Keeping track of all the variants from Avid Flyer and Kit Fox onwards, in every country where they’ve been built, would seemingly take a detailed study!
Well, the POH I cited from is of my Apollo Fox, which is a shameless copy of the Eurofox according to some, whereas others praise the small but clever improvements – not sure what to think about that. But I do know of several Eurofox and Apollo Fox living hard lives, as trainers, or in the hands of beginners like myself, with excellent performance. They can stand a lot of abuse and, if repairs are at hand, they are easy and affordable.
Just for the record, the aircraft Peter refers to in the OP is a Eurofox (Aerotrek in the US). Over 400 have been manufactured since 1990 (around 50 amateur-built), by the same family company in Slovakia. There have been zero structural failures recorded in this time, although many aircraft have had a tough life in the bush and many used for extensive glider towing.
Info here: http://goo.gl/5TZ7vw and http://goo.gl/bmExaW and http://goo.gl/5mQf59
I would be surprised if more than a handful of the 400 had pitot heaters fitted…..
A tubular frame is what made the Hurricane much better able to withstand battle damage than a Spitfire.
In case anyone’s interested!
Speaking about space frames and tubular structures: generally there are two load paths (perhaps two sides of the fuselage) and one path carries the load if a member is broken or very damaged. This also has an effect in terms of mitigating fabrication imperfections on the in damaged structure. If there are intact parallel load paths, one path will always carry more load than the other. It is absolutely correct that FEA/CAD has a hard time with that sort of thing, but there are factors that make it work out OK: ductile materials are used and the first time the structure is loaded up they locally yield (permanently stretch) in those areas where the stress in highest, working out kinks and redistributing the structure’s stress under subsequent loading forever. 4130 steel has over 20% elongation to failure, and steel generally is a remarkable material in its combination of strength and ductility. If you built the same structure out of glass it would fail at much less than the calculated load, due to imperfections. Ductility is what prevents that with metallic structures.
Knowing some of this stuff is what allows me to make a living even while being a total dunce with computers.
I found an interesting document that gives some insights regarding microlight/ultralight safety:
http://www.easa.europa.eu/rulemaking/docs/research/EASA.2009.C53/03%20-%20Final%20Report%2026%20Nov%2010.pdf
Having read it I conclude that this category is not significantly less safe than, say, gliders, which are generally certified aircraft (and flown by people with usually excellent piloting skills).
From my POH (which is factory built, but using the typical steel tubing, I reckon it is 10 mm but could be 12 too):
The operator is allowed to carry out only such repairs of lattice work in operation that do not
require either use of a welding equipment or application of a thermal treatment at straightening.
Straightening of such structural members is permitted, deflection of which does not exceed 3%
member length-member diameter ratio.
A local deflection (depression) not exceeding 5% of tube dimension in its diameter can still be
considered admissible provided this tube is not damaged by cracks or some other non-
reversible deformation.
The sort of thing I was thinking of in my original post was that the classical triangle structure (which is extremely stiff because all three members are either in “perfect” compression or tension) is trivially weakened if one of the three bits of tubing is less than perfectly straight – either as a result of fabrication or as a result of having been bent by somebody having kicked it.
If you make an airframe out of say 15mm or 20mm tubing then – even if the material is really thin – this is far less likely than if it is made out of 10mm tubing.
I don’t know how CAD software deals with this.
After reading the Swiss accident report, I wonder if the modern ability to calculate could produce a danger.
The typical problem there is that structural analysts using modern tools (and I have a couple of good ones who work for me) do very accurate calculations on the wrong thing. Understanding structures is slightly different than analyzing them.
Recently I had to explain to a guy that it didn’t matter if all the stresses looked OK if one of the parts could by hand calculation be shown to deflect so that it was no longer positioned to carry the load. He had made some simplifying assumptions on his model that completely changed the problem. The color stress plots looked OK, they would doubtless have passed a design review. 
You are exactly correct that defining the problem properly is Step 1. A wing precisely designed for certain limit load will not work if that’s the wrong load in actual service.
I recently heard second hand an interesting one involving glue. Apparently early RV3 wings were unpredictably strengthened by glue used (sometimes) to make assembly easier. One wing was plenty strong when tested, and a second wing of the same design not strong enough. I understood that glue between laminated spar caps was providing shear continuity where no glue was part of the documented design. I could be wrong on the details, but it’s an example of the kind of thing that can happen in both a helpful and unhelpful sense. Nature is a mother, and process control doesn’t help if you don’t understand what’s important.
If you think certification is in itself a guarantee of total structural integrity, look up how many people have been killed a result of Bonanza structural failure. My memory is that it’s in the hundreds of people, but I also think they’ve had it under control for the last 30 of 66 years in service. Also look at CAP 10 wing history.
Designing lightweight structures that work reliably (or even machinery) is pretty simple in terms of utilizing the now well established technology. The devil is in the details and I think the details tend to get worked out on a case by case basis, with how soundly depending on the skill and experience of the designers more than the code under which the design is certified. Just my observation.
After reading the Swiss accident report, I wonder if the modern ability to calculate could produce a danger. Older GA designs were over-built. But what is known about the actual forces on light aircraft in turbulence? How was the +3.5 value arrived at? If aircraft rated at +3.5g have in fact been much stronger, then aircraft designed to exactly that (+safety margin) may be unsuitable for severe turbulence.
I have heard that a deliberately badly glued Robin mainspar actually broke at +15g. (They were proving that the poor glue was irrelevant to a crash).
