In many aviation online communities I see many of the same fundamental question: why did this one type of airliner outlast newer ones? Why did they get rid of this one type? When are they finally going to retire this plane? And so on. The reasons why some planes get sent sooner than others depend on the airline itself, why it got that type in the first place and whether or not that justification or reasoning still exists. Regardless of why or when, all airliners share the same fate: sooner or later, they'll get to the Boneyard eventually.
Topshot by Dale Coleman via Wikipedia, used under GNU Free Document License
To demonstrate why, here's a fun little experiment you can do:
1. Get yourself a good pair of Kevlar work gloves.
2. Get an empty aluminum beer or soda can and use a pair of metalworking shears to cut a nice long strip from the can. Or if you don't feel like getting the aforementioned gloves, just play with the can's tab.
3. Use a can opener to remove the lid from a tuna can or a similar metal package (as long as it's a steel container).
4. Bend the aluminum strip repeatedly until it fails and make note of how long it took to fail.
5. Bend the tuna can lid until it fails and make note of how long it took to fail (hint: this is going to take a while)
Aluminum has a certain "shelf life" (or fatigue life, to use the technically correct term) where it can take a theoretically fixed amount of abuse before it fails, and depending on the nature of that use that can occur quickly. The number one reason why military aircraft get retired to AMARC is because they reach a state of being "surplus to requirements" either due to technical or performance obsolescence or budget cuts, but many F/A-18C "Legacy" Hornets are being sent there because the heavy toll of the wars in Afghanistan and Iraq have used up the "safe" hours available on their airframes. For most tactical fighters like the F/A-18, this is typically in the 5,000 to 7,000 flight hour range. If that seems low, that's because it is. Fortunately, fighter planes aren't flown as frequently as jetliners, so they can stretch those 5,000 hours out over years and even decades. Plus, upgrade and remanufacture programs (often in the form of a "Mid-Life Upgrade" or MLU) can add life back into the airframe, sometimes even doubling it, by completely tearing the aircraft down and replacing critical "timed-out" components like wing spars with brand new ones.
The general rule of thumb is that airliners (especially large and particularly sturdy ones like the 747) can last up to 100,000 flight hours or 100,000 cycles. What's the difference? Flight hours is exactly what it sounds like - the total amount of time the aircraft is in the air (well, not quite, but for our purposes we'll stick to this simplistic definition). A cycle represents one trip of the aircraft from Airport A to Airport B, or more specifically for our purposes, the time the pressurization system is activated on the ground to the time it's de-activated on the ground at the destination airport. Flying puts a lot of stress on an aluminum tube - turbulence for one, but also the simple act of moving through the air. Wings bend - actually, they bend a lot as part of how the aircraft defeats turbulence for the sake of passenger comfort and achieve high fuel efficiency. Things bend and move imperceptibly as flight attendants move about the aircraft, systems function within their normal parameters, and in just experiencing the act of existing as a component of a large moving machine. These are small, tiny forces, but after 20 years of flying across the Pacific Ocean for half a day each flight, they add up.
Image by Timothy Smith via Wikipedia, used under Creative Commons License
But wait! you ask. There are so many really old aircraft that have been flying around for decades! Like this Kenn Borek DC-3! What's so special about them? And don't give me any "They don't build them like they used to!" BS!
Well...the thing is, they really do...er, don't build them like they used to.
First of all, aircraft like the DC-3 actually do require complete tear-downs and rebuilds every so often, especially when they crash, which tends to be inevitable if an airplane flies long enough (not necessarily to the degree of having fatalities, but the aeronautical equivalent of a fender-bender). The Kenn Borek DC-3s (actually Basler BT-67s) in particular have been heavily remanufactured into new aircraft including new wing and fuselage plugs.
Secondly, what really does damage to the airplane is its pressurization system. Remember the secondary definition I gave for what a "cycle" is? The time the pressurization system is activated on the ground to the time it's de-activated on the ground at the destination airport. That's an important distinction because the pressurization system tends to literally inflate the aircraft's fuselage like a balloon - it's not enough for the average passenger to notice, but the aluminum structure does indeed get stretched to the same effect as if you were blowing up a balloon, and the cycle of pressurization from the inside when the aircraft starts boarding passengers against when it's facing pressure again from the outside atmosphere at rest on the ground results in major structural movements. Imagine taking a soda can and crumpling it just slightly, then stretching the ends to try to smooth those crinkles out, then compressing it again to create new crinkles. See how many times you can do that before the can completely fails. A major reason why the DC-3 can last so long is that it simply doesn't have a pressurization system, so the fuselage structure experiences comparatively stable forces. Tube-and-fabric aircraft like, say, a Piper Super Cub tend to have their frames constructed from mild steel which is closer related to that tuna can lid than the soda can, so they can keep on flying for as long as the owner keeps care of it (but have the downside of constant maintenance to their fabric covering).
It's not the only reason why airliners get sent to the boneyard, but it is a major one, and the reason why all airliners will inevitably be sent to the boneyard. With "plastic" planes like the Boeing 787 becoming popular, and Airbus sending its A350 into service, airliners might stick around longer, but if nothing else economic factors will take them out of service. Technology outpaces all aircraft and sooner or later said aircraft simply can't keep up with new frames. Even these carbon aircraft will reach a point of structural compromise eventually - right now, we're just not sure after what time and under what circumstances, but pretty sure the aircraft will simply become economically unviable before that. Financing structures also play a role, which is why Northwest Airlines and Delta Air Lines held on to its DC-9s longer than some of the far newer and fancier Airbus and Boeing aircraft it leased.
As for refurbishing airliners, that tends to be out of the question - well, not entirely. In industry parlance there's a thing called a "Heavy D Check" - the last of a sequence of maintenance procedures starting from A. The paint comes off, the old engines get swapped out for new ones and stripped of what can be reused, and even whole skin panels are removed and replaced. Basically, what needs to be replaced does get replaced - as long as it's economic to do so. Theoretically you can replace a timed-out wing spar on a 747 and keep it flying forever - but the process to do that is so complicated and expensive, all you've accomplished is to completely disassemble an airplane and reassemble a new one in its place using a mixture of old and new parts.
Image credit Julian Herzog via Wikipedia, used under Creative Commons License
Airlines are obsessed with fuel savings in order to stay relevant in today's economy, and they're willing to put down money for new planes in the name of less use of the gas pump. An old jetliner can't compete with a new one even with massive upgrades - a 737 "Next Generation" (600 through 900 models) for example has fewer holes cut into it because a combination of engineers and bean-counters decided those holes weren't needed after all (or rather, they decided it was preferable to sacrifice the cabin-air feed they provided for passenger comfort in order to make the fuselage more aerodynamic and instead continuously recycle the air already in the plane). An NG might have engines that belong in the same CFM-56 family as what hangs on a 737 "Classic" but said engines have undergone a decade's worth of evolution in the meantime. Some parts that used to be aluminum are now composite. The NG even has an entirely different wing design compared to the older model, and the new MAX models will have a further redesigned wing. Factoring all the major design changes, the hours that have accumulated on the old airframe, and other refurbishment costs, and that old bird is going to be sent to the boneyard.
Image from the official Nextant Aerospace webspage
And yet in the general aviation world there are quite a few total refurbishment programs available, such as from Nextant Aerospace specializing in a complete tear-down so extensive they're allowed to call the airplane something different than what it was called when it left the original production line and call themselves the original manufacturer.
A brand new engine pylon is manufactured and placed on this ex-Beechcraft 400 after the removal of the old one at Nextant Aerospace's remanufacturing facility as part of the "400XT" conversion process. The pylon incorporates structural and aerodynamic improvements for better efficiency and to accommodate the Williams FJ44 turbofans in place of the original Pratt & Whitney JT15D turbofan engines. The FJ44s incorporate new fabrication and software control technologies to leap ahead in efficiency, and are highly popular on the latest small bizjet designs. Image taken from the official Nextant Aerospace website.
The difference is the scale - in this case, smaller is better. Used Beech/Hawker 400s and King Air 90s are fairly plentiful, span decades of production and depreciation means they can be picked up on the cheap. Cheap enough such that they can have their engines replaced, their internal guts ripped out and new wingtips put on and still be sold cheaper than newer jets while offering the same performance and technological advantages. The flight hours/cycles aren't as much of an issue because of the infrequency of business jet use. Even after decades of use they still have thousands of hours available on the clock, and at the scale of these small planes even wing spars can be replaced with relative ease to create a "zero-time" aircraft. Remanufacturing isn't limited to former Beechcraft products - the Rockwell Aero Commander series of aircraft have had a few similar programs offered as well including the Grand Commander and Dash-10.
Airliners live a hard life, but it's possible to stretch a general aviation aircraft's life span for generations of use. A large remanufacturing and support base helps create highly skilled technical jobs in the aerospace industry, offer more affordable flight and transportation options for every price point, and keep many classic out-of-production designs flying. Remanufacturing can help the 172 Skyhawk pilot as much as it helps the corporate executive who loves NetJets. With engineers, technical advisers and number crunchers passionate about general aviation, expanding the GA remanufacturing base tremendously helps preserve aviation as an active flying art and useful means of rapid conveyance.