How to make engines be unable to start below set altitude and speed?

If both (height and speed) need to meet these requirements, you can use this program:
if (altitude ASL <= 15000 or (velocity)length < 2 * mach speed) {
Set part engine's part id activated to false;
}
If only one of these requirements needs to be met, you can use "and" instead of "or" in the if block. If you need help doing that, you can send me the craft if you want.
Edit: Forgot to mention before: You need to put all this in a "while (true)" block. Also, I just fixed a small mistake in the program.
Edit 2: Note: This deactivates the engine, so you'll have to manually activate them afterwards. If you want it to be automatically activated, you can add

else {
Set part engine's part id activated to true
}

Am not really sure how that would work, You better asking @TheVizzyLucky. Or someone else.

why don't you just make a AG that toggles them and just turn it off below that?