There are a lot of indirect subsidies going into air travel. To me this has always seemed rather perverse, and the money should have been invested in proper rail infrastructure instead, from a climate standpoint as well as for convenience (a proper rail network deposits you in the middle of a city without the need for a car, you don't have endless security controls etc.).
That's not really a subsidy for air travel, in fact it's a cost. People here are talking about how they would prefer rail so that they don't need to go through security at the airport. That's bad for airlines.