1. The aircraft operator shall determine the RFNBO or RCF fraction or synthetic low-carbon fraction of mixed aviation fuels containing RFNBO, RCF or synthetic low-carbon fuel. The aircraft operator may either assume the absence of RFNBO, RCF or synthetic low-carbon fuel and apply a default fossil fraction of 100 %, or determine a RFNBO or RCF fraction or synthetic low-carbon fraction in accordance with paragraphs 2 or 3. The aircraft operator shall use a default value of 100 % RFNBO or RCF fraction, or 100 % synthetic low-carbon fraction, as applicable, for neat RFNBO or RCF or neat synthetic low-carbon fuel.

By way of derogation from the first subparagraph, the aircraft operator using mixed aviation fuels containing RFNBO, RCF or synthetic low-carbon fuel, may choose to monitor the RFNBO or RCF content or synthetic low-carbon content and other fossil aviation fuel content, as separate source streams if the evidence provided by the fuel suppliers allows such approach.