What is the life limit due to metal fatigue in large bombers?

Question

This article explains how the B-52's will be kept flying for 100 years.

Replacing the engines, electronics, etc. is one thing, but when would metal fatigue become a problem? These planes are huge and, I expect, rather heavy (at least when loaded up) and although they fly fewer cycles than passenger airliners, a hundred years is a long time.

Obviously I expect the military to be aware of this, but I'd love to hear what you folks think.

Answer 1

1. Metal fatigue and fatigue corrosion are and have been a problem since the type's introduction. They always fatigue and corrode, they always undergo expensive repairs due to corrosion. DIEGME degradation of fuel tanks has been a particular culprit, and it has been reduced with new coatings. Bombers aren't pressurized, so the whole fuselage doesn't fatigue the way it does for airliners. The hard limit for wing spars fatigue can run into the centuries, considering that these planes don't fly all that much.
2. 100 years is a bit of an exaggeration. The oldest B-52 in service come from the late 50s to 1962, when the production ended. So at retirement time the planes will be just 90 years old. There is a lot of B-52s sitting idle in storage, since they used to be considered strategic and fell under SALT cuts, and they're available both as spare part sources and as restorable airframes.
3. Why shouldn't a 100 year old plane fly? The USS Constitution is 220 years old and she's still sailing the seas, a considerably more hostile environment. It took a lot of restoration, but not quite to the extent of the Theseus' ship. Not everything is built with planned obsolescence like cars or iphones; ships and planes are built to last, as long as the owner spends the money needed to keep them up.
4. Keeping the old planes intact may not be cheap, but new bombers are far more expensive. The B-1B is half a billion a piece, so is the Tu-160. At the same time, most of the work the USAF does is milk runs in a threat-free environment. It doesn't call for advanced supersonic or stealth machines, it calls for something that's easy to service and cheap to run. The B-52 fits the bill better than any other bomber the USAF has. The only extant competitor is the Tu-95.
5. The B-52 can in theory be replaced by something even cheaper to run. Plenty of Boeing manufacturing capacity will sit idle for a long time, and, given a supply of olive drab, they'd fit the job even better. But this can't happen, because every military procurement program since the WW2 begins and ends with feature creep, doubly so in the Air Force. Only existing inventory can be kept as simple as it is.

All of this essentially leaves no choice but to keep up the current planes. Aluminum is cheap, it's silicon that's expensive, and B-52s have very little of that.

Answer 2

Back in the day the BUF (correct acronym for "D" model) airframe usage was calculated using variable aspects of actual flight that were outside the assumptions that the engineers used to predict "ordinary" useful hours of the airframe. If for example the plane had a 20,000 hour useful life before major overhaul or permanent grounding, that figure could be less depending on actual flight usage. The 20k figure was on the assumption of 'ordinary' use.

Aircraft uses that were considered as "extraordinary" included flying at low level, air-refueling, carrying external (wing) munitions or shapes, flying in moderate or severe turbulence, hard landings and max gross weight takeoffs. Such uses or events or occurrences were recorded as "E hours" in the flight record book of the plane.

E hours consumed useful life hours of the plane at a higher rate. So, the total life of the airframe once operational would never match the predicted usage because of the "E" hours factor. (The effect of corrosion on airframe has been mentioned in an earlier post).

Answer 3

Lifetime limit depends on the design and on usage. Many aircraft have a lifetime limit given in cycles (i.e. number of takeoffs and subsequent pressurization cycles). For the B-52, flight hours are used instead (because the fuselage isn't pressurized).

Modern aircraft are designed to a specific limit, and modern CAD software allows you to predict this pretty well so the aircraft becomes no heavier than it needs to be. The B-52 didn't benefit from this, so it was overbuilt.

The move from high-altitude to low-level operations shortened fatigue life. Various modifications were carried out to strengthen the wing, for instance:

Intensive structural testing, conducted by Boeing and the Air Force in 1960, again confirmed that hard usage shortened the structural life of the B-52 aircraft. The B-52Gs and B-52Hs differed significantly from predecessor models, but design changes incorporated in the new bombers made them even more susceptible to fatigue damage. Briefly stated, the changes had been made to extend the aircraft's range, which essentially meant that while the B-52G and B-52H bombers were lighter than preceding B-52s, their fuel loads had been increased. Moreover, the overall decrease in structural weight had been achieved primarily by using an aluminum alloy in the aircraft's wings. While testing did not question the intrinsic strength of the wing, it pinpointed areas of fatigue. No one could forecast accurately when the wing failures would happen, but low-level flying and the structural strains that occurred during air refueling were expected to speed up fatigue considerably. It was estimated that under fairly similar circumstances, the operating stress placed on the new wing was approximately 60 percent higher than the stress inflicted on the wing of preceding B-52s. The anticipated problem appeared serious enough for SAC to impose stringent flying restrictions on the new aircraft, pending approval of necessary modifications. In May 1961, the Air Staff endorsed a $219 million modification program for all B-52G and B-52H wing structures. The wing structural improvement program, carried out as ECP 1050, replaced the wing box beam with a modified wing box that used thicker aluminum. It also installed stronger steel taper lock fasteners in lieu of the existing titanium fasteners; it added brackets and clamps to the wing skins, added wing panel stiffeners, and made at least a dozen other changes. Finally, a new protective coating was applied to the interior structure of the wing integral fuel tanks. The program provided for Boeing to retrofit the modified wings during the airplanes' regular IRAN schedule, except for the last 18 B-52Hs, which would get their modified wings on the Wichita production lines. Started in February 1962, the program was completed by September 1964, as scheduled.

According to GlobalSecurity:

Current engineering analysis show the B-52's life span to extend beyond the year 2040. The limiting factor of the B-52's service life is the economic limit of the aircraft's upper wing surface, calculated to be approximately 32,500 to 37,500 flight hours. Based on the projected economic service life and forecast mishap rates, the Air Force will be unable to maintain the requirement of 62 aircraft by 2044, after 84 years in service.

The B-52H was designed as high altitude aircraft, but was adapted to low level tactical maneuvers in 1960's. A number of structural improvements were made during the 1960s and 1970s to equip it to fly the more demanding low- level mission and to address other structural issues.

The airframe life for the current fleet is estimated to be between 32,500 and 37,500 hours, depending on the usage history of the individual aircraft. The estimate is based upon scaling measurements from a full-scale test structure using assumed mission profiles along with historical and projected usage information. The upper wing surface is expected to be the life- limiting structural member. As of 1999 the average airframe had 14,700 flight hours. Boeing believes with high confidence that the average number of flight hours left is 17,800, at a minimum. The "oldest" B-52H is at about 21,000 hours and only experiences about 380 flight hours per year.

Boeing makes an estimate of airframe life using fatigue testing on a test airframe, and/or modern CAD.

Answer 4

With some combination of regular Non Destructive Test inspections, patches and reinforcements, and in some critical areas replacement of primary structural fittings, panels and skins, and you can run an airplane forever if you can keep it safe from oxidation corrosion.

It only comes down to inspection frequency, and when and what structural parts to replace, as to whether it's worthwhile doing or not. On a commercial airliner, you get to a point where it's no longer feasible financially to either install all the patches, or inspect the airplane with the required frequency, and still make money with it. On something like the B-52, the business case environment is a complete alternate universe (in terms of financial payback and risk analysis) where it becomes worthwhile strategically in view of the alternatives.