When i was growing up I remember playing this futuristic flight sim game. You were on a planet and you had to find these caverns and fight bosses I think in them. Sorry its very vague but its been 15 years since I played it.

Could be any game, actually... based on that description, I'm going to say Hellbender, just because it was kind of a big thing back then and lots of people are still tracking down a proper (legal) working full version of it. Do some browsing on Hellbender, if it's not it at least it can juggle your memory, perhaps?

It isn't Descent is it?

http://www.gog.com/game/descent_1_descent_2

Maybe A.I.M ? But that's more recent than 15 years...

Thank you for the suggestion of Hellbender groze because when I googled it the game I was looking for was described being like it. The game was fury 3

I'm glad you found what you were looking for!

People generally look for either Fury 3, Hellbender or Terminal Velocity, when they talk about futuristic flight sims/shooters from that period, so I usually recommend browsing for one of them, as they were all developed by Terminal Reality. Hellbender is actually a sequel to Fury 3.