We don’t. We built wind tunnel models, we taxi them around at higher and higher speeds, we put them in machines that wiggle the wings at high loads. The first flight is a little hop and then right back down. Months later there might be a big ceremony with VIPs where the new plane takes a lap around the airport as it’s “first flight”.

And there are mistakes song the way, giant ones that add years to the schedule and tiny ones that engineers argue over even telling their boss about.

Sure there’s planning and experience, just like she knew to use epoll and not select, and that Linux can handle thousands of threads per process. But there’s no magic to it, just lots and lots of human attention and testing along the way.