As stated by Maoraigh the UK procedures by the LAA, and incidentaly the BMAA, during build and in subsequent annual inspections is thorough and has proved to be very effective. I can recall though an accident, said to be caused by sub standard materials supplied by the kit manufactuer, report below
http://www.sust.admin.ch/pdfs/AV-berichte/2053_e.pdf local copy
This caused the withdrawal of the Permits To Fly all aircraft of this type in the UK, and I expect elsewhere, untill all the wing spars had been examined. The results were quite eye opening with a reported nearly 40 per cent found to not meet standard that had been declared by the supplier.
While this is the only case like it that I can recall it goes to show that national measures to ensure safety while generaly effective, cannot cover all eventualities
That 40% does not suprise me at all.
I consider myself an “engineer” (college univ educated on electrical, electronic and mechanical engineering, with a lifelong interest in the stuff) and most of the “engineering” I see on a quick glance around the hangars at Friedrichshafen does not inspire me with confidence.
I am Czech myself and admire the long-history Czech aviation culture but…
Piper had a go with a Czech company a few years ago, exiting the deal abruptly. Different stories surfaced for the break-up, with some from a firm in the vicinity saying how CZAW got a load of locals in to “fill up the factory” during Piper’s visits. To me, such a comical description is, ahem, quite in line with expectations from an ex communist area.
Most certified GA types are significantly over-engineered on the basic structures. The problem with the minimally engineered structures common in the “lightweight” types is that the job becomes very dependent on the correct materials being used (source traceability), the correct heat treatment (source traceability if you are buying high tensile steel tube), correct welding (presumably they are not re-heat-treating the entire frame after welding), very good QA (source traceability, and everything afterwards), ongoing protection from corrosion, and regular inspections.
The UK and US ‘homebuilt’ categories are very different, the UK homebuilts being much less experimental than the US system allows. If I were a professional engineer like your father, I suspect I would find the UK system frustrating. As a reasonably technically proficient pilot with no specialist I find the oversight in the UK reassuring – particularly when it comes to purchasing an aircraft that someone else has built – not that I wouldn’t exercise due diligence, of course.
I think due diligence is the key no matter where you are, and if you want a proven design really not a lot of it is is required. Just buy an RV: they are the most produced aircraft design in GA now.
The benefit of UK style quasi-certification of homebuilts is in protecting fools from themselves, not the reasonably diligent who will have no problem. Community between home builders can achieve the same end. Regardless, I think the (vast) benefits that have come out of uncertified experimental category development have proven much more important, and in fact irreplaceably vital to the current survival of GA…. as well as being a lot of fun.
Peter – structures are not as critical as you have described. If they were everything in life would break all the time. I say that as a guy who runs R&D funded machinery design for a living, with the usual qualifications. Aircraft technology has been developed over a long period of time and things like 4130 welded tube fuselages are quite fault tolerant (and no you don’t have heat treat the whole thing, the welds strain harden in service if needed, 1920s welding technology) That’s a good thing for us: I have in my hangar a set of spare wings for one of my certified aircraft, stripped of fabric. If you saw the generally poor quality (extra holes redrilled here and there, for instance) I think you’d be surprised. Kit built stuff is a lot better.
The description of Czech ‘issues’ does make me smile. I think you have to look at anything you’re going to buy or fly and make your own judgements. And after doing so there will always be some level risk, it doesn’t matter how the thing was ‘certified’ and by whom. I just try to beat back each of the risks that is going to kill me eventually to a level where I die of natural causes first.
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).
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.
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.
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.
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).
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.
A tubular frame is what made the Hurricane much better able to withstand battle damage than a Spitfire.
In case anyone’s interested!
