Everything from pilots’ licenses to security arrangements need to be agreed. Much of this could be secured through mutual recognition of existing standards. But formalizing this cannot happen overnight. And even when that is done, there will still be an administrative burden for the airlines and governments involved that will take time and significant resources.
While we still hope for a comprehensive EU-UK deal, an assumption that “it will be all right on the night” is far too risky to accept. Every contingency should be prepared for, and we call upon both the EU and the UK to be far more transparent with the state of the discussions.