Just to be clear, ethical considerations aside, relying on fat from animals requires an extremely environmentally damaging industry (I.e. Livestock industry) to either sustain the current numbers or increase output, just to keep airplane fuel going.

The immediate solution, at least for regional airtravel is electric planes. This is already doable, so more investments should be put into that.

We can then look at solutions for international and long haul flight, even if that includes hybrid planes or fully electric, or a totally different (sustainable) fuel source.

If you consider that whatever solutions we come up with need to work in the long-term, it makes sense to prioritize more realistic goals, rather than create an entirely different set of problems.