What Exactly Is Sustainable Aviation Fuel?

Sustainable Aviation Fuel, or SAF, is not a single fuel but a family of drop-in replacements for conventional jet kerosene. It is produced from renewable feedstocks such as used cooking oil, animal fats, municipal solid waste, agricultural residues, and even captured carbon. The most common production pathway today is HEFA (Hydroprocessed Esters and Fatty Acids), which refines oils and fats into a synthetic blending component. Other pathways include Alcohol-to-Jet (ATJ) and Fischer-Tropsch (FT), which gasify waste or biomass before rebuilding the molecules. Crucially, SAF must meet the same stringent specification as conventional Jet A-1. It is blended with kerosene, typically at ratios up to 50%, and the resulting mixture is certified as a drop-in fuel. That means no changes to your aircraft, engines, or operating procedures are required. But while the fuel is chemically similar, it is not identical, and that is where your operational awareness begins.

How SAF Is Tested and Certified

Before any SAF blend reaches your wing, it goes through a rigorous approval process. Fuel producers must demonstrate that their synthetic component meets ASTM D7566, the international standard for aviation turbine fuel containing synthesised hydrocarbons. In the UK, the fuel must also satisfy DEF STAN 91-091 for civil and military use. Testing starts on the bench: rig tests check density, viscosity, freezing point, lubricity, and thermal stability. Then comes engine testing, where the fuel is run in a full engine to verify performance, emissions, and durability. Finally, flight tests confirm real-world behaviour. Once approved, the blend must meet ASTM D1655 (the standard for conventional Jet A-1) before it can be loaded. When you refuel, you can ask for the fuel certificate. It will state the blend ratio, density, and freezing point. That document is your first operational reference.

The Operational Limits You Need to Know

SAF blends are not identical to conventional jet fuel, and there are several limits you must respect. The most important is the blend ratio.

• Blend ratio: currently up to 50% synthetic component for most certified SAFs. This cap exists to maintain enough aromatic content for seal swell in older fuel systems.
• Density: a 50% SAF blend can have a density as low as 775 kg/m³, compared with 800–840 kg/m³ for pure Jet A-1. Lower density means less mass per litre, so range can be slightly reduced.
• Lubricity: SAF has naturally lower lubricity, but additives are used to meet the BOCLE wear scar limit of 0.85 mm.
• Cold flow: freezing points are often below -47°C, but always check the fuel certificate.
• Materials compatibility: if you fly an older aircraft, check the maintenance manual for any specific restrictions on low-aromatic fuels.

These limits are well understood by regulators and manufacturers, but as a pilot you should be aware of them when you plan and fly.

What This Means for Your Flight Planning

Because density varies, you must use the actual fuel density from the refuelling document when calculating performance. On many modern aircraft, you enter fuel density into the flight management system. If you use a standard figure instead of the actual value, your fuel quantity indication and range predictions will be wrong. For a 50% blend, the range penalty is typically small — around 1–2% — but on long sectors that can matter. Also consider volume limitations. If the SAF blend is less dense, you may need more litres to achieve the same fuel mass, but your tanks can only hold a fixed volume. In extreme cases, you might not be able to upload the required fuel mass. Check refuelling availability too. SAF is not yet at every UK airport. The UK mandate starts at 2% in 2025, rising to 10% by 2030 and 22% by 2040, so availability will grow. But for now, plan your fuel stops accordingly. Never assume SAF is available without checking.

UK Context and Practical Tips for Pilots

The UK is moving steadily towards higher SAF blends. From 2025, all jet fuel supplied at UK airports must contain at least 2% SAF, with targets increasing over the next two decades. As a pilot, you will see SAF more often, but the operational basics remain the same. Before flight, check the fuel certificate for blend ratio, density, and freezing point. Confirm your aircraft is approved for the blend — most turbine aircraft are, but always verify against the flight manual. If you notice any unusual fuel leaks after refuelling, report them; seal shrinkage is rare but possible with very low aromatic fuels. In winter, treat SAF blends like any other fuel: check freezing point and cold soak behaviour. Remember that SAF has lower sulphur content, which is good for emissions but means you cannot rely on the smell of sulphur to detect a fuel leak. Finally, if you are unsure, ask your fuel supplier for the safety data sheet. Being informed is the best way to keep SAF a safe, practical part of your flying.