A combination of both, aerospace is so safe, the edge cases and tiniest margins matter. Good quality parts of the non-flying variety are often good enough, they

A combination of both, aerospace is so safe, the edge cases and tiniest margins matter. Good quality parts of the non-flying variety are often good enough, they just aren't good enough all the time. Nor are they traceable. So luck, that the parts don't fail in cataszrophic fashion. And even bad quality "flying" parts are usually reasonable high quality, after all the folks handling the real parts on a daily basis have to be fooled as well.