: It outlines how major design changes must be substantiated with data and approved by the FAA to maintain safety standards.
In technical and regulatory contexts, "Part 21" refers to a critical set of aviation standards. If your file is a digital copy of these regulations, here is an informative overview: DODIGV164.part21.rar
: Approval of the manufacturer's ability to duplicate that design consistently. : It outlines how major design changes must