To have one air traffic control failure in a fortnight may be regarded as a misfortune, to have two looks like carelessness. The UK's air traffic services provider is coming in for a wave of criticism at the moment, so where is it all leading?
The incident at the National Air Traffic Services (Nats) main control centre in Swanwick earlier this month was extremely serious. It caused more than 2,000 flight cancellations over two days, affecting hundreds of thousands of passengers. Some were left sleeping on airport floors, others were stranded in foreign airports.
Monday's failure at the Prestwick centre, which looks after air traffic in Scotland, northern England and northern Ireland was not as dramatic. But according to the aviation analytics company Cirium, it still led to more than 200 flights being cancelled and many other delays affecting tens of thousands of travellers.
The first failure this month was attributed to a previously undetected software flaw, which had an impact due to a very specific set of circumstances combining in a millisecond. Today's issue was blamed on an unspecified "connectivity issue" specific to the system used in Scotland. The two, Nats says, were unrelated.
But this is becoming worryingly familiar territory.
In August 2023, on a bank holiday weekend, a problem with a single aircraft's flight plan caused a key system at Swanwick to break down, leading to cancellations and delays that affected 700,000 passengers.
Two years later, a "radar-related issue" at disrupted air traffic for four hours, and led to the cancellation of 150 flights.
The question is whether all of these incidents were isolated and unavoidable, or whether they point to something more systemic.
There is no question that the job Nats has to do is fiendishly complicated, looking after some of the busiest airspace in Europe - including the approaches to Heathrow and Gatwick - which means handling up to 8,000 flights a day.
Visiting the Swanwick centre, it's hard not to be impressed by the sheer scale of the place, where hundreds of staff sit behind an array of large radar screens, calmly making sense of what looks to the untrained eye like utter chaos. But these controllers need reliable equipment, and the concern is that they aren't getting it.
Steve Heapy, chief executive of Jet2 made the point: "It is simply not good enough for this critical national infrastructure to keep letting customers down."
Easyjet said the latest incident "calls into question the resilience of Nats' systems and demonstrates the need for firm actions to prevent these repeated failures."
A Nats spokesperson, apologising for the latest issue, said technology "can and sometimes does fail. Our job is minimising disruption where possible."
They continued: "What isn't acceptable is any incident where safety is compromised - this is always our first priority."
Sources say that behind the scenes, airlines are "furious" that disruption keeps on happening, and that they have to bear the costs of refunding passengers and looking after those who are affected by delays. There are deep concerns that not enough resources are being put into ensuring the air traffic control system has the resilience it needs.
Some have gone further. Ryanair, for example, has repeatedly called for the Nats chief executive Martin Rolfe to resign, or failing that for the Transport Secretary Heidi Alexander to remove him from his post.
But although Alexander has described the disruption in early September as "completely unacceptable", there seems little appetite to force through a change of leadership at Nats.
The government has asked the Civil Aviation Authority (CAA) to carry out an independent review of the first incident from this month, which is expected to take six months to complete.
Action is unlikely before the CAA's report appears, and today's events do not appear to have changed that.
Meanwhile, Rolfe made it clear on Friday that he has no plans to resign. He emphasised that in systems of such complexity, obscure faults are extremely hard to prevent.
"We spent so much time making sure they work that anything that does happen of this nature is, by its very nature, going to be incredibly complicated and probably difficult to understand at the time," he said.
Nevertheless, it is clear that impatience with the Nats failures is growing, both among the airlines that pay for air traffic services and among passengers who bear the brunt when systems do not work as they should.
