Well, to determine if JL 50 is a real story, we need to look at its source, the details presented, and any available background information. But as of now, with just the name, it's impossible to tell.
The 'jl 50' real story remains a mystery without further details. It could potentially be a story in a specific community or field. For example, in a small town, 'jl' could be a family name and '50' might be related to a family anniversary or a particular event that happened 50 years ago within the family. It could also be related to a business where 'jl' is the initials of the company and '50' is a significant number for the company's history, like the 50th product launched or 50th employee hired.
Madeline Beck is indeed fictional. Usually, such characters are invented to drive the plot and engage the audience in a made-up narrative. In this case, she exists only within the realm of fiction.
I'm not sure specifically which 'jl 50 plane' you are referring to. There could be many planes with various designations. It might be a very specific model within a certain region or a project that is not widely known. Maybe it's a military - related plane or an experimental one.