Heads up: Kaurit glue deterioration in wooden aircraft

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14 years 11 months

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Thought I'd post a heads-up here for those of you who happen to own or operate a wooden aircraft. Whilst glue joint integrity in ageing wooden aircraft is always an issue to be aware of, there has been a spate of cases more recently where one particular glue has been the culprit. Kaurit, also known as Klemm-Leim or Beetle-Cement. This late 1930s synthetic ureaformaldehyde resin glue was used widely in the 1930s through to the 1960s. In particular in a whole range of German glider types, continental homebuilts and French wooden touring aircraft. Others types however have not been ruled out. Its British counterpart Aerolite seems to age better than Kaurit.

For those of you sceptical of what the problem is with Kaurit, some videos online: https://www.youtube.com/channel/UC3coEbYm2HRprIc3Xf3JfPA and https://www.youtube.com/watch?v=JPCj_yY58WU&feature=youtu.be

The British Gliding Association has quite a neat inspection program that is mandatory for wooden Schleicher gliders, and gives a lot of useful information on what to look for. https://members.gliding.co.uk/wp-content/uploads/sites/3/2015/04/042-07-2004-issue-6-16th-March-2017-r3.pdf

Now, having Kaurit glue joints in your aircraft does not mean it is bad or unsafe. It does mean that glue joint integrity needs to be checked to ensure continued airworthiness. Sometimes failing joints can be quite tricky to spot, not the least because the often used Kaurit WHK has a bakelite filler that makes it look suprisingly like Aerodux (a very good glue). The reddish die used in the acid hardener of Kaurit tends to colour a joint pink in the same way Aerolite is also pink. And the white bakelite filler sometimes used in WHK makes it look suspiciously like casein... Get somebody to look at it who is familiar with the way these glues look and behave over time...

More information on the Kaurit type glues in the excellent Hans Jacobs book 'Werkstattpraxis für den Bau von Gleit- und Segelflugzeugen'.

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12 years 11 months

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Very useful info. Thank you.

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14 years 11 months

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Very useful info. Thank you.

Most welcome John. Can't believe that such a widespread and wellknown problem has not triggered an AD yet...

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12 years

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I really hope the german LBA will not disable the whole Schleicher and Scheibe fleet from the sixties. You can imagine what happens if they are waking up.

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14 years 11 months

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I really hope the german LBA will not disable the whole Schleicher and Scheibe fleet from the sixties. You can imagine what happens if they are waking up.

LBA does not have that authority. EASA now. They are Part M aircraft. Regardless, I would like an AD that mandates glue joint inspections. The problem is serious, is widespread and will not go away on its own...

Anyway, not all were built with Kaurit. So an AD is unlikely to permanently ground a type.

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12 years

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You won´t believe it, but I am flying my SF27m for several years now. I have no idea if she is built with Kaurit.

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14 years 11 months

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Might be worth contacting the tc-holder to ask what glue they used. They usually are pretty quick to respond. From what I have seen up to now, Scheibe is one of the manufacturers thas has had little or no problems... Great glider BTW, an SF27M.

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18 months down the road. I have since done structural surveys on 11 aircraft glued with Kaurit-WHK glue ('Klemmleim'). Results below:
- 1 aircraft with delaminated rudder spar web only. Rudder changed, flying again.
- 1 aircraft with desintegrated rudder spar, failing ribs in rudder, delaminated elevator spar web, failing ribs in elevator. Controls rebuilt, aircraft flying again.
- 3 aircraft with extensive delamination of wing ribs, trailing edges, aileron skins, both requiring extensive ground-up restoration. One is flying again, the other two will be.
- 1 aircraft with accident damage to a wing, which revealed extensive glue failure throughout the wing structure, and additional glue joint failure in the aft fuselage. Wings scrapped, fuselage repaired, now flying with new set of wings.
- 1 aircraft with glue failure in the vertical fin, this is awaiting restoration
- 2 aircraft checked out OK without issues
- 2 aircraft with extensive glue joint failure in wings, elevators, rudder. These were both scrapped.

9 out of 11 inspected had structural integrity issues. All of them were flying beforehand. Apart from the two scrapped examples, all were in well-maintained condition. Shows the extent of the problems with this widely used glue...

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Eric,

Thank you for the results of those rather worrying examinations. If, as I imagine, these a/c were all hangared, do you know the particular conditions in which these a/c were stored ?

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11 years 8 months

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A worrying subject but a fantastic, useful and generous post.

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14 years 11 months

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Very happy to share the information.

John: storage sometimes does play a role. One of the aircraft had a documented three-year poor storage period. However, two of the worst affected aircraft regarding wing glue joint deterioration were aircraft built by the manufacturer (not from kit), with no damage history, low hours, well-stored and well-maintained, always indoors. The problem seems to be either in the glue quality itself, or in the way individual batches were mixed and used, combined with ageing (brittleness) and occasionally poor storage or poor repairs.

More videos here: https://www.youtube.com/channel/UC3c...HRprIc3Xf3JfPA

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24 years 2 months

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As far as I know, the oldest gliders (pre-war and immediate post-war) were scrapped due to this in the 1960-ies but that glue may have been of a different type.

It also affected wooden aircraft, like the Fw 44 Stieglitz. Those still flying have been rebuilt using different glues.

Isn't "Kaurit" a filler but the glue is "Kasein"?

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Casein and Kaurit are substantially different glues. Casein is formulated from casein, water, hydrated lime and sodium hydroxide. A very strong glue that holds up very well when ageing, provided the wooden structure is kept dry at all times. With water ingress, moulding of the glue is an issue (causing mass scrappings in the 60s). Kaurit-WHK (also Klemmleim) is an ureaformaldehyde resin glue, synthetic. Kaurit-WHK is often used with a bakelite powder filler, casein is not. Apart from the differences in origin (biological versus synthetic), methods of application and clamping, as well as shelf life and pot life are wildly different between casein and Kaurit-WHK... Recent research shows that UF-glues are much more prone to ageing effects than casein, provided casein glue structures have been kept away from the elements. Strangely enough Aerolite, which is also a UF-glue, ages completely different to Kaurit-WHK...

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Eric, thanks for the clarification!

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Eric,

Do you think that storage with the convenience of temperature and humidity would have delayed or prevented degradation ?

Having been imprinted at an early age with the caveat: - among others, "Don't use a chemical solution to solve a mechanical problem", I became interested enough to build and repair wood boats in the size range 20 - 70ft. The odd aircraft inserted itself at regular intervals.

Some forty or more years ago, the principal tools in use were Cascamite, Aerodux and Aerolite and in the marine environment I can't recall a single failure, admittedly within a contracted timescale.eg. if a mistake had been made resulting in a partial or total failure, we'd long departed the scene !

I do recall that at critical points in the boat skeleton, accumulative weight not being a consideration, half, butt, scarf and tenon joints would be supported by the use of either galvanised or aluminium metal flitch plates of varying shapes. The idea being that if the glue failed, the joint would still maintain at least some integrity until the next inspection - if ever.

Which brings me neatly to my next point : Inspection. I've never yet seen any light aircraft which to my eye featured an adequate number of inspection ports giving ready access to all parts of the interior. Even allowing for the use of extended inspection tools, as far as I am aware, it is not possible to examine the interior of most GA a/c without creating significant additional reinstatement work.

It seems that over the years, there was some questioning of the use in the marine environment of the glues mentioned above. Notwithstanding, urea formaldehyde is still a favourite. Resorcinols are top of the list being waterproof and unaffected by UV, tho' Aerodux and Cascophen are temperature sensitive when curing. Epoxies are well up the list tho' water resistant not waterproof and liable to degrade when exposed to UV.

I've always thought that thorough maintenance of wood aircraft requires more frequent intrusive internal inspection. I don't know whether there is any clear statistical information that supports.

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Ex Brat,

Many thanks for that suggestion. I will follow it up. I have previously read largely anecdotal reports concerning problems applicable to the glued joints on Mosquito's used in the tropics.

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14 years 11 months

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Good to see you know your glues John!

It is always good to store wooden aircraft in an environment which has a constant temperature and humidity. Some of the aircraft were actually stored under cover in a warm dry place for most of the 50+ years they have been around, yet still they had issues. I have worked with a fair number of glues in structures (casein, Aerodux 185, Araldit, Aerolite, and the likes), but have never come across a problem so consistent as age-related glue-joint failure in Kaurit-WHK. It is not a new problem; I have seen references to it in 1960s research and technical literature. It was indeed the reason some national authorities put a moratorium in Kaurit-WHK for new-built aircraft, with manufacturers using Aerolite, Araldit and Aerodux instead, in the early 1960s already.

With regards to ageing, there is no ureaformaldehyde glue that ages gracefully. Kaurit-WHK can be very strong after decades, but I have seen some shocking failures, as mentioned above. The glue just becomes brittle and turns to powder on the touch. Aerolite, another UF-glue, does not behave that way, but tends to glass out and becomes prone to microcracking following peak loads (such as heavy landings). Aerolite however holds up much much better than Kaurit. My all-time favourite is still Aerodux. Waterproof, practically uv-resistant,easy to use. Just takes some care to only use it on wood with the correct moisture content, and above a certain temperature. I use it for almost all repairs - except to repair in casein-glued aircraft as casein residue and Aerodux don't go well together.

Re inspection holes: couldn't agree more. The problems in Kaurit-WHK glued structures are not detectable using for instance an endoscope or mirror. They require applying gentle force to structure (gussets, ribs, ply joints), which requires access holes to be cut. A more permanent access would be nice...

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14 years 1 month

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Curiosity I suppose, but what if any are the merits of using PVA adhesives? They seem to be good for domestic use.

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Thanks Eric, that’s a really interesting read. I’ve repaired two, scrapped two and have one currently on the go, which will hopefully fly at the end of March.

We’ve undertaken a number of tensile coupon tests on glue scarf coupons taken from a spar of a wing wiith significant trailing edge glue failure and this looks quite encouraging that the spars are safe. Hopefully the results will be published shortly.

I’ve had some correspondence with a long time American inspector who believes the basic problem was in the management of the kaurit at the factory. ie When mixed, it has a pot life but only when kept in an air tight container. If not stored in an air tight container, a skin would form on the surface and it was common practice to stir this back into the liquid. Although an instruction was produced not to do this, it was not particularly well respected.

The whole issue certainly has a strange randomness.

As for interior inspections, I’m now using a small CCD camera with a built in light, attached to the end of a fishing pole. The camera has a long lead hooked up to a laptop. It works a charm.

I’m also an Aerodux fan but recently I’ve done a bit of work with West Epoxy on a US home build , and it’s very impressive.

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14 years 11 months

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I think your American friend may be right. I too have long suspected the glue skin issue, it is indeed mentioned in the manual. Problem is that if you stir the skin back in, glue joints appear really solid. But not maybe so 50 years down the road as they degrade chemically. The D-noses and spars seem to have been built with much more care regarding glue handling. I have rarely come across problems with Kaurit-WHK joints in D-noses (and then only in ribs) of wings. Never in spars of wings.

Regarding ribs, the issue may well be in the way they were produced. Schleicher had several local contractors make these, based on prices per rib. This of course promotes working too fast by sometimes unskilled labour not used to working with synthetic glues.