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For example the DC3. The way I understand it, the only thing that "ages" the airframes of DC3 is the landing/takeoff cycles and the accompanied periodic force loading/unloading. Do planes such as this really have a meaningful definition of lifetime of its airframe?

As a side note, is loading/unloading cargos on the floor another kind of cycle too?

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Yes they do, and here is why.

Airplanes are made mostly of aluminum rather than steel. It is possible to design a steel part in such a way that it will never succumb to fatigue failure (crack propagation at a stress significantly below the yield stress) but this is not true of aluminum and its alloys. For this reason it is necessary to periodically tear down and inspect aircraft for fatigue cracking of their aluminum structure- in essence, safety is inspected into (aluminum) aircraft because from a design standpoint, it is not possible to design fatigue failures out of their structure.

During that teardown, any part that shows fatigue cracking (or corrosion, or physical damage, etc.) is removed and replaced with a new part that is certifiably fault-free. To some extent this means that a DC-3 with 40,000 hours on it has had so much of its structure replaced over the years that the joke is told that the only part that actually has 40,000 hours on it is the serial number plate.

Flexure of the structure during turbulence in flight is a more significant source of fatigue cycling than cargo loading & unloading cycles.

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When you subject aluminum to cyclic loadings, there is a relationship of cycles it takes to crack/ vs material thickness, or structural meat you might say. If the structure is sufficiently meaty, the number of load cycles to generate cracks become effectively infinite.

Modern airliners have structures carefully designed by analysis to last a certain number of load cycles, then subjected to a fatigue test in a rig to simulate the loads, then whatever cracks turn up have to happen at at least 3 times the number of cycles you are designing for. Put another way, if a crack shows up at cycle X, the life of the structure is set at 1/3 of X.

Airplanes like the DC3 were designed before all this was well understood, and in the slide rule era, so they added lots of meat to be safe, and the result is the 3 has a fatigue life of more or less infinity.

What brings them down, besides accidents, is corrosion in its various forms. Deal with that and a DC3 will theoretically run forever.

In my book the '3 is the world's greatest engineered transportation device. I can't think of any other machine that, 85 years after it was designed and 74 years after it was last built, businesses still, in 2019, buy these ancient airframes not to fly around as toys or museum peices, but to put them to work to make money.

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Older planes like the DC3 undergo 100 hour and/or annual inspections when used for hire just like other planes, and Airworthiness Directives for maintenance issues are sent out to the fleet as things crop up as well. If the inspections and required maintenance do not show anything, they are signed off to go until the next inspection period. There are also shops that specialize in tearing down DC3s down to their frame and rebuilding them so they end up like practically a new airplane. Example https://www.airspacemag.com/flight-today/high-mileage-1053125/ 40,000 hours!

So effectively, there is no lifetime to the airframe.

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