Keeping aging systems on their feet is a daunting and resource-intensive task. The U.S. Air Force, for example, continually wages an internal battle to keep its weapons systems in fighting form. One enormous and often overlooked factor contributing to the early demise of military technologies is the problem of unavailable parts. Take the B-2 Spirit, a stealth bomber that first flew in 1989: by 1996, significant components of the aircraft's defensive management system, just one small part of its electronics, were obsolete. Repairing the system entailed either redesigning a few circuit boards and replacing other obsolete integrated circuits for US $21 million, as the B-2 program officers chose to do, or spending $54 million to have the original contractor replace the whole system. The electronics, in essence, were fine--they just couldn't easily be fixed if even the slightest thing went wrong.

Although mundane in its simplicity, the inevitable depletion of crucial components as systems age has sweeping, potentially life-threatening consequences. At the very least, the quest for an obsolete part can escalate into an unexpected, budget-busting expense. Electronics obsolescence--also known as DMSMS, for diminishing manufacturing sources and material shortages--is a huge problem for designers who build systems that must last longer than the next cycle of technology. For instance, by the time the U.S. Navy began installing a new sonar system in surface ships in 2002, more than 70 percent of the system's electronic parts were no longer being made. And it's not just the military: commercial airplanes, communications systems, and amusement-park rides must all be designed around this problem, or the failure of one obsolete electronic part can easily balloon into a much larger system failure.

The crux is that semiconductor manufacturers mainly answer the needs of the consumer electronics industry, whose products are rarely supported for more than four years. Dell lists notebook computer models in its catalog for about 18 months. This dynamic hurts designers with long lead times on products with even longer field lives, introducing materials, components, and processes that are incompatible with older ones.

The defining characteristic of an obsolete system is that its design must be changed or updated merely to keep the system in use. Qinetiq Technology Extension Corp., in Norco, Calif., a company that provides obsolescence-related resources, estimates that approximately 3 percent of the global pool of electronic components becomes obsolete each month. PCNAlert, a commercial service that disseminates notices from manufacturers that are about to discontinue or alter a product, reports receiving about 50 discontinuance alerts a day. In my capacity as an associate professor of mechanical engineering at the University of Maryland, College Park, and a member of the university's Center for Advanced Life Cycle Engineering, I have been developing tools to forecast and resolve obsolescence problems. To deal with that growing pile of unavailable supplies, engineers in charge of long-lasting systems must basically predict the future--they must learn to plan well in advance, and more carefully than ever before, for the day their equipment will start to fail.

The systems hit hardest by obsolescence are the ones that must perform nearly flawlessly. Technologies for mass transit, medicine, the military, air-traffic control, and power-grid management, to name a few, require long design and testing cycles, so they cannot go into operation soon after they are conceived. Because they are so costly, they can return the investment only if they are allowed to operate for a long time, often 20 years or more. Indeed, by 2020, the U.S. Air Force projects that the average age of its aircraft will exceed 30 years--although some of the electronics will no doubt have been replaced by then.

Some of the best examples of obsolescence come from the U.S. military, because it has been managing long-cycle technology programs longer than just about any other organization in the world. But there are also commercial aircraft that fall into the same, almost incredible age range of 40 to 90 years. The Boeing 737 was introduced in 1965 and the 747 in 1969; neither is expected to retire anytime soon.

When those systems were first built, the problem of obsolete electronics was only vaguely addressed, if at all, because the military still ruled the electronics market, and the integrated circuits they needed remained available for much longer than they do now. In the 1960s, the expected market availability for chips was between 20 and 25 years; now it's between two and five. Call it the dark side of Moore's Law, which states that the number of transistors on a chip doubles every 18 to 20 months: poor planning for parts obsolescence causes companies and militaries to spend progressively more to deal with the effects of aging systems--which leaves even less money for new investment, in effect creating a downward spiral of maintenance costs and delayed upgrades.