Joseph Epperson, Materials Group Chairman Metallurgical Findings

Recovered Jackscrew Assembly Screw attached to horizontal stabilizer Separated from acme nut Nut thread remnants on screw

Closeup of Thread Remnants

Jackscrew with Lower Stop No grease in working area of screw Sandy grease on lower end of screw Oil sheen on upper end of screw Grease on lower stop with nut contact marks Fractured torque tube

Lower Mechanical Stop Lower stop recovered separately Grease on stop Marks on upper surface from contact with acme nut after nut thread separation Created offset loading of the torque tube Spline damage from rotation of screw

As Recovered Acme Nut Threads completely stripped No grease on interior or adjacent surfaces Grease fitting passage blocked Red and green grease on surrounding structure

Used Nut with Worn Threads Accident Nut with Stripped Threads

Torque Tube Fracture Fractured in threads at lower end of the acme screw Low cycle fatigue cracking Similar fractures could be produced by high stress in a few cycles

Two & Three Dimensional Finite Element Analyses to Determine Thread Loading and Contact Pressure Distributions  22 MPa u y =0  Screw Nut -70 MPa  22

Conclusions from Finite Element Analyses Approximately 90% of screw load reacted in four threads Loads shift as thread wear progresses The maximum value of the contact pressure does not change as wear progresses - 3,000 to 4,000 average over contact area At large levels of thread wear (>  ) under normal operating loads, large bending deformations develop which are consistent with the thread remnant wear shape Approximately 90% of screw load reacted in four threads Loads shift as thread wear progresses The maximum value of the contact pressure does not change as wear progresses - 3,000 to 4,000 average over contact area At large levels of thread wear (>  ) under normal operating loads, large bending deformations develop which are consistent with the thread remnant wear shape

Thread Remnant SHEAR FRACTURE At major diameter Less than 10% of the cross section remained WEAR Multiple bevel/stages Unique wear / deformation shape Wear occurring during initial deformation Wear occurring after additional deformation Shear fracture surface

Thread Deformation Sequence

Summary of Findings Material and structural conditions did not contribute to the acme nut wear No grease in working area of the screw Grease not removed by ocean impact, exposure, or recovery Jackscrew grease not contaminated The wear is caused by sliding contact and is consistent with an unlubricated condition Torque tube fractured by low cycle fatigue Material and structural conditions did not contribute to the acme nut wear No grease in working area of the screw Grease not removed by ocean impact, exposure, or recovery Jackscrew grease not contaminated The wear is caused by sliding contact and is consistent with an unlubricated condition Torque tube fractured by low cycle fatigue

Metallurgical Findings

Additional Photographs Lower stop in contact with the acme nut

Additional Photographs Interior of lower stop

Remnant Positions vs Trim Positions