This is something we used to discuss at length back when I worked in aerospace design and manufacture. To break it down into two parts, hydrogen and biofuels the answers looks as follows: Hydrogen is likely very far away and possibly never at scale. Firstly the aircraft needs to be drastically redesigned to use hydrogen. The fuel storage is completely different and usually requires on board cryogenics to keep the power density on par with jet fuel. So firstly you’re talking about designing a completely new plane just to deal with an alternative fuel (and it will not be compatible with jet fuel if there’s a hydrogen supply issue). Secondly, the fueling of planes at airports will require new fueling infrastructure just for the hydrogen planes. That means storage. That means getting the hydrogen to the airport. It’s not honestly that difficult, but at scale being the key phrase to your questions: there’s a lot of inertia working against you. Lastly is the manufacture of the hydrogen itself. Airbus plans to use hydrogen as part of a co2 neutral stance. But typical methods of hydrogen production are not green and absolutely have a carbon footprint. So to get it to net zero, the hydrogen production needs to be done by renewables. It’s been done and is doable. But it’s expensive. So overall going to hydrogen will have a lot of inertia and design issues and hugely expensive. I don’t see it as commercially viable or even available at all for a decade. At scale, I would say unlikely before 2050. Biofuels is a totally different and more promising story. Airbus already has their planes certified to fly on up to 50% biofuels. The infrastructure is there. If that fuel isn’t available they just switch to jet fuel and are good to go. No redesign of the plane. No fancy cryogenic gas storage. They aim to get certified to 100% biofuels quite soon and I don’t see any reason why they wouldn’t get there. The only barrier to this happening at scale is the supply of the fuels themselves. Right now, supply is limited and price can be high. So the aircraft side of the equation is actually pretty simple. It’s the fuel creation and supply issue that will likely require government policy changes or subsidies to get up to scale. But this is a much more realistic target than the hydrogen plane.