Mundane matters such as the continuance of the international carriage of goods and services, the preservation of aviation jobs and aviation related employment, the maintenance of reciprocal aviation agreements and treaties all wrapped up in a multi billion pound international aviation industry.
If that were the case, why would the EU have different terms for trading with members and non-members? If you're part of the club then you have to play by the rules and contribute to the running costs. Making an exception would put the whole project at risk. Plus, for many EU members the blow will be softened by the relocation of British industries to countries remaining in the EU.
The EU can afford to lose us, but can't afford to let us become freeloaders.