Originally Posted by
Adlerdriver
Biff,
We may just have to agree to disagree, but I’ll come at it one more time just in case. If you still want to wave the BS flag, I’ll give you the last word.
All these facts you list simply bolster my argument. The choice to fly the Eagle beyond its original lifespan and utilize OWS to expand the G envelope was done using what was assumed to be accurate design information and engineering data. Had the longerons on this aircraft been IAW the original design, this “abuse” and exceedance of the lifespan/G limit wouldn’t have resulted in the failure. The reason the rest of the properly built Eagles aren’t breaking up in flight is because they were over-engineered and able to exceed the original expectations. This Eagle and the other problem children are “under-engineered” and obviously unable to meet the demands of flying beyond the originally projected lifespan. Again, if all the Eagles were cracking, the above “facts” would come into play.
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Exactly – The choice to fly the Eagle as a “9g, 7.5K jet” AND fly it differently from the way the “USAF envisioned” was made with the incorrect assumption that the longerons were the proper thickness. If all the longerons were designed and documented like the one on the failure jet, we’d probably have a boneyard full of Eagles. MD and the USAF wouldn’t have been able to make the numbers support a 9g/7.5K program.
The rest of the stuff you mentioned falls into the apples and oranges category, IMO. I can’t speak very well to the engine issue. I first flew the jet in ’89 and honestly don’t remember duel engine flameout being an issue. If you’re talking problems from the initial fielding in the ‘70s, then I really don’t see the point. The engines weren’t de-tuned then, they had turkey feathers that were flying off and any new aircraft is going to encounter some issues. Were these flameouts a result of improper design? No. Improper use? Yes. Big difference. The fact that an analog engine control can’t hack the daily abuse of a BFM engagement and a digital control can really has no bearing on our longeron discussion.
Water intrusion can have many causes but I know for a fact, that vertical stab didn’t roll off the assembly line in St. Louis with that problem. The choice to not fly the Eagle up to 800 CAL any longer may simply be an acknowledgement of all the exceedances you’ve mentioned. Not that guys were bumping up against that limit on a daily basis, anyway. Choosing to impose additional limitations on a 30 y/o airframe based on solid engineering data and consideration of the use/abuse it’s gotten up to this point is one thing. That’s significantly different than a catostophic failure resulting from parts that never met original design specs – never mind attempting to tap into the 25% of a 125% design and finding out it was never there to start with.
Cheers.
AD
AD,
My point is if the jet weren't over engineered by some margin when it was first designed (as a 7.33g jet) then the envelope would never have been enlarged to 9g's.
Obviously MD thought something might give eventually or they wouldn't have asked the USAF for one to destroy.
My point with the engine analogy is that regardless of how well something might be designed the engineers can't think of everything.
Yes, that longeron was not up to manufacturing specs for a 9g jet. However, it did last longer 5k and 7.33g's. The USAF has a program to determine the lifespan of a jet. All recorded G's, over G's, total hours, etc are entered. That jet was with in a small hours margin of the end of it's lifespan regardless of where it stood on the 7.5k front.
Biff
PS The USAF can track G's on the Eagle up to 7.5, then again at 9.0 (the counter readings from 5R). They have no ideal what goes on between 7.5 and 9.0 as the OWS tells the pilot only.