I read in a recent interview that Clint Eastwood was surprised that

the Soviet Union allowed him to film this film on location in the Soviet Union. This was at the height of the Cold War. It was surprising to Eastwood because of all the anti-Soviet sentiment in the film. The studio even gave the Soviets a copy of the complete script and asked them if there was anything troubling. After two hours of reading the Soviets answered no. They even let them film the Firefox on the ground in the hanger. They still had to use a model for the shots in the air. The studio even asked if they could have the design schematics for the Firefox so they could build a realistic full sized aircraft to use in the snow scenes and the Soviets gave the studio this.