The thing is, it just sort of happens. It's not some massive effort to remain calm, but rather you just automatically focus. That's how people have described it, and it's been my own experience. I've never experienced anything close to this bad, but I've had a couple of aeronautical close scrapes and there's just no room for fear.
Edit: had the same experience in a car once as well. Total focus on solving the problem. Terror comes after.
Edit: had the same experience in a car once as well. Total focus on solving the problem. Terror comes after.