Most sustainable airlines rated on what is actually in the tank. An airline is an operator, not a machine, so it is read on three measurable things: the share of sustainable aviation fuel genuinely loaded rather than contracted, how new the fleet is, because newer aircraft burn 15 to 25 percent less, and whether a transition plan carries dates and volumes. Transport and Environment found only ten airlines worldwide with serious plans to move to green fuels, so the gap between a pledge and a litre of fuel is where this category does its work.