This web story continues from our previous one, which covered the ideas and plans for certifying SAF and Hydrogen propulsion by 2030. Research into seal and material compatibility with SAF is ongoing. While the IATA Aircraft Technology Net Zero Roadmap was released earlier in this decade, we now report on the current progress with one of these challenges: the development of seals and related materials that are compatible with 100% SAF.
The Airbus path to 100% SAF, going across three of their key airplanes, is indicated in Figure 1.
Figure 1: The Airbus 100% Sustainable Aviation Fuel Journey
As the aviation industry intensifies its push toward decarbonization, sustainable aviation fuels (SAFs) have emerged as a vital pathway. These fuels promise up to 80% lifecycle CO2 emissions reductions over conventional Jet A fuel. However, SAFs differ significantly in chemical composition from Jet A fuel, particularly in their lack of aromatic compounds. The absence of aromatic compounds in SAF raises critical compatibility questions for existing aircraft fuel systems, specifically for elastomeric seals such as O-rings.
This article synthesizes findings from three key sources: a recent peer-reviewed study on elastomer behaviour in different aviation fuels, an overview of fluorosilicone elastomers (FMVQ), and a manufacturer’s profile of Viton™ fluoroelastomers. Together, these materials highlight the need to reevaluate elastomer selection as SAF adoption expands.
The Problem: Aromatic Content and Elastomer Behaviour
Jet A, the industry-standard fuel, contains around 17% aromatic hydrocarbons. These compounds swell elastomer seals, maintaining tight fuel system connections and preventing leaks. In contrast, HEFA-SPK and ATJ-SPK SAFs—two of the most prominent sustainable fuels—have virtually no aromatics. This compositional difference impacts elastomer swelling, or lack thereof, leading to potential leakage when systems transition from Jet A to 100% SAF.
A 2024 study by Hamilton et al., published in The Aeronautical Journal, utilized a novel flow-through test rig to measure dynamic seal behaviour over 100 hours. Four O-ring types were tested: three nitrile-based elastomers (N0602-70, NM072-70, and N0674-70) and one fluorosilicone (LM100-70). The study revealed that in Jet A, nitrile elastomers exhibited varying degrees of swell (up to 26%), while the fluorosilicone shrank slightly. However, when exposed to HEFA and ATJ fuels, all nitrite elastomers showed little to no swelling, and in some cases, shrinkage.
The results indicate that aromatic compounds are essential for maintaining elastomer seal effectiveness in current systems. Without these compounds, nitrile-based elastomers risk shrinking, increasing the likelihood of leaks and component failures.
Variability Within Elastomer Families
One of the most striking findings is the intra-family variability among nitrile elastomers. Although chemically similar, each nitrile formulation behaved differently in response to the same fuel. For instance, N0602-70 swelled the most in Jet A but shrank in SAFs. NM072-70 exhibited moderate swelling in Jet A and minor shrinkage in SAFs. This diversity challenges the current ASTM testing standards, which often rely on a single representative elastomer per material class. The assumption that one nitrile O-ring typifies the entire family is demonstrably flawed.
Fluorosilicone (FVMQ): A Promising Alternative
Fluorosilicone elastomers, such as LM100-70, offer a more consistent response across various fuel types. Though LM100-70 shrank modestly in all fuel environments, it did so uniformly, suggesting that its performance can be predicted and engineered around. Fluorosilicones combine the low-temperature flexibility of silicones with added resistance to fuel and oil due to their fluorinated side chains. A snipped section of the FVMQ is indicated in Figure 2.
Figure 2: A Snipped Section of the FVMQ Molecule
The performance characteristics of FVMQ make it an attractive option for aerospace applications that involve variable fuel chemistry. It operates effectively across a wide temperature range and resists fuel-induced degradation. Although more expensive than nitrile elastomers, fluorosilicones may provide reliability in SAF-dominated operations and in switch-loading scenarios where aircraft may alternate between Jet A and SAFs.
Editor’s note: The possibility of switch-loading fuels is not indicated in any industry presentations until now. However, this becomes a very real possibility as different customers and countries align themselves to potential SAF mandates.
Viton™ Fluoroelastomers: High Performance with Caveats
Viton™, a trade name for fluoroelastomers (FKM), is another high-performance material widely used in aerospace. These elastomers are prized for their chemical resistance, high-temperature stability, and minimal compression set. However, their compatibility with aromatics and performance in SAFs lacking these compounds is nuanced.
While Viton™ shows excellent resistance to degradation in Jet A and blended fuels, the data on its interaction with 100% SAFs is limited. Viton™ materials typically resist shrinkage and maintain dimensional stability better than nitriles, but may not offer the same uniform shrinkage behaviour observed in fluorosilicones like LM100-70. For mission-critical applications involving pure SAFs, additional testing may be required to validate Viton™ formulations.
Implications for Industry and Standards
The current ASTM D4054-22 Tier 2 fuel approval process requires testing with just one representative elastomer per polymer family. This standard, although efficient, is insufficient for the adoption of SAF. Hamilton et al.’s study makes it clear that elastomer response is highly formulation-specific, even within the same polymer class. Broadening the testing protocol to include multiple elastomers per family is critical.
From a systems design and maintenance standpoint, aircraft operating with high-SAF or pure SAF blends should reconsider their seal materials. Using legacy nitrile-based O-rings developed for Jet A can lead to seal shrinkage and fuel leakage when used with low-aromatic SAFs. Fluorosilicones like LM100-70, or potentially tailored Viton™ fluoroelastomers, offer more predictable performance in these conditions.
Figure 3: Airbus A350 first in-flight study using 100% SAF
Source: Airbus.
Airbus in-flight measurements from an A350 aircraft using 100% sustainable aviation fuel (SAF) showed a significant reduction in soot particle emissions and formation of contrail ice crystals compared to using conventional aviation fuel. Global model simulations estimate a 26% reduction in the climate impact of contrails when using 100% SAF.
Conclusion
The transition to sustainable aviation fuels is not just a matter of chemistry or emissions—it’s also a matter of material compatibility. The elastomer materials chosen for fuel system seals must perform reliably across a broader range of chemical environments than ever before. The data underscores that:
- Aromatics are crucial for maintaining seal swell and leak-tightness in nitrile elastomers.
- Elastomer response is not uniform within material families, demanding broader testing.
- Fluorosilicones (FVMQ), such as LM100-70, offer more stable performance across various fuel types.
- Viton™ fluoroelastomers provide excellent resistance but require further study for 100% SAF.
As the aviation sector accelerates toward net-zero goals, refining elastomer compatibility standards will be essential to ensuring safe, leak-free adoption of SAFs in both commercial and defence aerospace operations.