Aviation biofuel is a biofuel used for aircraft. Some consider it to be the primary means by which the aviation industry can reduce its carbon footprint. After a multi-year technical review from aircraft makers, engine manufacturers and oil companies, biofuel was approved for commercial use in July 2011. Since then, some airlines have experimented using biofuels on commercial flights. The focus of the industry has now turned to second generation sustainable biofuels that do not compete with food supplies. “Drop-in" biofuels are biofuels that are completely interchangeable with conventional fuels. Deriving "drop-in" jet fuel from bio-based sources is approved via two routes.