"Combat Aces" came out in 2001.  70 missions including night-time, 26 planes-yes, you can fly dirigibles too, new scenery including a choice of summer or winter landscapes.  There was a patch issued too, to fix a collision bug.

I don't know if the add-on is still available, although I have it.  One of the great things about it is you can match up WW2 Zeros and Corsairs against WWI Fokkers and other planes.  Cross-era battles, if you want. I've done it many times. You'd be surprised at the outcome!

Go here, if you don't believe me. It's a review:

   http://www.netwings.org/pages/default.asp?page=review/combataces1.htm

I don't know if the game is still available anywhere.