Nearly all of the F15s in the US have been grounded for the past several weeks due to structural failures in flight. Here is a video reenactment of the latest incident. This has kind of missed headline news, but it's rather important to our national defense. Obviously there was tremendous cost in obtaining these F15s. There would be tremendous cost to replace them with new ones...it sounds like the entire airframe has to be replaced, so that would mean they could probably salvage the seats and engines, or something. It should be noted we also sell these things to our allies. Also, it should be noted that whoever is elected president next will be handed this problem.

What should be done? What will be done? Had you heard of this before I started the topic here?

What should be done? What will be done? Had you heard of this before I started the topic here?

Wow.

No, I'd not heard anything about this at all. On first hand, coming now in the current global climate, it sounds pretty serious, but on rethinking, several questions spring to mind.

First, isn't the F15 a child of the 1970's? How old are the failing aeroplanes? Second, isn't the F15 being replaced by the F22 anyway? How great a short fall will this grounding actually result in?

Here is an article from the Los Angeles Times that goes into a little more depth in describing the problem. Apparently the problem has appeared primarily in some of the older airplanes in the fleet and my guess would be it's a fatigue problem more than any kind of structural design flaw. As an air superiority fighter, the F-15 airframe is subjected to much higher stresses than different kinds of airplanes are and I wouldn't be surprised that it ages a little faster than most.

Having said that, it seems to me we have a couple of options here. We either come up with a retrofit program to repair the existing fleet, build additional F-15s to replace the existing ones, or bite the bullet and build additional F-22s to phase out the F15. The F22 option is the most expensive one in the near term and it doesn't address the problem for the countries we're selling F-15s to today. I read on Boeing's website (I can't find the link right now) that Boeing intends to continue the F-15 program into 2030, so a fix to this problem pretty much has to be in the works.
Here is another link to another article about this problem in the LA Times. I cited this article because of the following quote.....



Uh, I don't necessarily agree with this statement. Quite frankly, it sounds like a little bit of politics is being played here by certain "senior officials" in the Air Force to justify more F-22s. If fatigue cracks are being found in the F-15 longerons, then you replace those longerons and upgrade your fleet inspection procedures to check for this problem. This "senior official" is right about one thing. It won't be the same airplane it was before because it won't break up in flight.

Personally, I'd go with the inspection/repair option in the short term, perhaps a re-design for future export airplanes and continuing production of F-22s at the present rate.


Aquilla

Thanks for the link to that (more informative) article, Aquilla. Things aren't as bad as I thought. Not good, but it sounds as though all but 180 or so of the planes are scheduled to be operational again. I was under the impression that all of the planes were grounded indefinitely. That's an awful lot to make up for with only three operational F22 squadrons in the entire US. And of the 90 F22s total (I think) they have produced so far, some have been grounded as well due to maintenance issues. Over at Nellis, in Las Vegas, most haven't flown in weeks. Mr P's squadron just deployed to Red Flag with an extra two jets so that the pilots over there can use them to stay current.



I'm sure you're right about the politics.



If the problem isn’t design flaw, but age, why would this problem be restricted to longerons? I’m not an aerospace engineer as you are, but it seems to me the same problems will affect every other piece of aluminum in the aircraft that is also continuously subjected to extreme loads of stress. Could it be that this part is just the first to go and shouldn't we expect others?

Longerons are specific load-carrying members of the structural design. Thus, they are subjected to stresses that other no-load bearing members aren't. Back when the F-15 was designed, the primary design tool was a computer program called NASTRAN which took wind tunnel data and used it to predict the loads (in the form of a tensor) on various parts of the airplane. A tensor is essentially a vector with an additional dimension. It may well be, and I'm just speculating here that there may be a stress component in the failed longerons that was missed in the initial wind tunnel and subsequent flight testing. It may have been so small that it took 25-30 years to eventually manifest itself in the form of a fatigue failure. My gut tells me there's some sort of a torsional component (twisting) that over time causes a fatigue failure. Metal doesn't mind being pulled, but it hates being twisted - that's why pop-tops work so well on cans.

As far as other failures are concerned, and I know you have a personal stake in this since Mr P is likely to be flying an F-15 from time to time (wish I could), that's entirely possible. However, now that the problem has been flagged, I'm sure that both Boeing and the Air Force are taking a good hard look at things. Stress fatigue is actually pretty easy to find if you have a rigorous inspection procedure that's looking for it. I'd be doing that right now on every F15 in the fleet before I signed off on it to be flight worthy. I wouldn't put Mr P in a defective airplane and I don't think the Air Force will either.


Edited to add......

For those of you having trouble getting to sleep at night, I'd recommend this this Wikipedia discussion about tensors. Read through this and it's lights out.

Aquilla

Hmm, I got this second hand, but from what I understand, all F-15s have a design flaw of some load bearing unit ( sorry I didn't catch it now) and that the supplier may have even supplied lower grade materials than specified, and it is starting to show now with the long term streses on it. Politics may be a part of it- but my buddy seems genuinely concerned. I do know one thing- I will have some intimate knowledge of this in about a week. One of the professors, and my advisor, at my school is Air Force civilian worker (fed service) and he is a well known, in certain circles, to be quite a genius in metulurgy and welding and other issues. Being in Alaska, we have some very unique testing conditions. I will be torturing some US military exotic and non-exotic metals, and getting some really good grades in the process, while having the time of my life!