The companies that will make the most impact on clean aviation are those addressing either of the two issues mentioned in the title. A sustainable supply of an alternative fuel to kerosene, preferably one that mitigates emissions during both its production and usage, or a zero-emission powertrain system that can utilize the fuel. Once these issues are addressed, the sector as a whole will be able to move toward achieving sustainability. This is what the leading companies in clean aviation are doing – see ZeroAvia for example. They realized that a full redesign for the single-aisle market is stretching it too far. Getting a competitive clean powertrain past certification and towards commercial operation is a hard enough task. This is…