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.

Researchers / sources featured

• No individual researchers or external sources named in the provided subtitles.