Issues for airlines that operate within the UK — or between the UK and another country — will have more to worry about as the countdown clock on the stopgap agreements begins.
Because the UK participates in the European market, UK carriers are allowed to fly between any points within the EU. That means that an airline like EasyJet, which is based in the UK, is allowed to operate flights between other EU countries — for example, between Paris and Berlin, or Madrid and Milan.
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Although EU rules stipulate that airlines flying within the European bloc must be at least 50% European investor-owned, the fact that the UK is in the EU market allowed British-owned and -based carriers to operate within it too.
That wouldn't be the case anymore once Brexit happens.
Because of EU ownership rules, combined with a complicated set of rules known as the "freedoms of the air," which date back to 1944, and various air-services agreements, an airline would not be allowed to fly between two countries that are not its home country.
In other words, easyJet — a British airline — would no longer be allowed to fly between Amsterdam and Budapest, Hungary, and British Airways could not, theoretically, fly between Paris and New York.
This gets complicated by who owns the airline, and what their nationalities are.
To get around these issues, easyJet has gotten a secondary air operator's certificate based in Austria but has a large operation hub at London's Stansted airport. Ryanair has obtained a UK air operator's certificate, essentially becoming partly based in the UK.
Things become even more complicated for airlines owned by the International Airlines Group (IAG), the parent company for British Airways, Iberia, and Aer Lingus, since investors who are UK nationals will no longer count toward that 50% requirement, and European investors won't count toward Britain's similar requirement.
IAG has previously argued that its individual airlines are technically independently owned domestically because of a series of ownership trusts and companies, the Financial Times previously reported.
However, Charlton said, this may not hold up under scrutiny.
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"To be frank, there's going to need to be a lot of study done on ownership," he said. "I suspect that it's one of those things where no one has actually asked the question for fear of finding out the answer."
"It's going to be very interesting, [IAG] is going to spend a lot of time making sure that it satisfies both [the UK and the EU], for British Airways on the one hand, and all its other carriers on the other. Somehow, it will need to satisfy requirements, which appear to me, to be contradictory."
Ultimately, the ownership issue will be complicated at best, according to Charlton, and will need to be closely examined and renegotiated as the no-deal Brexit occurs, the backstop measures expire, and long-term agreements are negotiated and implemented.
"There are some serious questions in all this," Charlton said. "How f---ing stupid are these? That we continue to use a regulatory framework from 1944 and try to run the modern aviation industry in that framework. That's genuinely unconscionable. That's genuinely stupid."