1.The aircraft operator shall determine the biomass fraction of mixed aviation fuels containing biofuels. The aircraft operator may either assume the absence of biofuel and apply a default fossil fraction of 100 % or determine a biofuel fraction in accordance with paragraphs 2 or 3. The aircraft operator shall use a default value of 100 % biomass fraction for neat biofuels.

By way of derogation from the first subparagraph, the aircraft operator using mixed aviation fuels containing biofuels may choose to monitor the biofuel content and fossil aviation fuel content as separate source streams if the evidence provided by the fuel suppliers allows such approach.