What are “fit-up forces”?
Fit-up forces are applied to parts to ensure they mate up properly prior to any shimming or assembly. These small forces are used to fit the general shape of the structure.
Long-wave gaps are less of a structural concern than potential “short-wave gaps”
Long-wave gaps extend across several fasteners or stringer bays.
Short-wave gaps extend only over one fastener location.
Short-wave gap tests & results
Boeing conducted configured element testing to understand the effects of extreme short-wave gaps on static and fatigue performance. The forces induced in these tests were more than 10 times the maximum fit-up force that has been allowed in production. This testing considers a more severe condition than anything induced by the application of the comparatively small fit-up forces. The report for these tests was finalized Nov. 10, 2020.
Findings
These tests confirmed the fuselage skin would not be susceptible to fatigue degradation, even for these short-wave gap conditions.
These tests also confirmed the static load carrying capability of the structure met requirements.