Aircraft can run on several fuel substitutes, but every option must meet strict chemical and performance standards before it’s legal to use. The answer depends on whether the aircraft has a turbine engine (jets and turboprops) or a piston engine (most small propeller planes), because the two categories use completely different base fuels and have different substitution rules.
Turbine Engine Fuel Substitutes
Commercial jets and turboprops burn kerosene-based fuel, most commonly Jet A in the United States or Jet A-1 internationally. Jet A must have a minimum flash point of 38°C (100°F) and a maximum freezing point of -40°C (-40°F). Jet A-1 has a lower freezing point of -47°C (-53°F), making it better suited for long polar routes. These two grades are largely interchangeable, and Jet A-1 can substitute for Jet A in any situation.
Beyond that, the main substitutes fall into three categories: sustainable aviation fuels, military-specification fuels, and wide-cut fuels for extreme cold.
Sustainable Aviation Fuel
Sustainable aviation fuel, or SAF, is the highest-profile substitute today. It’s not a single product but a family of fuels made from non-petroleum sources, including plant oils, animal fats, municipal waste, agricultural residues, and even captured carbon dioxide combined with green hydrogen. Seven production pathways are currently approved under the ASTM D7566 standard, and each has a maximum blend limit with conventional jet fuel:
- Fischer-Tropsch kerosene: made by converting carbon-rich materials (biomass, waste, or captured CO₂) into synthetic hydrocarbons. Approved up to 50% blend.
- Hydroprocessed fats and oils: produced from used cooking oil, tallow, or plant oils by removing oxygen with hydrogen. Approved up to 50% blend.
- Alcohol-to-jet kerosene: converts ethanol or isobutanol into jet-range hydrocarbons. Approved up to 50% blend.
- Catalytic hydrothermolysis kerosene: uses high-temperature water and catalysts to convert fats into fuel. Approved up to 50% blend.
- Fischer-Tropsch kerosene with aromatics: a variant that includes ring-shaped hydrocarbon molecules. Approved up to 50% blend.
- Fermented sugars to synthetic fuel: approved up to 10% blend.
- Hydrocarbon-hydroprocessed fats and oils: a newer variation of the fat-to-fuel process. Approved up to 10% blend.
None of these substitutes are approved as 100% drop-in replacements yet. They must be blended with conventional Jet A or Jet A-1. The reason is partly chemical: many SAF blends are almost entirely paraffinic (straight-chain hydrocarbons) and contain very little aromatic content, sometimes less than 0.5% by weight. Conventional jet fuel typically contains 15 to 23% aromatics, and that matters because aromatic compounds cause rubber seals and fuel tank sealants to swell slightly, creating a tight seal. Research published by SAE International found that when aromatic content drops below about 8%, certain polysulfide sealants actually shrink, which can lead to fuel leaks. Blending SAF with conventional fuel keeps the aromatic level high enough to maintain seal integrity.
The energy density of SAF blends is comparable to conventional jet fuel. ASTM D1655 requires a minimum energy content of 42.8 megajoules per kilogram for Jet A and Jet A-1. Most paraffinic SAF blends meet or slightly exceed this on a per-kilogram basis, though their lower density means each liter contains slightly less energy than conventional fuel.
Military Kerosene Fuels
JP-8 is the standard military jet fuel and is chemically equivalent to Jet A-1 with a few additions: it contains a corrosion inhibitor and an anti-icing additive that civilian Jet A-1 does not require. Military aircraft can burn Jet A-1, and civilian turbine engines can generally accept JP-8 in emergency or logistical situations. The additives in JP-8 don’t harm civilian engines; they simply aren’t part of the civilian specification. This interoperability is by design, allowing military and civilian operations to share fuel infrastructure during deployments or emergencies.
Wide-Cut Fuel for Extreme Cold
Jet B is a wide-cut fuel that blends kerosene with lighter gasoline-range hydrocarbons. It has a much lower freezing point than Jet A and ignites more easily in extremely cold conditions, which makes it useful in Arctic and subarctic operations. The tradeoff is greater volatility and a lower flash point, which increases fire risk during ground handling. Jet B is rarely used outside of northern Canada and similar environments, but where it’s needed, it substitutes directly for Jet A in approved turbine engines.
Piston Engine Fuel Substitutes
Small piston-engine aircraft traditionally burn 100LL (100 octane, low lead) aviation gasoline, or avgas. It’s one of the last leaded fuels in widespread use, and eliminating lead has been a major goal for decades. Two main substitutes exist today: unleaded avgas and automotive gasoline.
Unleaded Aviation Gasoline
G100UL, developed by General Aviation Modifications Inc. (GAMI), is an unleaded avgas with 100 motor octane that can fully replace 100LL. The FAA has issued a Supplemental Type Certificate covering a broad list of spark-ignition piston engines, from common Lycoming and Continental models to older radial engines. G100UL can be mixed with 100LL and with other gasolines rated at 100 motor octane or below, including approved automotive gasoline. Engine power settings certified for 100LL operation remain unchanged when running G100UL.
Automotive Gasoline (MoGas)
Some lower-compression piston aircraft engines are approved to run on automotive gasoline, commonly called MoGas in aviation. This is typically limited to engines that don’t need the full 100 octane rating of avgas, such as certain Rotax, Continental, and Lycoming models operating at lower compression ratios. Using MoGas requires a specific Supplemental Type Certificate for the airframe and engine combination.
The biggest risk with automotive gasoline is vapor pressure variation. Avgas has a fixed vapor pressure year-round, but automotive fuel changes seasonally. Summer blends mandated by the EPA typically have a Reid Vapor Pressure around 7 to 8 psi, while winter blends can reach 14 psi or even 15 psi when ethanol is added. Higher vapor pressure means the fuel releases more vapor at a given temperature, which can cause vapor lock: bubbles forming in the fuel lines that starve the engine. If you use winter-blend automotive fuel during warm weather, vapor lock becomes a serious risk.
Ethanol is another concern. Most automotive gasoline in the U.S. contains 10% ethanol, which raises questions about compatibility with aircraft fuel system components. A long-term study at South Dakota State University tested ethanol-blended fuel in a Teledyne Continental Motors fuel system, including the fuel pump, flow controller, flow divider, and associated hoses. After extended operation exceeding two recommended overhaul intervals, the fuel servo showed a 12% drop in outlet pressure and the throttle adjustment torque increased by 27%, suggesting some seal deterioration. However, no external leakage was observed, and the researchers concluded that ethanol-based fuel caused no abnormal wear overall. Still, many aircraft fuel system components, particularly older rubber seals and certain composite fuel tanks, were never designed for ethanol exposure. Unless your engine and airframe have a specific ethanol-compatible approval, ethanol-free automotive fuel is the only safe choice for MoGas operations.
Why You Can’t Just Pick Any Fuel
Switching to a non-standard fuel in any aircraft is a legal change to the aircraft’s type design. Under FAA regulations (14 CFR Part 21), using a fuel not specified in the engine’s type certificate requires either an amendment to that certificate or a Supplemental Type Certificate. The applicant, whether a fuel manufacturer or a modification company, must demonstrate that the aircraft and its fuel system meet all applicable safety, emissions, and performance requirements with the new fuel. Flying with an unapproved fuel substitute violates airworthiness regulations and voids your insurance coverage.
This is why every substitute fuel discussed above comes with specific approvals tied to specific engines and airframes. The chemistry has to match: the fuel needs the right energy content, volatility, freezing behavior, and aromatic balance to work safely in systems designed decades ago for petroleum-based fuels. Even fuels that seem chemically similar can cause subtle problems, from seal shrinkage to vapor lock to unexpected combustion characteristics, that only show up after thousands of hours of testing.