- The outer race lubrication groove:
  - about 1/4 inch wide
  - centered along the race width
  - extends around the inner circumference
- A grease fitting connects to internal passages that feed grease throughout the assembly.

Investigative findings (failure mechanism)

Fatigue crack initiation and propagation

From the outer race at the accident:
- fatigue cracking initiated from multiple locations around the inner surface
- initiation occurred at both the forward and aft corners of the lubrication groove
The fatigue cracks grew outward over time through the thickness of the outer race.

Loss of even load distribution after outer race fracture

Eventually, the crack at the forward corner grew large enough that the outer race split into two pieces.
Once split:
- loads were no longer evenly distributed across the lugs
- the separated outer-race pieces could be forced out of the lug bore
- leading to abnormal loading on the lugs

Secondary fatigue in the lugs and catastrophic fracture

Fatigue cracks then initiated within the bore of both lugs:
- on both inboard and outboard sides
The fatigue cracks grew outward through the lugs.
During the accident flight:
- lugs failed completely on the inboard side
- then bent and separated on the outboard side

Separation event sequence

With the pylon aft mount lugs fractured:
- the aft end of the pylon was no longer attached to the wing clevis
The remaining pylon connections failed afterward.
Result: separation of the left pylon and engine from the airplane during takeoff.
Evidence mentioned:
- an airport surveillance video recorded takeoff rotation and the separation event.