NTSB Press Release
"... NTSB Materials Laboratory, microscope examination of the fracture faces of the ruptured skin revealed fatigue cracks emanating from at least 42 of the 58 rivet holes connected by the fracture. Electrical conductivity measurements, hardness tests, and X-ray energy dispersive spectroscopy elemental analysis of the skin in the area of the fracture revealed that the aluminum skin material was consistent with the specified material. The skin was the specified thickness.
"Non-destructive eddy current inspections conducted around intact rivets on the removed skin section forward of the rupture revealed crack indications at nine rivet holes in the lower rivet row of the lap joint. To assess the condition of the intact rivets and the skin rivet holes, X-ray inspections were performed on the skin located forward of the rupture location. This inspection revealed gaps between the shank portions of several rivets and the corresponding rivet holes for many rivets associated with S-4L. Upon removing selected rivets, the
holes in the upper and lower skin were found to be slightly offset relative to each other and
many of the holes on the lower skin were out of round.
"In this ongoing investigation, the NTSB Materials Laboratory work is actively conducting additional inspections and examinations in the following areas:
- Removal of rivets and examination of rivet hole dimensions, rivet dimensions, and rivet hole alignment between upper and lower skins.
- Detailed fractographic analysis of the skin fractures emanating from the rivet holes using optical and scanning electron microscopes.
- Fatigue striation analysis using a scanning electron microscope of specific skin fractures to determine the rate of crack propagation.
- Additional portions of the lap joints from the accident aircraft...."