I love this game concept, there's nothing quite like it. I especially like the commitment to 1998 period realism. After playing for a couple hours I have a couple user interface suggestions for your consideration:

On the title screen it's not immediately obvious what the difference between the game modes is.
Combat Ready - It's not intuitive that this is what other games call Career Mode
Instant Action - Obvious what this is, however the action isn't quite instant when this game mode starts.
Trade Training - This must be what the Royal Air Force calls advanced training?
Basic Training - Self-explanatory

If I pre-emptively launch tankers, the voiceover says "tankers airborne to support the intercept" even though there aren't fighters airborne or an intercept in progress.

On the Flight Information window, I'd like tooltips/hover hints on the U A X S buttons to remind me what each one does. When I haven't played the game in a while, I don't remember what all of them are and could use a reminder.

In the ASMA window scroll goes opposite of what I expect. I look for newest messages at the bottom, not the top, and instinctively scroll up for previous messages.

Maybe not realistic, but a way to display flight history would be helpful. I'd like to be able to compare what a track is supposed to be doing with what I've seen it actually do. This should be a button or function similar to calling ATC, not an always on thing. There are some tracks that look like they're merging into established airways instead of having always been on that airway. This looks suspicious but in the game it isn't. A way to view more of the track history would let me determine if this is a bogey trying to sneak into a normal flight path, or a real scheduled flight that's just a little off course.