Flying on french friesKLM, a Dutch airline, has begun powering some transatlantic flights with a fuel mix that of 25% cooking oil and 75% jet fuel. 25 roundtrip flights between New York’s JFK and Amsterdam’s Schiphol will take place every Thursday for the next six months. So while I'm not a big fans of biofuels in general for the reasons stated by Dr. Hartmut Michel, this is actually a good use of them; aviation needs energy-dense fuel and can't be electrified in the near future, and waste cooking oil that might otherwise be thrown away is a great "free lunch".
The leftover waste oil comes from restaurants in the southern US state of Louisiana, where it’s used to fry up cracklins, catfish and other Cajun treats before being refined at a plant near Baton Rouge and trucked to New York to fuel the flights. (source)
The next step would be for KLM - and other airlines - to increase the ratio of biofuels and find supplies that are actually carbon-neutral (or close to it). In fact, most of the waste cooking oil that is currently going into making biodiesel for terrestrial vehicles could probably be better used in planes. We don't really have other good substitutes for jet fuel, but we have lots of ways other than waste oil biodiesel to make cars & trucks greener.