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RedditA 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."
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very interisting article.although why would a newly built airplane after only flight tests would be scrapped,is a waste of time money labour and why not put the airplane on display in a museum for others to enjoy?
Posted by carmine pelaia on December 11,2008 | 10:27AM
I have to agree with Carmine, great article, but what a tremendous waste of time, efford and money. And forget about a museum, they could have put that baby right in my back yard.
Posted by Bo Green on December 24,2008 | 03:03PM
i am glad to know that you care about safety,and what the genaral public thinks.it may be very difficult and expensive to determine an air plane's flight worthiness,but sometimes we have to think outside the box.i am not sure how pressurization work in an air plane,or the process that has to be use.perharp the design and complexity of its mechanical and the sensitivity of the wireing make it difficult to perform other less expensive test.maybe the airplane companies need to consider designing their planes differently.
Posted by giddian francis on January 14,2009 | 06:29AM
More info on the ultrasonic phased-array technique for NDE can be found at http://www.bercli.net/documentation/phased_array.htm (updated link).
Posted by Guillaume on January 16,2009 | 09:14AM
With newer materials like carbon fiber and other composites, why can not aircraft manufacturers make use of them instead of the perennial aluminum that is less durable? Then they should also design the aircraft that sections can easily be replaced thus extending their flying lives longer. When I see aircraft bone yards split familiar aircrafts in only a few minutes for recycling a part of me is also dies with the plane especially if they have made historical flights in the past.
Posted by Ray Kabigting on January 30,2009 | 08:30PM
Boeings new airplane the 787 which has not flown yet is made with composite materials and may not suffer from the fatigue problems todays aircraft have.
Posted by Larry Waggerman on February 3,2009 | 10:19PM
Boeings new airplane the 787 which has not flown yet is made with composite materials and may not suffer from the fatigue problems todays aircraft have.
Posted by Larry Waggerman on February 3,2009 | 10:19PM
How frequent do the airliner conduct an inspection to an airplane? Do they conduct it on daily,weekly,monthly or distance of flight basis?
Posted by Ron Ting on February 5,2009 | 05:50AM
I found this article very interesting. This to me brings up another question. Why is it some airplanes last longer than others? Like, has anyone figured out why the DC3s can still fly while planes far younger are long gone?
Posted by Louis Steiner on February 22,2009 | 05:23PM
I agree. A very interesting article. Could anyone tell me how to get into aircraft recycling career?. As a Senior Project Manager with Aviation in my heart I would love to be managing projects to preserve aircraft.
Posted by Darren Price on February 27,2009 | 09:32PM
Re. Louis, for one thing, DC-3s weren't pressurized airplanes, nor were B-17s or piston fighters. B-29s were pressurized but in a year or two's service I imagine they didn't go through nearly as many pressurization cycles as a jetliner. In regards to fatigue cracks, it is interesting that the first DeHavilland Comet jetliners had rectangular windows - the window corners were the perfect place for cracks to propogate from, which caused the infamous catastrophic failures in early Comets. Metals and their properties is a fascinating subject.
Posted by Jerry P on March 12,2009 | 11:25PM
The answer to the question “why do some planes stay aloft longer than others?” is an easy one. Money money money! Is it worth it to keep em’ flying? Look at the balance sheet. Airlines and museums work on the same principle; they do what they can with the money they have. Of course, the military is a little different….
Posted by clay on April 1,2009 | 07:42PM
At first one would think with today's technologies, different materials could be used to increase the length of time planes remain in service. But after reading clay's comments, he hit the nail on the head, it's all about money... -Stephen Personal Injury Lawyer NJ
Posted by Stephen on April 7,2009 | 06:14PM
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. I think that it is great that they do rely on manufacturers. Why not? Mikhail at fat loss 4 idiots guide.
Posted by Mikhail on April 15,2009 | 02:15PM
Yeah, the limiting TECHNICAL factor may be metal stress and fatigue, but the possible FINANCIAL limits are endless: engine fuel consumption vs upgrades vs scrappage, the structure of the original purchase [actual ownership vs lease], who owned the airframe and for how long, which airline had it last and for what purpose, etc. ad nauseum. Most of the aircraft now in long-term storage in the desert will never fly again, though most of them are or could be made to be airworthy - just not economically. These airplanes are technically not "worn out", but no one wants to spend the money to keep them flying commercially, because there are more economical alternatives. AA just announced a major conversion to 737-800s to replace a bunch of their MD-80s, not because the latter are so near to their technical demise, but because they use more fuel and are starting to become a maintenance nightmare. Same reasons you trade a used car for a new one....only in this case the used one never goes back on the road because no one wants it.
Posted by John Robison on April 15,2009 | 03:19PM
I have to agree with some of the other postings here. With today's technology and materials, one would think planes should last longer. If the lifespan of a plane can't be lengthen, there won't be enough museums around to showcase all the scrapped planes. Scott Philadelphia Wedding Photographer
Posted by Scott on April 15,2009 | 05:48PM
In response to Louis Steiner's question about why DC-3's are still flying while many younger aircraft are not, I recall a conversion between the co-pilot of a DC-3 that a group of us was going to fly in and one of the passengers. He asked: "Is it safe to fly on an airplane this old?" to which the co-pilot responded:"How do think it GOT TO BE THIS OLD?!!" Probably the main reason that the DC-3 is still in the air is because the aircraft is so underpowered that the airframe doesn't get a stressed as aircraft with more power. (I wonder if anyone ever though about replacing the Pratt & Whitney R-1830's with R-2800's during the war.) Also, the DC-3 is built like a tank with rivets to match. Flush-riveted, the DC-3 could probably fly 30 knots faster.
Posted by H. Steven Dittrich on May 26,2009 | 10:37AM
The DC3 is not underpowered. Like all designs it was built to fly at the speeds it works at. It can fly on 1 engine. I flew in a Super DC 3. It was a C 56(?)(Navy R-6D or some number). It was a Navy version and took me from Okinawa to Kyushu in 1953. It was a DC 3 with different wings and empennage, powered by R-2800s. Those engines drank gasoline and the airplane was more expensive to operate, but it flew faster. No commercial versions were sold that I know of. Ask Douglas about their swept wing Super DC-3.
Posted by robert E. henry on June 18,2009 | 04:04PM
During mountain travels in a motorhome, the stored water bottles fatigue from constant exposure to various altitudes and temperatures. They all eventually leak from the constant flexing. Same effect but more drastic for aircraft components. DaFlikkers
Posted by Blogengeezer on June 23,2009 | 06:57AM
My brother is an aircraft mechanic who repairs smaller jets that fly domestic routes from Newark Liberty Airport. Due to the metal fatigue and the pressurization that the aircraft is put under, he tells me he is constantly changing fasteners and rivets on these aircraft. There is even a special technique to fasten the fasteners.
Posted by Luke Lagis on July 7,2009 | 12:54PM