I love that so many people here think they can think about the problem for 10 seconds and come up with a solution that hasn't already been considered a thousand times and discarded.
Any well-written article should anticipate, ask, and answer the obvious questions. For example: Why not use alphanumeric characters? And if the answer isn't known by the sources the writer quotes, the writer should say so.
Someone hardcoded that field 50 years ago and it's impossible to change the type. It would be easier to just abandon planes altogether, and set up a parallel transportation system.