Poorjohn
In answer to your question.............Because a properly executed scarf joint replicates the load transfer property's of the original structure.
When doing such repairs normally a test sample is made and sent to the aircraft manufacturer for destruction testing, we have never had a test sample fail to meet the new specification and on average the test samples are 1-3 % stronger than new items.
Last edited by A and C; 28th Oct 2013 at 13:26.