The air traffic control meltdown that left thousands stranded in airports across Ireland and across Europe last week has been called a "one in 15 million" event.
The UK's National Air Traffic Services (NATS) says its system shut down after software failed to process a flight plan properly.
NATS says it's permanently changing the system to stop it ever happening again.
The bug was triggered by two waymarkers on the route with identical names.
It resulted in flight cancellations and delays, with the average number of flights being handled per hour dropping from 400 to 60.
NATS says the issue led to systems being unable to process routes automatically for several hours.
Martin Rolfe, the chief executive of NATS, says it won't happen again.
"Even though we haven't completed every single step yet, because we are still performing that really critical testing, the problem has been isolated and we know how to deal with it, and it will not reoccur in a way that causes disruption to the travelling public in the future".
