A couple of days ago, a Pioneer 300 lost a wing in the approach somewhere in Italy. (
Both occupants lost their lives. Pictures show a broken wooden spar of the ULM, what might indicate fatigue. Plus, the Aircraft seems to have flown many high wingload missions before. Not jumping to conclusions (I think there will be an accident investigation), but please all be careful how you load your wings and how you load your spars. Something that might work for a flight, might have an impact on the fatigue endurance limit of your aircraft. This is true for all aircraft, especially for those which are easy to overload. Don’t jump to conclusions by similarities to other aircraft. Consider your POH for the limits on your aircraft and keep in mind that aircraft are allowed and even expected to break beyond those limits. And with fatigue, it might not even be your skin, you put in jeopardy.
Take care of yourself. It’s hard to lose an acquaintance through something perfectly avoidable.
The manufacturer is not far from there, so hopefully they can help figure out why the structure failed. Fatigue failures are a rare occurrence with wood structures, but sometimes its the attach fittings. Having watched Pioneer 300 display videos its not hard to see how some of the aircraft are used in Italy.
My aircraft was likely stressed hard early in its life, several decades ago, and one example had a similar incident in the ‘80s. I’ve tried to be mindful of that when inspecting the structure.
Wood makes for great structure but behaves differently from Al. One nice thing about wood wings is they don’t have a fatigue life like metal spar wings do, but on the other hand you’ve got issues like delamination (happened to a glider that I own, we found it during its annual inspection – the damage was repairable), and things like compression fractures (which happened to a friend of mine – he taxied into a pole at walking pace, with the wingtip of his Taylorcraft just clipping the pole – hardly a scratch on the paint but it caused a compression fracture on the inboard part of the rear spar which let go in flight – but fortunately the wing didn’t separate or my friend wouldn’t have been able to tell the tale of gingerly flying to the nearest airfield).
Some of the videos on Youtube are of the “Acro” version, which is a purpose designed aerobatic version. I do not know what the accident aircraft was, who built it etc. There are lots of questions and few answers. Sad accident.
Wood is a marvellous structural material and better engineers than I will probably chip in on this, but I too was under the impression that wood does not fatigue, it therefore does not have a life like metal. It is either good or broken, although as described above you can break part of the structure leading to a later failure.
Oh, I am not arguing against wood, on the contrary, I think wood is a prime material to use in aircraft structures. And often, the adhesives are the problem. The fact remains, that certain structures are built to resist certain loads and if these are overstressed, the fatigue endurance limit, even if you use wood. Plus, as I said, Aircraft are expected to break past ultimate load.
Modern adhesives are not a problem in the way that the Pre WW2 adhesives were, most of the problems were solved by DH when they introduced Aerolite.
Most people are now using Aerodux, a glue that has very few problems, there are also some epoxy glues that are used mostly in the homebuilder sector.
The problems with Adhesives are usually related to poor bonding surface preparation or not assembling the parts within the working time for the adhesive.
The problem with wood is that it can be damaged but the damage doesn’t show. There was one incident of a chap who hit his wing on some object on the ground, but due to no obvious damage he flew the aeroplane. Later that flight the wing snapped and he was killed.
A friend of min had a Cessna 140 (I think it was, but anyway it had a wooden spar). The co-owner managed to clip with wingtip on the ground and there was a bit of damage to the skin. They asked the engineer to repair the cosmetic damage but luckily he said “no” and that he needed to check the inside of the wing structure. Good job too as it had suffered spar damage and as such could have suffered from failure in flight.
I think with metal, you often get rippling on the surface of the wing which may indicate an over stressed spar. Of course this is not always true but if you fly a plane with ripples along the wing skin, then I’d be wary, especially if an aerobatic type (I remember by Aero’s FI was very keep to stress this point….so to speak….! when I briefly tried aerobatics).
Alan, I think that’s a very good point and one that I hadn’t thought about before. While wood structures are certainty resistant to fatigue in its classic meaning, they can get over stressed one time and not show it until later. Its tempting to conclude that may have happened in this case, but who knows?
Years ago my dad went flying with a guy in his ratty old Aeronca Champ and they did some mock dog fighting with a Cub. A few months later the Champ was re-covered and they found a long crack in the wood spar. My dad is very capable at structural analysis, and he estimated the spar was good for 2G – which was roughly what they pulled during the flight. Later on some Aeroncas and Citabrias (at least the latter) got replacement metal spars, and perhaps for them it was a good thing. On the other hand some planes have the most amazingly robust looking wooden spars, for instance the One Design aerobatic plane has what looks like beam from the roof of a cathedral running through the cockpit and its hard to imagine it could ever be broken, by anything.
How exactly do you check wooden spars for hidden damage?
There is a particular incident I have in mind, concerning a friend of mine, and given the lack of confidence I have in the competence of whoever inspected it, I have suggested that this pilot doesn’t fly it for as long as possible, allowing others to try to “bend” it first… not exactly a “nice” policy but probably a practical one.
The famous case of the Robin which hit a bale of hay, unreported, and later the wing came off, obviously fatally, comes to mind. But that one was never inspected – not even by a monkey. But had it been inspected, would it have come to light? We will never know.
With metal aircraft, if somebody gave the spar a good bending, you should see at least cracked paint (on the skin joints), loose rivets and probably skin ripples. These are really obvious if you actually look for them. There is a great one of a King Air in Ireland which got well and truly bent in IMC and then landed, without anything being said. I have the pictures somewhere…
Wood does show structural damage, but one has to know what too look for and it can be hidden under paint etc. But it’s even worse with composites – they might not show any signs at all before they fail catastrophically, so I would worry more about that. Wood is a great structural material and especially today when you can combine it with composites. Many composite structures use wood cores – Kaman Helicopters blades, MT Props, Extra Aerobatic aircrafts and many other aerobats.
