Aviation Biofuel Standards and Airworthiness Approval
The chemical properties that control the performance of aviation fuel have evolved in parallel with the aircraft and engines that use these fuels. The initial focus was on improving piston engine fuels to meet the increasing performance demands of combat operations. This early experience established a precedent for the evolution of industry and military specifications used to control the properties of jet fuel used on today’s aircraft.
Jet fuel specifications have been refined and revised over the years leading to a commonly accepted set of properties now listed in those specifications. They are utilized by the aviation fuel industry stakeholders to control the performance and properties of the fuel to support a myriad of activities, such as aircraft and engine design, aircraft operation, fuel production, fuel transport, fuel commerce, and fuel storage.
Airworthiness authorities accept industry fuel specifications as operating limitations when specified by engine and aircraft manufactures for use on their products. Airworthiness authorities do not specifically approve aviation fuels, but rather they approve engine or aircraft models to operate on a specified aviation fuel.
The alternative jet fuel approval process evolved from the approval of Sasol’s Fischer-Tropsch (FT) coal-to-liquid (CTL) semi-synthetic jet fuel in 1999. This represented the first approval of a material not produced from petroleum for use in jet fuel. These early efforts by Sasol provided the specification-writing organizations (ASTM International and the UK MOD Aviation Fuel Committee) with a basis for development of alternative jet fuel approval methods.
Alternative jet fuels are approved as “drop-in” fuels, which possess identical fuel properties and performance as petroleum-derived jet fuel. Aircraft operations are unaffected when using drop-in fuels and recertification of existing jet-powered aircraft or jet engines is not required. This drop-in fuel concept has been accepted by the Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA). The Commercial Aviation Alternative Fuels Initiative (CAAFI; www.CAAFI.org) in partnership with the US military has worked with the aviation fuel community to help develop this qualification and certification process to support development and deployment of these alternative jet fuels.
A generic specification for FT fuel at a maximum 50 % blend level was issued in 2009. A specification for Hydroprocessed Esters and Fatty Acids (HEFA), also at a 50 % blend level, was added in July 2011, followed by Synthesized Isoparaffins (SIP) at a 10 % blend level in July 2014. Other prospective alternative fuel producers have initiated the ASTM International qualification process to approve new renewable jet fuel pathways. A variety of feedstocks and associated conversion technologies are being proposed for these new jet fuels.
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