Bolts and screws

Maanantai 3.4.2023 - Erja Reinikainen


Suomeksi

In Arlanda the main parts of the Caravelle were disassembled so that the fuselage, wings and stabilizers could be placed on special trailers before transporting them to Finland. The engine nacelles were also dismantled along with a number of smaller parts and equipment, such as the wing root fairings, inner flaps and equipment from the landing gear wells. This means that thousands of screws, bolts, nuts and rivets were unfastened in Arlanda, and they need to be put back into place when the aircraft is reassembled in Turku in early June. This blog tells about a small but an important part of the Caravelle’s restoration project: about the different kinds of fastening accessories.

Kari Nyman from the Caravelle technical team has been interviewed for this blog.

An additional challenge to the jigsaw puzzle of the fastening accessories is caused by the Caravelle’s special feature: the screws and bolts for the aircraft’s structural parts have metric dimensions, but the majority of them are a special design from the Sud Aviation aircraft factory which don’t follow the standard types and sizes. This can be seen in the aircraft’s technical specifications, which have dozens of pages defining the different “210.-series” fastening accessories. Here the 210. refers to the type of the aircraft. France has been a strong supporter of the metric system and the competition in developing a passenger jet airliner has played a significant part in these choices. The various system equipment in the Caravelle were mainly produced in the United States or Great Britain, so their fastening parts are naturally based on the standards of their country of origin, and having in this case usually US or imperial sizes, i.e. inch based sizes. For example, the hydraulic system pipes, flexible tubes and fittings are based on the American AN standard. This means for the disassembling and reassembling work that the fasteners which may look similar may have metric or imperial dimensions and the tools to be used have to be chosen accordingly.

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Photo by Reino Myllymäki

The disassembling work in Arlanda went surprisingly smoothly. The parts which had to be disassembled could be unfastened without major difficulties or damages. However, some screws had to be drilled out and there was more work needed especially on the underside of the fuselage. There the screws have corroded and rusted more than in average. The most difficult fasteners had to be treated with rust removing chemical and eventually all necessary joints could be opened.

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Photo by Reino Myllymäki

In Arlanda all disassembled fastening accessories were collected into separate plastic bags, based on their origin. Each bag was marked clearly with the information where this batch of screws was from, for example: rudder lifting point cover screws, LH wing innermost leading edge section, horizontal stabilizer, etc.

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Photo by Erja Reinikainen

In Pansio we have a small ultrasonic cleaner where all disassembled fastening accessories will be cleaned. Originally, we aimed to put into the cleaner only a bagful of items at a time, separated as they had been packed in Arlanda. This would have been an ideal procedure to keep track of the origin of each batch of accessories. However, the ultrasonic cleaner proved to be so slow that for time saving reasons several bags of fasteners had to be put into the cleaner’s basket at the same time. This leads to the need to sort these “mixed batches” after cleaning. The “mixed batch” screws, for example, have been sorted by their length for future use. Larger labelled batches of accessories can be cleaned one or two bags at a time and in these cases their origin is known. If the origin is known, after cleaning the accessories are put into new plastic bags which are marked with the location, for example: nose bulkhead screws, LH engine nacelle fairings and supports, LH wing upper side panel, etc.

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When the Caravelle is eventually re-assembled, there will be new and old fastening accessories in use. The aircraft will never fly again, so it will be possible to build some details in a different way than it has been. All joints and seams which will be visible, are fastened using original screws or screws resembling the original ones, if possible. In the locations which won’t remain visible, and which possibly will never be opened again, pop rivets will be used to speed up the re-assembly. There are some fairings, visible ones, which have originally been riveted and now they have been fastened with pop rivets, and the appearance is quite like the original one.

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Photo by Jouko Tarponen

One of the most difficult details in re-assembling the aircraft will be getting the panel under the wing joint back into its place. There the original screws have been mainly drilled out, the original flange nuts have been damaged and the panel metal has corroded.

Will the new and old fasteners look different on the re-assembled Caravelle? The aircraft will hardly be examined so close up that the difference could be seen. The new screws have a yellowish corrosion protection and the old ones are oxidized and darker, so both are different from their surrounding surface. The ultrasonic cleaner doesn’t remove rust or oxidized surface. This means that in painted areas the screw heads will have to be sanded before painting so that the paint sticks on them in the same way as on the surrounding aluminium surface.

We managed mainly to drill out the tricky screws quite nicely, but in the vertical and horizontal stabilizer the drilled holes will have to be repaired during the re-assembly. Cup washers will be placed in the drilled holes to cover the damaged hole edges.

Aircraft fasteners can’t be bought in an ordinary hardware store, nor in a car accessories store. Usually the countersunk screws on aircraft have a different countersunk angle than the ones used in cars. US standard inch-based screws are still available in the same dimensions as the originals and they can be found on the US market. However, the special Sud Aviation’s own screws are difficult to be replaced with new ones resembling the original. For example, the panel covering the main wing joint is a part of the fuselage bulkhead, and it has been fastened using metric French screws with a countersunk angle of 120 degrees. This is different from all other countersunk screws on the aircraft and similar new screws can’t be found.

Ronema Oy has provided valuable help in aquiring inch-dimensioned fasteners. They represent the American company Aircraft Spruce, which imports spare parts and pilot accessories. Through them we have ordered screws from the US.

Naturally one had to ask if such special screws are very expensive when they are imported for the Caravelle. Mainly they are not, we talk about 10-30 (euro) cents per screws. So with a hundred euros you already get quite a lot. The Caravelle technical team has also investigated whether it would be possible to have the metric special screws specially made for the purpose. Yes, it would be possible but we haven’t got the cost information yet.

Re-assembling the Caravelle will be a challenging jigsaw puzzle, but the fastener team is prepared, and they will do their best to foresee all coming details of the hectic assembly week in June.

Photos by Ismo Matinlauri expect if otherwise separately mentioned.

Muokattu 9.4.2023 9.25: Terminology enhanced

Avainsanat: aviation history, restoration, Caravelle, SE-DAF, Sven Viking, OH-LEA, Sinilintu


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