A lack of planning and engineers working from home caused a major delay for over 700,000 passengers.
A big problem with the air traffic control system caused hundreds of flight cancellations and stranded thousands of people on a busy holiday for air travel.
Nats, the air traffic control provider, had a technical problem while processing a flight plan.
The airlines lost about £100 million on refunds, rebookings, hotels, and food due to the big consequences.
A report found that over 300,000 people had cancellations, 95,000 faced long delays, and another 300,000 had shorter delays.
The investigation found there was no practice for managing such a big incident, even though other sectors regularly do this kind of preparation and planning.
The report mentioned a serious lack of planning and coordination for major events and incidents to fix things.
Nats wouldn’t usually plan maintenance work on public holidays, so engineering staff would usually be available at home.
It took 90 minutes for a Level 2 on-call engineer to get there and do a full system restart, but it took over three hours to call a more experienced engineer after the initial failure.
The flight plan had two locations with the same abbreviations, causing a system error and shutting down to stop sending bad flight data to the air traffic controllers.
Tim Alderslade, chief executive of Airlines UK, said the report showed Nats' basic resilience planning and procedures were not good enough for such an important national infrastructure.
A Nats spokesman said they were already working on making things better.
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