Aviation Fuel Guide: 100LL vs Mogas for GA Aircraft

1. Understanding Aviation Fuel Types

General aviation aircraft primarily operate on two fuel types: aviation gasoline (avgas) 100LL and automotive gasoline (mogas). While both are petroleum-based fuels, they differ significantly in formulation, octane rating, and regulatory approval for aviation use.

100LL (100 Low Lead) contains 100 octane rating with reduced tetraethyl lead content compared to its predecessor, 100/130 avgas. The lead additive serves as an anti-knock agent, preventing detonation in high-compression aircraft engines. Mogas, or automotive gasoline, typically ranges from 87 to 93 octane and contains no lead additives, instead using alternative anti-knock compounds like ethanol or MTBE.

The fundamental difference lies in their intended applications: 100LL is specifically formulated for aircraft engines, while mogas is designed for automotive engines with different operating parameters and environmental conditions.

2. Octane Ratings and Engine Performance

Octane rating measures a fuel's resistance to knock or detonation. Aircraft engines, particularly those with higher compression ratios or turbocharging, require higher octane fuels to prevent destructive detonation under high power settings and elevated operating temperatures.

100LL provides consistent 100 octane performance, essential for aircraft engines designed around this specification. Many certificated aircraft engines require minimum 100 octane fuel, making 100LL the only approved option. Mogas octane ratings vary by brand and seasonal blend, typically ranging from 87 to 93 octane.

The lead content in 100LL also provides superior lubrication for valve seats and other engine components in older aircraft engines designed before unleaded fuels became prevalent. This lubrication property can be critical for engine longevity in certain aircraft.

3. STC Requirements and Legal Considerations

Using mogas in certificated aircraft requires a Supplemental Type Certificate (STC) approved by the FAA. The most common mogas STCs are issued by Petersen Aviation and EAA, covering hundreds of aircraft models. These STCs specify approved fuel grades, operational limitations, and required placards.

STC approval considers factors including fuel system compatibility, vapor pressure characteristics, and engine octane requirements. Not all aircraft are eligible for mogas STCs due to fuel system design, high compression ratios, or turbocharging that demands higher octane fuel.

Experimental and light sport aircraft (LSA) often have more flexibility in fuel selection, as their operating limitations may specifically allow mogas use without requiring an STC. However, pilots must still ensure the chosen fuel meets the engine manufacturer's specifications.

4. Fuel System Compatibility Issues

Mogas contains ethanol in many markets, typically up to 10% (E10). Ethanol can cause compatibility issues with certain fuel system components, including rubber seals, gaskets, and fuel lines not designed for alcohol-containing fuels. This phase separation concern is particularly problematic in aircraft that sit inactive for extended periods.

Ethanol also increases fuel's affinity for water absorption, potentially leading to phase separation where the ethanol-water mixture settles at the bottom of fuel tanks. This condition can cause engine roughness, fuel system corrosion, and potential engine failure if the water-ethanol mixture reaches the engine.

Vapor pressure differences between mogas and 100LL affect high-altitude operations. Mogas typically has higher vapor pressure, increasing the risk of vapor lock in fuel lines at altitude or in hot weather conditions. This characteristic makes mogas less suitable for high-altitude operations or aircraft with fuel systems prone to vapor lock.

5. Cost Analysis and Availability Factors

Mogas typically costs significantly less than 100LL, often $2-3 per gallon cheaper at retail. This cost difference stems from higher production volumes, simpler refining processes, and lack of specialized aviation distribution networks required for 100LL.

However, availability presents challenges for mogas users. Most airports don't offer mogas, requiring pilots to seek automotive fuel sources near airports. This limitation affects cross-country flight planning and may require additional fuel stops or carrying extra fuel.

Ethanol-free mogas, preferred for aviation use, has limited availability and typically costs more than standard E10 gasoline. Some regions have better ethanol-free availability than others, making mogas viability location-dependent.

Weather planning becomes more critical when using mogas due to its vapor pressure characteristics. Understanding density altitude effects and temperature variations helps pilots avoid vapor lock conditions that could affect flight safety.

6. Operational Performance and Limitations

Performance differences between 100LL and mogas are generally minimal in approved applications. Some pilots report slightly better fuel economy with mogas, though this varies by engine type and operating conditions. The absence of lead deposits may result in cleaner combustion chambers and spark plugs over time.

Cold weather operations may favor 100LL due to its consistent formulation and additives designed for aviation use. Mogas seasonal blends can vary in cold weather characteristics, potentially affecting engine starting and operation in freezing conditions.

Flight planning considerations include fuel source locations, storage time limits for ethanol-containing fuels, and weather conditions that might affect vapor pressure. Pilots should also consider backup plans for obtaining approved fuel if mogas isn't available at the destination.

Proper weather briefing procedures become even more important when using mogas, as temperature and pressure variations can significantly affect fuel system performance and vapor lock potential.