Fun little flight sim. Looks like there might be a math error in your physics though. cutting your engine with enough height lets you go infinitely fast backwards.
Edit: Took a look at your code, and I think I've found the issue. Check plane.gd line 94. Your drag calculation squaring your forward velocity before flipping its sign likely means that your velocity is always considered positive for drag purposes, which would mean the faster you're going backwards the more drag you accumulate, which would make you go even faster backwards, compounding the issue.