@nxobject I meant litterally. The ATC system is derived from the IBM 9020 which was itself a set of interconnected System 360 systems: https://en.wikipedia.org/wiki/IBM_9020

I believe it's all virtualized today

Microsoft 363

That's a little unfair to IBM – sure, you're just as beholden to them for life, but at least the service engineer gets to your rack before you know something's gone wrong!

That explains the outage. We're just in one of the 5 days of the year they don't cover...

I did a tour of Adelaide airport with Engineers Australia in the 1990s. We visited their obsolete IBM main frame, still running ATC in their computer room. They were tied to it by the certification of the software it was running. Just as you described.

In naming the platform, IBM built in some expected downtime.