The model aircraft used in the series is designed and constructed by famous Swedish model aircraft designer Bo Gårdstad. It's called Josefin (girls name Josephine). Three exact copies were built for reliability reasons. The scenes were shot during two weeks in the summer of 1985 with Bo and his son Mats piloting the aircraft. (About 30 liters of methanol fuel was used up during training and filming).
Fun fact : During a scene the plane lands on a narrow country road between high pine trees. Villagers in the neighborhood called the local air traffic control tower reporting UFOs...