I thought the game idea turned great! What I didn't like was the slow part of the game, I could see that you tried to tackle this problem with adding the SHIFT "frame-skip", I'm used to playing games where the main objective is upgrade, but I think that getting the initial upgrade takes too much time, if you take a look on "Learn to fly" it could give you some ideas
Other thing, the first time I lost the game I didn't saw that it was because of gas, I recommend showing an alert the first time you're running out of gas, and I think the Hud is a bit too hidden