Factual error: In the scene where Bobby and Mike go into the comic book store, you can clearly see a Transformers comic on the shelf. But the movie is set in the late 1960's/early 1970's....and Transformers did not come out until the 1980's.
Factual error: The walkie-talkies the kids use at the end of the film would not make the squealing noise they do. The walkie-talkies they have transmit on high enough frequencies that this would not happen. Mediumwave and Shortwave have this effect, not walkie-talkies.
Answer: The script's ending was ultimately changed by director Richard Donner from what writer David Evans had originally written. In the original ending, it was made clear that the Radio Flyer had actually worked and it was seen hanging (actually floating) next to the Wright Brother's plane in the Smithsonian. Donner instead wanted the ending to be a "Rorschach Test", meaning the viewer sees the ending as he or she wants. So if you perceive it as working and Bobby being free or you see it as Bobby committing suicide, etc, it's meant as a reflection on you. However, Evans has said that he never intended the ending to be, or allude to, suicide, but because of the script changes, he understands how people could make that conclusion.
Bishop73
I have watched this movie 3x in the last 2 days. It has emotionally exhausting and I have done more research on this film, the actors, the original intent for the films direction by David, I mean you would think I'm nuts! But after paying close attention, 28min into the movie, the boys discuss exploring the hill where Fisher jumped off one day. The look and sound of Bobby's voice when he said "he died, no-one can make that jump" is what leads me to believe he was telling us the viewers.
His fate. I think Mike tried to help him, and describes the idea as a "dangerous" one. In their attempt to fly, things of course go wrong and Bobby dies during the crash. However mike in his young mind, created the story of Bobby actually getting away. A grown up mike seems sad, depressed and a little off. He still struggles with his trauma.