Superstress
Right on! Couldnt agree with you more. I worked for a company who recently went thru the same exact experience. They argued and argued with the certification authorities that due to their state of the art finite element modeling capabilities including linear and non-linear analysis, that no full scale ultimate test of their wing was required. Well......, the wing failed at about 137% of limit! So much for state of the art. This ended up costing them millions.
In the past 20 years, I have developed several airframe FEMs and IMO here is what much of the industry has forgetten about analytical methods and particularly FEM from its original inception and use in the aircraft industry. Back in the olden days, EVERYTHING was tested and the results evaluated (that is what all of the OEM stress manuals are based on - the famous Lockheed bathtub fitting method was based on P38 and B17 wing attach fitting tests). Analysis was used only for preliminary estimates until the test could be completed. THEN, and ONLY THEN, would methods be reviewed, modified, altered in order to predict the test failure modes. Having done this and established these methods, THEN the methods could be used to help evaluate new designs without repeating the expensive testing. LESSON NUMBER ONE - ALL methods must be correlated and validated. The FAA does not accept FEM without test validation, period. IMO the industry has become far too arrogant.
Now keeping this in mind, the experienced engineer knows that he must validate his method/FEM throughly at least ONCE and capture all of significant failure modes. Then, the method/FEM can be used subsequently for any new designs, mods, etc. This is the proper way to approach the issue. Most OEMs have spent millions of dollars validating their methods and have a large volume of substantiation data (even if there arent any engineers left at the OEM who know where the find the data). In fact, Boeing has a wonderful manual on exactly how to idealize airframe FEM which is based on decades of validation.
Unfortunatly, the people running companies today have no clue about this. Worse yet, many management teams of upstart companies think ANY testing and/or methods development to be a complete waste of money since CATIA with FEA is the solve all solution. Worste yet, young impressionable engineers start to believe this as well because "senior management must know what they are talking about", NOT. Unfortunately for them, they end up paying thru the nose when the day they submit their mod for an STC, the FAA says the following "...thats all fine and dandy but where's your validation to test data?".....The engineers are left bugeyed and the management team disappers or better yet blames the engineers. Shortly thereafter, the company folds up and lays everyone off. No one benefits from this.
One last comment then I'll get off my soap box. Many an old stress engineer always told me: "If half the design team and management arent pissed off at you, then, youre worthless to me as a stress engineer and you should call yourself one". I believe this to this day. Stress engineers must have a backbone and stand up when need be. Stress is the last defense line for a safe airframe.
Good luck all.
James Burd
