Fuel costs for an airline operating a regular schedule are <i>much</i> higher than their aircraft&#x27;s capital cost, so changes to airlines&#x27; fleet plans induced by unexpected breaks in rising fuel price trends only really happen at the margin.<p>Where an airline chooses less-efficient designs it&#x27;s usually because it&#x27;s realising [partially] compensating efficiency savings by sharing pilots&#x2F;engineers&#x2F;equipment with similar aircraft it&#x27;s already operating rather than simply because they cost less to buy.<p>Given the variety of specific notionally pro-environment levies on airlines and their passengers as well as on the fuel, figuring out how much per seat mile airlines actually pay is nearly as complex as guesstimating how much that aircraft&#x27;s externality might be....