To find a “defective part by design” is rare because of the way parts are analyzed, design margins (safety factors) are applied during design, first articles are build in model shops, and tested for strength and environmental conditions. Such defects are usually the result of not understanding the environment where the part will be used (e.g. believing the pilot will pull 9 Gs on only 5% of flights instead of 90% of all flights , an F-16 issue, or buffet loads during high AOA flight exceeding the capabilities of vertical tail attachment fittings, an issue for both F-18 and F-22).
Well, it should be very rare.
The F-16 example doesnt really fit here, because it was designed good enough for the original specs. Defective by design is like the F-22 problems with the bulkhead between the engines for example.
It is far, far more common to find parts that are not manufactured according to the engineering drawing (e.g. tool broke during machining, dull tool causes rough surface finish, wrong datums during setup causes CNC machining offset and thin web thickness, voids in composite laminates, inadequate adhesive flow during composite fabrication, Etc.). As I stated above, most discrepancies are repaired/reworked, some are “accepted as is” after engineering analysis, and very few are scrapped.
Agree, thats why I think the F-15 issue is a major mistake that should have been discovered. I tend to think McDD and the USAF just agreed to accept it, maybe they came to the conclusion it was ok for 4000 hours. Of course, 4000 hours quickly became 8000…
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