SAF Pathways: Making inroads into Clean Fuels markets | Insight Report | June 2022

For SAF to substitute conventional jet fuels, it must have characteristics similar to existing petroleum-based fuels. This is to ensure that aircraft systems need not be significantly overhauled to accommodate fuels processed out of biomass.

The term “drop-in” refers to SAF that is entirely replaceable with conventional aviation fuels, requiring no alterations for existing infrastructure and equipment. The ASTM D7566 lays down the standards that SAF needs to meet in order to qualify as a drop-in fuel. Any SAF that does not qualify for the ASTM D7566 technical certification could pose significant safety concerns. Lack of “drop-in” capabilities could also result in SAF pushing up costs, as airports might then be required to build parallel systems for handling renewable fuels.

Out of the pathways approved thus far, SAF produced can be dropped in with conventional jet fuel up to a maximum blending capacity of 50% by volume. This limit on SAF blending has been imposed in order to prevent any fuel leakages in aircraft systems. Conventional fossil fuels contain contaminants such as sulphur and aromatics, which cause the seals present in jet engines to swell, thus ensuring that no fuel gets leaked out. Compatibility between seals and fuels is especially pertinent for legacy engines and fuel distribution systems, which is a key consideration for the blending limits put in place by ASTM. Current standards require a minimum aromatics content of 8% to ensure sufficient seal swelling. Using unblended SAF with incompatible systems could significantly compromise the safety of jet engines.

Several test flights have already been conducted with 100% “drop-in” SAF. However, the jet engines in such cases usually had to be modified to include seals that did not require aromatics for preventing leakage.