[RogerL]

I worked a program in the early nineties. We all met the test pilots that would fly the bird. It was abundantly clear (they made it clear) that lives were at stake, and that they wouldn't step inside unless they trusted you and your work. They had the unilateral right to declare the entire program 'no go', and rightly so. This is formalized as a "flight readiness review", but of course you meet the pilots before that. This was a Navy helicopter, not space, I can't speak to what happens at NASA these days, but I can't imagine it's much different.