ericmunkSadly the Indian Air Force Hawk formation team have had a mid-air today. One dead, two ejected safely.
During a display the aircraft flying inverted in mirror formation above the other appeared to descend into the lower aircraft’s tail (or the lower one climbed, or both), causing that the lower to nose up into the inverted upper aircraft. Too close indeed.
ericmunkJohn Green wrote: Coefficients of expansion and contraction differ with the type of timber and the influence of temperature and humidity. Any adhesive used for joint bonding on an aircraft should possess – among other characteristics – a gap filling and elasticity function. If some of the earlier adhesives tended to lack these qualities, then maybe a repetitive cycle of expansion and contraction could account for the failure of joint bonded elements critical to the integrity of the airframe.
You are so right. Problem is that Kaurit W with the bakelite filler added has a joint filling capacity of up to 2.5 mm. It wouldn’t be certified if it had no gap-filling properties… And when applied correctly it is absorbed by the wooden contact surfaces, and the resulting fully cured joint is quite strong and elastic. So in theory quite suitable. However, the process of storing, mixing, applying, clamping and curing the glue joints is somewhat complex when compared to say Aerodux. And with it it is prone to errors, which may only now show when the glue fails with age. Worrying, but with the correct inspection techniques it is not a problem to catch this in time. Problem is that for a wide range of aircraft there is no mandatory inspection program, and many owners/LAMEs are not aware of it either. I understand some TC-holders in Europe are working on an inspection program, but that will not catch all those experimental aircraft…
ericmunk@Powerandpassion: Beetle Cement is the British trade name for Kaurit WHK glue. So material specifications should match between the two. They were both initially known as Klemm-Leim, after the inventor of it, Hanns Klemm. His 1937 dissertation on this glue is here: https://hannsklemm.files.wordpress.c…21-10-1937.pdf.
Re Beetle glue AP.2662A (http://vhjet.com/wp-content/uploads/2017/08/AP-2662A-Standard-Repairs-for-Airframes-OCR.pdf) is helpful, and contains further references.
ericmunkThe 90 Squadron ORB would be a start for research. The ORB is in the Kew archives.
Reason I asked about the individual aircraft is that there’s a fair amount of local sources available usually. Take this one, https://db.wingstovictory.nl/pdf/514-story.pdf. The library in Middelburg also has an extensive collection on wartime losses, https://opac.zebi.nl/vubis/PDF/32551058X.pdf.
The Lancaster was part of the first wave of an attack on batteries in Vlissingen, in support of the Allied ground campaign to conquer the righthand bank of the Schelde river to secure the supply port of Antwerp. It shot down by an AA (Flak) battery in Vlissingen, coming down just off the Buitenhaven (‘Outer Harbour’) in the Westerschelde, around 10:02 LT. The source above also contains detailed information on the fate of those onboard.
ericmunkWould help to know which one.
is this the loss you mean? https://www.verliesregister.studiegroepluchtoorlog.nl/rs.php?aircraft=&sglo=T4581&date=&location=&pn=&unit=&name=&cemetry=&airforce=&target=&area=&airfield=
ericmunkRegarding PVA glues, as fsr as I know these are not certified for aviation use. They are certainly not approved for the types I worked on. Apart from that, I would imagine that they are not water resistant and susceptible to moulding…
ericmunk(Would be great if you can share the results of the spar tests when they are available!)
ericmunkI 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.
ericmunkis also very nice reading material.
ericmunkGood 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…
ericmunkCasein 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…
ericmunkVery 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
ericmunk18 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…
ericmunkAnd it’s not the first KLM one either. A 1978 example went to the Aviodrome museum at Lelystad in 2004. Another 400-series – or rather the front half of the fuselage – is sitting across the street from here in a school… Hardly preservation, but good use for a tired airframe!
ericmunkIn the Cairns area, go to Sid Beck’s museum in Mareeba.
Melbourne: I would suggest the RAAF Museum at Point Cook, the Australian National Aviation Museum at Moorabbin, the B-24 at Werribee.
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