What is the real impact of biofuels on aviation?
The "real" (or at least ideal) impact of biofuels in aviation is a reduction in CO2 emissions. When they burn, they produce exactly the same amount of CO2 as fossil fuels, but, all this CO2 came from CO2 that was already in the atmosphere. So it's only putting back what was already there:


  From the standpoint of human-released carbon dioxide, other greenhouse gas emissions, and contributions to climate change biofuels have one large advantage over gasoline, diesel and other fossil fuels: The feedstocks for biofuels are part of the above-ground carbon cycle. Unlike petroleum or coal, the soybeans, corn, switchgrass and other biological materials that are made into biofuels are not dug up from underground, nor do they release long-stored carbon as carbon dioxide into the atmosphere when burned. Instead, when biofuels are burned, carbon dioxide they recently captured is release back into the atmosphere.


This is sometimes claimed to mean biofuels are carbon neutral. For example, it's claimed here a 95% reduction occurs. But it's a bit more complex than that:


  Producing biofuel with crops like corn often requires converting land from food to fuel production or destroying natural ecosystems that provide valuable services, including carbon sequestration. Crops also require fertilizers, pesticides and large amounts of water, as well as machinery for planting, growing, harvesting, transporting and processing. If forests are cleared for fuel crops and if the entire lifecycle of the fuels is taken into account, biofuels don't always reduce overall greenhouse gas emissions. Palm oil, used for biodiesel, is especially bad, because valuable carbon sinks like peat bogs and rain forests are often destroyed to grow palms.