Air traffic control chaos that struck British airlines last summer was made worse by delays in verifying a password for an engineer working from home, an inquiry has found.
The Civil Aviation Authority (CAA) carried out an investigation into the 2023 disruption - when thousands of passengers were left stranded abroad, and had their flights severely delayed or cancelled.
It took place on a Bank Holiday Monday, one of the busiest days of the year for flights, and caused airlines to lose around £100m in refunds, rebookings, hotel rooms and refreshments
Widespread disruption broke out when air traffic control provider National Air Traffic Services (NATS) suffered a technical glitch while processing a flight plan.
In their final report into last year's incident, published on Thursday, the CAA found a Level 2 engineer was working remotely rather than on-site at NATS' headquarters in Swanwick, Hampshire, on 28 August.
As soon as automatic flight planning systems failed at 8.32am, a junior Level 1 engineer working on-site began checks.
The Level 2 engineer was contacted 34 minutes later, but the report said their password login details "could not be readily verified due to the architecture of the system".
It was then agreed that the senior engineer would go to the control centre, but it took another hour and 30 minutes for them to arrive.
By that time, it had been three hours and 15 minutes since the incident began.
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The CAA also noted that support from Frequentis Comsoft, which manufactured the automatic flight planning system, was "not sought for more than four hours after the initial event".
The firm found a solution to the glitch within 30 minutes of being contacted.
The regulator recommended that NATS consider rostering a Level 2 engineer on site during busy periods such as the summer, which they accepted would be a "significant" expense.
However, the CAA added the cost should be viewed in "the context" of the overall cost to the industry and passengers from the failure.
Jeff Halliwell, chair of the CAA's Independent Review Panel, said the incident "represented a major failure on the part of the air traffic control system".
The regulator's chief executive Rob Bishton added: "It is vital that we learn the lessons from any major incident such as this."
A previous CAA report estimated more than 300,000 people suffered cancellations as a result of the glitch.
Approximately 95,000 endured delays of over three hours, and at least a further 300,000 were hit by shorter delay, the regulator added.
(c) Sky News 2024: Air traffic control chaos that left thousands stranded made worse by WFH enginee
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