Originally Posted by
Tourist
they know which is the nearest suitable airfield
What defines the nearest suitable airfield then?
It's not just the nearest airfield that the aircraft can safely land at.
It depends on a whole host of different factors.
In a medical emergency one airport further away may actually be the quicker option for the necessary hospital care.
You're right, humans do make mistakes. Thats what cross-checking is for, and it does a good (not perfect) job of eliminating or identifying as many mistakes as possible.
But a computer is programmed not by a computer but by a human. They can make mistakes too.