The aviation industry’s efforts to reduce carbon emissions have driven the rapid development and scale-up of sustainable aviation fuels (SAFs). SAFs have the potential to significantly reduce CO2 lifecycle emissions by up to 80% in comparison to Jet A and other conventional fossil-derived jet fuels. For multiple logistical and practical reasons, it is preferable to ensure that SAFs are ‘essentially identical’ (also referred to as ‘drop-in SAF’) to conventional jet fuel in terms of their performance, durability and compatibility with existing hardware systems. Because the majority of SAFs are not identical (non-drop-in) to conventional jet fuel, they have not been approved for use in their neat (100%) form. Instead, these non-identical SAFs are named synthetic blend components (SBC) as they are blended with conventional fuels to different extents per ASTM D7566-23a. It should be noted that there are on-going efforts to develop non-drop in SAF specifications to broaden their proliferation and maximise the aviation industries’ ability to reduce CO2 lifecycle emissions. One very important area of focus is the compatibility of SAFs with engine and fuel system seals, specifically understanding the dynamics of elastomeric seals. To address this, a novel approach has been developed to measure seal dynamics in flowing fuel. This technique has been applied to study the dynamic seal behaviour of four industrially relevant elastomer seals commonly employed in aviation fuel systems. The study involved three test fuels: (i) conventional fossil-derived Jet A, neat hydroprocessed esters and fatty acids (HEFA) SAF, and neat alcohol to jet (ATJ) SAF. Notably, both HEFA and ATJ fuels contain 0% aromatics, in contrast to Jet A, which typically contains around 17% aromatics by volume. The novel fuel-elastomer test rig used in this study was designed to simulate a practical scenario in which fuel flows through the inner surface of a pre-loaded static O-ring. The results of these tests demonstrate that the behaviour of different nitrile elastomers is unique to their formulation, and in all cases, the behaviour in HEFA and ATJ SAF differs significantly from that in Jet A. However, new fuel approval tests may only list one type of elastomer for evaluation, for example the ‘Fit-for-Purpose’ test in ASTM D4054-22 Tier 2 lists one specific nitrile. The findings of this study highlight the complexities of fuel-elastomer interactions within nominally identical chemical families and emphasise the potential risks of assessing compatibility based on tests conducted with a single member of a chemical family.
