Last week Stuart Koehl shared his thoughts here on the Air Force’s decision to ground its entire fleet of F-15s. We got some interesting feedback from the piece, and I thought I’d post one exchange here for those who find the technical details of these matters as interesting as I do. From one reader who is recently retired from the Air Force and currently working in the defense industry:
Having spent 20 years in the Air Force as an aircraft mechanic and my last 5 working F-15s (a couple of years running a Periodic Phased Inspection Dock and then as a flight line Production Superintendent) at Tyndall AFB, Fl, I can maybe add some info on the recent crash. Tyndall had F-15 C/Ds that made in the late 70’s early 80’s. The F-15 has a “honeycomb” mesh filler system in its wings (mainly tips) and particularly its vertical stabilizers. These areas are x-rayed periodically for evidence of internal damage, external cracks, etc. In particular they are inspected for evidence of “water logging’ of the honeycomb mesh. When the filler material becomes waterlogged, the area expands and weakens and causes internal separation to various degrees. It is like comparing a dry sponge, which is hard and has load-carrying capabilities, to one that has been wetted a bit. The water gets in through seams and such and is hard to detect and also hard to stop. That’s why they paint these jets. It helps seal them. If evidence of water is found the usual method for repair is evacuation. Basically the water is slowly sucked out by vacuum action over several days and then the areas reexamined. It’s a fairly routine procedure. For more extensive damage and cracking there are more intensive repairs for damage to the honeycomb areas with a bondo-type material, and this repair is performed at a higher level of maintenance such as depot level by civilians. Or a contractor team on base will be dedicated to performing such repairs. When I was there, Tyndall had a dedicated contractor team, constantly performing these types of repairs.
In one instance (a few years ago) a vertical stabilizer partially exploded under high Gs, but the jet was able to land without event. An immediate action inspection was performed of the fleet and eventually it was discovered that the cause of the vertical stabilizer coming apart was due to improper repair at the depot level facility. Numerous other aircraft were found to have this improper repair and action had to be taken on those aircraft. So I would speculate that this crash was most likely related to one of these honeycomb mesh areas coming apart during flight ( I would guess the vertical stabilizer again–they take a REAL BEATING at 9 Gs). This time it caused an unrecoverable condition and a class A mishap. If this is indeed the case, there is much cause for concern. Perhaps the entire repair procedure and any and all processes relating to structural repair of this type of damage will be called into question. In that case it is a big deal because by now, a huge proportion of the F-15 fleet has almost certainly had structural repairs of this nature performed to one degree or another. But the generals do not lie when they say high ops tempo plays a significant factor. These type of repairs can only be done at depot level facilities or by depot level teams in the States. Perhaps they are doing extensive depot level repairs overseas somewhere but I wouldn’t think we would have the facilities to sustain such an operation. The jets would have to be flown back stateside to receive attention (and the downtime is considerable on average for each extensive repair). The age of the fleet means that eventually all F-15s would have gotten some type of honeycomb repair eventually. It’s just the nature of the design and limitations. Anyway I will follow this story to see if I was on the right path.
To which Stuart Koehl responds:
This is a very interesting and informative letter. I knew that honeycomb composites were used on most modern fighters because they are light and structurally strong, but they do, over time, have a tendency to de-bond. The problem of waterlogging is more common among carrier-based fighters. I also know that F-15s have suffered some structural failures as a result of debonding of honeycomb in the wingtip and tail, but as the F-15 has two tails these incidents have usually been recoverable. Wingtips are more problematic–when you lose the tip, you usually lose the aileron. Still, the F-15 has proven remarkably resilient–one lost an entire wingtip (due to a midair collision, as I recall) including the aileron and still landed safely. There are big margins built into the plane. Still, if the problem is a systemic debonding of the vertical stablizer, it could be a very difficult problem for the Air Force. There are, though, a number of quick fixes that could be employed. First, as a result of inspection, they could identify all vertical stabilizers with problems, and remove them either for repair or scrapping. Then the remaining stabilizers would be consolidated over the fleet to create a pool of aircraft with certified stabilizers (giving priority to deployed or rapid reaction squadrons). The next step would be to grab all the stabilizers sitting at the Aerospace Maintenance and Regeneration Center (AMARC, aka “The Boneyard”) at Davis-Montham AFB, ship them off to various Air Logistic Centers for inspection and repair, and use those to fix the F-15s still in need of stabilizers. Long term, the Air Force may actually need to commission Boeing to produce new stabilizers (and possibly wingtips) to replace those still in service. Fortunately, the F-15 is still in production, so the jigs and tooling are still available. Boeing will be glad of the work. Regarding the need for the F-22 and F-35, I have never denied this, and have in fact pushed for accelerated production. Still, there is no way that we can afford to have a 1:1 replacement of either the F-15 or the F-16 with the new aircraft, due to the cost differential. The capabilities of the F-22 and F-35, however, are significantly greater than those of their predecessors, so we actually get more capability out of a smaller number of airframes. On the op tempo being a factor, the author of the letter is absolutely correct. We are paying now for the “lost years” in the mid-1990s when we cut back our force structure more than was really prudent. There is nothing we can do about this, at least not quickly. Production of the F-22 and the F-35, and more importantly, their integration into the force (setting up squadrons, training pilots and ground personnel, establishing logistic support systems) probably can’t go much faster than they are right now. So the question is what to do over the next 2-3 years, while we bring the new planes on line. We could reactivate planes now in AMARC and use these to replace some of the ones now in service, so those can be sent to depot for overhaul, but (assuming there are sufficient birds in AMARC), it would take time and money to bring these up to standard and deployed to the squadrons. We are in a very uncomfortable place right now, and our best hope is a resolution of the ground situation in both Iraq and Afghanistan that will reduce the demand for air support and give the Air Force some breathing time.
