DC-3 ground power unit to be restored by the Tuesday Club

Suomeksi

Aviation Museum Society Finland received a C-28 ground power unit for a Douglas DC-3 from the South Karelia Aviation Museum in Lappeenranta. The unit originally belonged to Finnair. Its destination will be the display area of the Caravelle III “Bluebird” in Turku, where it will be delivered after restoration. The fuselage of Douglas C-47/DC-3, former Finnish Air Force DO-5, is displayed next to the Caravelle, making the ground power unit a fitting addition. The unit will be externally restored to its original appearance. At the same time, we will investigate whether it can also be returned to working order.

The ground power unit arrived from Lappeenranta at our workshop on Puusepäntie on a car trailer. A group of us gathered around to inspect the device. After examining it for a while, we unloaded it from the trailer and pushed it into the Puusepäntie workshop.

In the workshop, we examined the unit more closely. Structurally, it is in quite good condition. However, there is extensive rust on its painted surfaces. The main work involved in restoring this DC-3 ground power unit will be removing rust from its components and repainting them in the blue “Finnair colour.”

Restoration of the ground power unit began by removing the components that could reasonably be disassembled for cleaning. First, the thick twin cable leading from the unit to the aircraft was removed. Next, we removed the top cover and the side panels protecting the sides of the unit.

The top cover was secured with quick-release screws and came off easily. Attached to its surface is a luggage rack made of wooden slats and fitted with a metal railing. The cable leading from the ground power unit to the aircraft was coiled and stored on this rack. The wooden rack was detached from the metal top cover so that the cover could be cleaned of rust and the rack dismantled for painting.

Removing the wooden rack was difficult because its fastening screws had seized. The screws were treated with rust remover and heat. Eventually, all but two were removed; the remaining two had to be drilled out. The top cover was now ready for rust removal, and the wooden rack was ready for restoration.

The wooden rack on top of the ground power unit’s top cover was dismantled. We found that its slats were made of very close-grained wood and were still sound. It therefore makes sense to restore them rather than build a new rack. The wooden supports underneath the rack, however, are so rotten that they will have to be replaced.

The old paint was scraped from the wooden slats. Their surfaces were then sanded with a random orbital sander. The slats are now ready for painting. The rack’s metal frame was thoroughly cleaned of rust and paint and is also ready to be varnished and painted.

With the top cover removed, we were able to look inside the ground power unit. It revealed electrical technology from the 1950s, which looked quite unfamiliar to us. Nothing appeared to be visibly broken, so we connected the unit to a three-phase power outlet. This caused the residual-current device to trip, meaning that the unit is not operational. We will continue trying to identify the fault, but because we have been unable to find a wiring diagram for the ground power unit to guide the investigation, the unit will probably remain non-functional.

The protective panels on both sides of the unit were fastened to its frame with screws. Removing them caused no difficulty either and the side panels were soon ready for cleaning.

Once the sides of the ground power unit had been opened, its wheels were fully exposed. They are very likely motorcycle wheels. One tyre still held pressure, but the other was flat. We inflated the flat tyre, but it did not retain the air properly. Either the inner tube has a small puncture or the valve is leaking. Both wheels were removed for cleaning. Behind them, a completely rust-covered wheel housing was revealed, also requiring cleaning. The tyres were removed to make it easier to clean the wheel rims and spokes.

The wheel rims were cleaned with a nylon brush attached to a drill. The brown paint on the rims is still in good condition, and they do not need repainting. After cleaning, it will be sufficient to treat them with clear Isotrol varnish. The wheel spokes were cleaned and sanded using both an abrasive pad and steel wool.

We began removing rust from the side panels and top cover of the ground power unit. Their surfaces were cleaned with various rotary brushes attached to a power driver, as well as with random orbital sanders. The panels were not sanded down to bare metal throughout. For the forthcoming paintwork, it is sufficient to remove the rust and any loose paint. Flaking paint was scraped away with scrapers and the tip of a knife, while paint that was still firmly attached was merely roughened on the surface.

After cleaning the top cover and side panels, we turned to the surfaces of the ground power unit’s main structure. As with the protective panels, the rusted areas were cleaned, and flaking paint was scraped off. The surfaces of the ground power unit itself are now also ready for painting.

The cleaned and sanded surfaces of the ground power unit and its components will first be treated with clear Isotrol varnish and then painted with blue Isotrol paint matching the unit’s original shade.

Photos: Lassi karivalo

Translation to English: Erja Reinikainen