Two questions -- one, why is survey / pipeline inspection more stressful on the aircraft than other flying? Are you doing a lot of tight, higher-G turns?
Two, is it purely hours in service on the airframe that contributes, or is calendar age a factor (albeit smaller) also? Is your 1953 airframe with 3500 hours in the same situation as your 1998 airframe with the same 3500 hours? Corrosion aside - assume they've both been in kind environments and cared for.
Subject to cyclical stresses due to turbulence from a lot of low level, near to the ground flying hours. That puts a high number of stress cycles on the airframe. Most pipeline patrol and survey flying is boring, straight and level flying and high-Gs aren't part of the normal curriculum.
Aluminum doesn't have a fatigue limit. With enough cycles it will eventually fail. From a practical standpoint, if the stress levels are low the number of cycles to failure can be very large (hundreds of thousands, or even millions of cycles) so its not an issue. But higher cycle stresses lower the number of cycles to failure. That's why an Embry-Riddle trainer can have an airframe failure at 7000 hours, while other, identical airframes can amass double or triple that without a problem.
This is not the first time 210s have lost wings though, right? I seem to recall there were some other issues as well, however those were the result of flying into thunderstorms, etc...
I believe you are recalling Scott Crossfield's accident.
Metal fatigue is a scary killer, and I was always surprised that GA airplanes don't have cycle and fatigue life spans like commercial airliners do. Sure they're (mostly) not pressurized, but just surprising that there is no real life limit on these planes. Even if there is no corrosion and things "look" fine there are fatigue cracks that start accumulating and each landing event does put a higher stress level on the plane...
And how would that figure be determined?
In the case of a pressure hull the range limits of the cyclical stresses imposed are highly predictable and a life limit due to that fatigue source can be determined with a suitable safety margin, with high confidence. I believe the Piper PA-46 pressure hulls have something like a 14,000 flight hour limit on them.
How does one assign the same, single life span to a conventional GA airframe of the type most of us fly, and used in training or pipeline or low-level survey work, compared to the exact same airframe used "by a little old lady to fly to church on Sunday"? Should every airframe be scrapped based on a severe service training usage assumption? See the dilemma?
From the day an aluminum airframe leaves the factory, every single structural aluminum component subject to cyclical stresses starts to "wear out". We have proved that for most of those components, on most of those airframes, that rate is not high enough to worry about in our lifetime. The dumbest thing the FAA could do is force the scrapping, or extensive rebuilding, of every airframe of a given type, regardless of its service history, based on a small sample of failures from airframes in the most severe service. Accepting that on some airframes, in some types of service, perhaps prudence should govern.
It's not just Piper's or Cessna's we are dealing with here. The much admired, "answer to all What-plane-should-I-buy" queries, Bonanzas have long been the subject of web spar inspection ADs and Service Bulletins due to cracking. There's a fix for it, but it ain't cheap. I recently replaced the RH flap on my Aztec due to fatigue cracking of the actuator attach bracket. The flaps are directly in line with the slipstream from the wing mounted engines/props. These planes are all getting old, cracks develop in aluminum parts over time. It does not mean the part is underdesigned, it could be it just reached enough cycles at the stresses it was subjected to (for example, I suspect prior owners/pilots habitually extended the flaps on my Aztec at the upper end, or above the max flap extension speed as a method of slowing it down quickly entering the pattern.
