My guess is that the highest loads are on bad landings. Wing mounted main gear would load the wing more unless designed to fail before overloading the mainspar.
Is the PA 28 gear (which is attached to the main spar) designed to fail before overloading the spar?
I’m not knowledgeable on engineering stuff but I’d guess even if it were designed that way, repeated hard impacts that are below such limit, will cause gradual fatigue on the spar and attachment points.
Some more PA 28 break ups
http://www.kathrynsreport.com/2022/05/aircraft-structural-failure-piper-pa-28.html
And an interesting comparison of Lock Haven Pipers (Wing spar meets at fuselage center) vs Vero Beach Pipers (Spar carry through design) – attached as pdf.
Thanks.
Writing generally so as to make the point, strut-braced wing structures are built as light as they can be but often its determined that the issue preventing further reduction in weight is something other than static strength or fatigue. Examples would be to withstand ground handling damage or to be adequately stiff, i.e. to withstand buckling. Structural design has multiple requirements and constraints, in flight stress and fatigue are not the only ones. Conversely, cantilevered wing structures tend to operate at higher material stresses, so stress and fatigue become bigger factors. If those stresses are reduced by heavier spars, thicker section wings etc. the payload and/or performance of the aircraft is reduced, so a compromise is found.
There are those who think the pre-1967 Cessna 210 with strut braced wings was a pretty good going places plane, at least structurally. But they have other problems, and fly like a truck. Retracting the landing gear in combination with a strut braced high wing was problematic. Nothing is perfect 
Yes, clean aircraft with relatively highly stressed structures are more easily broken by aerodynamic loads, especially since the Normal, Utility etc. G loads for certification are the same regardless of aircraft performance. Obviously early Bonanzas and their contemporary era of pilots are the poster child for this issue. Literally hundreds had in flight structural failures, the result of loss of control, overloading and static strength failure and not a fatigue cracking issue in normal service like the PA28 that originated this thread.
Silvaire: To summarise, strutted structures are mechanically more efficient than pure cantilever structures, less prone to fatigue, and easier to analyse.
But given that this is the case, why are cantilever structures not built more strongly to compensate; why not profit from the structural efficiency of strutted wings to build them lighter?
I suspect I would be right in predicting that very clean airframes will be more prone to structural failure, because it will be easier to inadvertently overspeed in them.
My guess is that the highest loads are on bad landings. Wing mounted main gear would load the wing more unless designed to fail before overloading the mainspar.
@kwlf, the highest stressed area in a strut braced wing is typically where the the strut attaches to the wing spar, at which point the wing outboard of that area is cantilevered, although with much lower bending moment than would be case at the wing root, minus the strut, for a cantilevered design. This often means there is a localized wing spar reinforcement near the strut attachment, but due to the stubby length of wing that is outboard it doesn’t take much added material to lower the stresses a lot in that area, and the strut itself and its pinned joints are generally not the issue. People do in-the-field weld repairs on steel wing struts, supported by guidance in AC 41.13. That’s not exactly a precise or well controlled operation, but its rarely an issue. Strut bolts are usually hugely over strength too, relative to the load applied.
At the wing root of a strut braced wing the spar is in compression, not bending, and given that the spar maintains the same overall dimensions relative to e.g. the area around the outboard strut attachment, the stresses on everything in that area are pretty low. Typically a single pin (bolt) in shear per wing spar takes the wing load into the fuselage. Compressive buckling as opposed to stress can be a bigger issue in this area, as with struts in compression (typically meaning negative g wing loading), but its obviously not an issue for fatigue… which is generally the cause of cantilevered wings failing, if they fail. Look at the aluminum carry through structure above the cabin in a strut braced design and you’ll see that despite carrying the entire compressive load between wings its a pretty lightly built structure. This is why corrosion in the carry through area can be a concern – because the material thickness is so small, not because its highly loaded.
Basically, except at the wing spar in the area of the outboard strut attachment, a strut braced wing is not highly stressed and for a light plane with a strut braced wing fatigue is not so much a major issue. That’s why strut braced designs often don’t have life limits, and why its harder to design and operate a cantilevered wing. When fatigue is the dominant design issue as it more often is with a cantilevered wing you have to do more analysis and/or more testing to determine the useful life, you can’t just do a single load to failure test and know the structural margin quickly.
All of the above is why the Cessna Caravan and Sky Courier have strutted wings, even though they were designed after Cessna had produced a lot of planes with cantilevered wings. For a customer that wasn’t so concerned with cosmetics, it made more sense according to a guy I know who did that structural analysis on the Caravan.
Yes I agree, in certified aircraft, the design is done based on high level rules and principals and likely to lead to similar guarantees across all CS23 airframes, anything else is ‘just noise’ but some, including FAA, would even go to attribute even the tiniest of noise to specific type based one single rare events !
On the modelling side, things like robustness to corrosion and abuse are not well understood in Mechanical Engineering even with ‘rule of thumbs’ and plenty of margins on loads…if something ‘wrong in the data’ of Piper wing vs Cessna wing, it will have to do with water? rough surfaces? history of flight? or type of pilots?
PS: while ago when working on jet engines, ‘technically’ one could predict 3D crack propagation in titanium alloys fans for given jet engine based on it’s unique recorded usage and maintenance history, obviously, you can’t use that (non-approved) model to certify an engine with 300 pax on it, even if the maths add up, you have to stick ‘rules of thumb’ with load of margins and throw in ‘regular inspections’ 
Ibra wrote:
I was expecting a conclusion on C172 vs PA28 to involve mechanics & physics (structural analysis, sheer & load tensors, crack propagation, elasticity-plasticity, solid-fluid harmonics…)
I found it interesting to contemplate the possible value of a paper that didn’t.
In engineering, we work on the assumption that everything is more-or-less understandable. In biosciences, it’s much harder to work anything out from first principles. Even when you can, you don’t trust it until it’s been validated experimentally – and it’s much harder to do that rigorously because there are rightly lots of rules on how much you can experiment on people, and because people are far more diverse than rivets so you need larger sample sizes to reach firm conclusions. But what if engineering is actually more like the biosciences than we like to admit? We all know there are some unknown unknowns, but perhaps they are more numerous than we realise.
My understanding of aircraft design is that you have structures that are designed analytically, but padded with safety margins and design decisions that are perhaps based more on rules of thumb, and hopefully take into account the fact that people don’t always treat the structures in the manner for which they were designed (see Silvaire’s excellent point about some structures being sized in practice to resist hangar rash rather than structural loads). Wing roots with all their holes and bolts strike me as something that will only partially yield to analysis (particularly back when the Cessnas and Pipers in question were designed), and the safety margins are likely to be drawn as much from experience as theory.
No aircraft should fall apart in reasonable use, so if cantilevered wings are significantly more likely to do so then perhaps our rules of thumb are not as conservative as they should be. On the other hand, if we’d found that strutted wings were more likely to fail due to the end fasteners on the wing struts being prone to failure due to stress concentrations that weren’t fully accounted for, or moisture flowing to the bottom of the struts and causing corrosion – then post-hoc we would have said ‘sure, that makes sense’. Perhaps sometimes it makes sense to throw the theory aside and ask, ‘what is?’
My question would be how generalisable the findings are. If all the companies building planes are using similar rules of thumb to design their aircraft, then we might find that Eurostars and low wing Zenairs are more prone to wings breaking than C42s or high wing Zenairs. However, I would not jump to this conclusion because I suspect that there will be cultural and temporal differences in how wings are designed (i.e. Zenair might use different rules of thumb from the legacy Cessnas, partly because it’s a different company and partly because the designs are more modern, and materials and design philosophies have changed). Personally I’m happy to fly any individual design with a decent track record.
By9468840 wrote:
Has anyone seen anywhere Piper or any authority publish the results of all eddy current inspections done so far?
This is based on hearsay, but it is actually not low single % figures, but somewhere between 5% and 10%, in some cases the NDT inspection failed on relatively low time examples not covered by the AD. It would be helpful if Piper did publish the actual failure rate, so that you are not relying on local engineers word of mouth/gossip.
As a consequence pretty well all PA28 have, or will have the NDT inspection to maintain their marketability.
I was expecting a conclusion on C172 vs PA28 to involve mechanics & physics (structural analysis, sheer & load tensors, crack propagation, elasticity-plasticity, solid-fluid harmonics…)
Not rare events stats with 2 data points where one go to look into NTSB database to extrapolate Gaussian variables beyond the 0.01% quantiles !
