You don't even have to be zero emissions. You just need net zero. If you're putting carbon into the air, you need to sink an equivalent amount of it. If you use biofuels for instance, so long as you replant/regrow the same biomass as you convert to fuel, you're closing the loop.

Zero emissions technologies are great where they're practical. Aircraft are one of those places where the energy density and overall density of your fuel source is very, very important.