Board Thread:General Discussion/@comment-26257239-20150419194332/@comment-25045164-20150423014129

I advise that you use Non-standard upgrades for higher end planes or special aircraft. If you spend real money, perhaps you can save it for XFA-27 or R-101, the planes located at the far right of the tree. You just can't waste the upgrades on low and middle tiers because higher end planes and special aircraft cost more to level up.