A reader asks: "Two articles in the Feb./Mar. 2007 issue of Air & Space raised a question. One was about the last flying examples of a number of classic planes ("And Then There Was One"). The other was about newer jetliners, too old to fly, being chopped up to make skateboards and soft drink cans ("We Recycle"). It struck me as odd that the old planes are still airworthy, while the jetliners are fit only for the scrap heap. Why can some planes seemingly keep flying forever, while other, newer ones are already used up?"

An aircraft's lifespan is measured not in years but in pressurization cycles. Each time an aircraft is pressurized during flight, its fuselage and wings are stressed. Both are made of large, plate-like parts connected with fasteners and rivets, and over time, cracks develop around the fastener holes due to metal fatigue.

"Aircraft lifespan is established by the manufacturer," explains the Federal Aviation Administration's John Petrakis, "and is usually based on takeoff and landing cycles. The fuselage is most susceptible to fatigue, but the wings are too, especially on short hauls where an aircraft goes through pressurization cycles every day." Aircraft used on longer flights experience fewer pressurization cycles, and can last more than 20 years. "There are 747s out there that are 25 or 30 years old," says Petrakis.

How do airlines determine if metal fatigue has developed in their passenger-liners? Bob Eastin, an FAA specialist on aircraft fatigue, says, "[Airlines] are really relying on the manufacturer's maintenance programs. The manufacturers design the aircraft to be trouble-free for a certain period of time. There are maintenance actions to preclude any catastrophic failures, but that's not to say that the aircraft might not [experience metal fatigue] before those times…. When you get to a certain point [in the aircraft's lifespan], you need to inspect or replace certain parts."

Nondestructive evaluation (NDE) inspections are used both during production (to ensure that components start out free of defects) and during an aircraft's service life to detect cracks as small as 0.04 inch. Inspectors might, for example, take a close look at fastener holes located at the wing and spar junction.

We contacted NDE experts Deborah Hopkins of Lawrence Berkeley National Laboratory and Guillaume Neau, of Bercli, LLC, who together answered in an e-mail: "The challenge in developing an easier and less expensive inspection strategy is to design a technique that can be used from the skin side (of the wing), that does not require removal of the fastener, and that provides the same or better resolution than the conventional method of removing the fastener." Not having to remove the fastener is a big money-saver.

One commonly used method of NDE is ultrasonic phased-array testing, which analyzes the echoes from ultrasonic waves to reveal imperfections inside a material. By using several ultrasonic beams instead of just one, then varying the time delays between the beams, inspectors can look inside a material at different locations and depths, thereby determining the size and shape of any defects.

At present, million-dollar robotic inspection systems equipped with phased arrays are being used to inspect wings and composite fuselages for large commercial aircraft and jetfighters before they fly. "Most aircraft manufacturers and service providers—Dassault Aviation, Airbus, and Boeing, for instance—ensure the quality of their production with large-scale non-destructive testing systems," Neau wrote in an e-mail. And while a million dollars may sound like a lot, "when put in perspective, the number is not so large," he says. "If manufacturers discover a problem after assembly, the cost of dismantling and redoing the part or the scrappage waste is much higher than the inspection cost."