The engine cover and landing gear wheel covers are repaired

Suomeksi

The PIK-21 “Super Chug” OH-XTM is equipped with upper and lower engine covers made of fiberglass, using a mould. The landing gear wheel covers are also constructed from fiberglass. In the aviation accident that occurred in August 2024, the lower engine cover was damaged, though not severely.

Photo: Harri Kaikkonen

Upon examining the damage to the lower engine cover, it was found to have dents and one larger fracture, from which some fragments had also come loose. Both wheel covers had minor dents, while one of them had a somewhat larger damage.

The repair of the engine and wheel covers began by sanding off the paint from the damaged areas to reveal a clean brown fiberglass surface underneath. This step is essential to ensure that the new fiberglass mat adheres properly when repairing the damaged spot. Sanding was carried out manually with sandpaper, but also with a belt sander and a multi-tool fitted with a triangular sanding head. All damaged fiberglass was cut away.

Photo: Reijo Siirtola

On the lower engine cover we sanded the edges of the damaged areas both from the inside and outside, so that several centimetres of clean bonding surface for the new fiberglass mat was exposed on both the inner and outer edges of the damaged area. The same procedure was applied to the edges of the damaged areas on the wheel covers.

Photo: Reijo Siirtola

Once the damaged areas on the lower engine cover and the wheel covers had been sanded to clean fiberglass, their repair was begun with fiberglass laminating. Laminating was carried out using cross-woven fiberglass reinforcement fabric and epoxy resin. We laminated the fiberglass mat with epoxy, as the mat does not absorb water when laminated with epoxy, even if the surface of the laminate becomes damaged. For the laminating, we used Biltema’s two-component “Laminating Epoxy Art. 30-103” epoxy-based resin.

Photo: Reijo Siirtola

On the lower engine cover we started laminating the damaged areas from the outside on the corner where there was a major damage. For this, we made a mould from a plank, and the lower cover was fixed around the mould with clamps onto the workbench. This ensured the cover remained in the correct shape while laminating it from the outside. No stresses are left in the laminated area that could later break. Between the clamp and the mould, we placed yellow plastic tape, to the backside of which the epoxy resin would not adhere. This made it easier to detach the cover after laminating.

Photo: Reijo Siirtola

After laminating the engine cover’s fractured area from the outside, we laminated fiberglass mat onto the corresponding damaged spots on the inside of the lower engine cover. This ensured that the repaired area achieved at least its original strength. In the same manner, we laminated all other damaged areas on the lower engine cover as well as on the wheel covers.

Photo: Meeri Siirtola

After laminating the fiberglass mat, sanding was started. This was done with a flat sander using grit 60. Such a coarse grit can cause deep and difficult-to-repair pits and scratches if used incorrectly, so care must be taken during preliminary sanding. However, sanding with coarse sandpaper levels the materials with different hardness at the repair site, without leaving harder material protruding. Additionally, sanding is quick and the result is already relatively neat.

After the initial sanding, the laminated areas and smaller repairs made without laminating were filled with epoxy-based filler. For this, we used “Biltema Lightweight Epoxy Filler Art. 30-648”, a two-component filler. After filling, we sanded the surfaces with a flat sander, using grit 180. This grit smooths out any unevenness in the epoxy filler as well as any scratches that may have occurred during the initial sanding.

Photo: Reijo Siirtola

After the treatment with Lightweight Epoxy Filler, we moved on to priming. For the priming, we purchased grey “Sparymax 1K AC-Fyller” primer from Pintaväri shop. This spray-applied primer evens out the remaining small irregularities and covers well the mottled appearance of the sanded fiberglass surface. An added benefit is the short time before re-sanding can be done; sanding can be carried out just 15 minutes later.

Once the primer had dried, we sanded the repaired spots by hand, first with sandpaper of grit 360 and finally sanded the entire surface of the lower engine cover and wheel covers using wet sanding with water sandpaper of grit 1000. By doing so, we achieved a smooth and clean surface on the lower engine cover and wheel covers ready for the upcoming surface painting. The surface painting will be performed when the restoration of the aircraft frame has reached the painting stage.

Photos: Lassi karivalo, unless otherwise mentioned

Translation to English: Erja Reinikainen