Took 26 Days until 6000 feet. It's easy. Once your fully upgraded (or close to it) just make sure your resistance is 3% during flight and KEEP it there, then keep tapping turbo to maintain a speed of 150 mph. I would tap mine every second or so. By the time you run out of turbo you should be almost to 5000 feet so the rest is smooth sailing. :) I get nearly 7k every time now.
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