New Advisory Circular AC 20-24C The Result Of 11-Month Process

The FAA has responded to a rapid increase in the pace of development of alternative fuels for aircraft with an updated version of a 25-year-old advisory circular, AC 20-24B. The new AC 20-24C took almost a year to finalize including the public comment period. It is dated July 29, 2011, but is just now showing up in electronic document databases.

The new AC specifically accepts military, ASTM and other industry-consensus standards as acceptable for defining fuels and oils for turbine engines, as well as piston engines made by TCM and Lycoming, and makes clear that other standards are acceptable to the FAA provided they adequately define performance, and how it is to be measured.

The complexity of the topic is underlined by many of the public comments, which mischaracterized the circular as a regulation, or misinterpreted it as a requirement to use ASTM's process.

The new guidance allows definitions of new fuels based on performance criteria without regard to the feedstocks used to create them. This may lower the barriers to introductions of drop-in fuels made from biomass for both turbines and piston engines. It also provides a system for commonality in specifications for developing type certificates and STCs for fuels which may not work in legacy engines fleet-wide.

While traditional fossil-based oils used for engine lubrication are considered a lower priority for phase-out, for both economic and environmental reasons, the new AC 20-24C also covers their specifications.

FMI: www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2020-24.pdf