Well it's not really green if they do it. It will have to be done worldwide.
Otherwise all the operators will fuel up to the brim at tax havens and be much heavier (and thus burn more fuel which is bad for the environment). And skimp on fueling at expensive locations and as such introduce dangerous situations (like RyanAir was already caught doing!). I think this was also the reasoning behind the no-tax thing.
They could make it mandatory for intra-europe flights for example but then the airlines will just introduce little side-hops to places like Dubai and Northern Africa for the sake of it. There just is no good solution unless the world agrees on taxation everywhere. That would have been a good point to raise at Glasgow but it was once again all form over function.
You could first raise the tax to just the point where detours do not make sense. It's the same with petrol and here local governments tax. IMHO the effect is minor. The solution would be to collect the tax like an import tariff even if you have fueled up elsewhere. Fuel usage should be well documented. There will be ways. I guess this is however not covered by the EU directive i guess. But that shows the problem of a green new deal. New agricultural subventions can also be used in a green way. Nobody does really...
