I used to work on the website for a utility company that insisted on ridiculous uptime. Our major deployments risked taking the system down for a few seconds, though in practice that never happened.

As a result, they made us do deployments starting at 3:00 in the morning since that was the least active time on the site. Fuuuuuuuuck that place.

My brother is an air traffic at one of the world’s busiest international airports and I can assure your their system uptime is no where close to 3 9s.