What’s The Difference Between 100LL & Jet-A?

The two main types of fuel used for private and commercial aircraft are jet fuel and Avgas. Jet fuel is more commercially used, colorless, and contains Jet A and Jet A1 fuel. It’s also more simply known as diesel fuel.

However, following troubling industry failures using the word “diesel,” people refer to it as Jet-A fuel to avoid further conflict. Jet A presents a few issues for aircraft owners:

  • It’s more expensive to use over time
  • Modern aircraft engines can’t run on diesel
  • It has no overhaul limit meaning more frequent engine maintenance

Avgas is for small-piston aircraft, available in Avgas 100 and Avgas 100LL. Avgas is the only fuel option that contains a lead additive called tetraethyl.

Brief History Of Avgas

During and after World War II, consumers noticed Avgas’s ability to diminish the chances of engine failure, prolong an engine’s life, and provide proper lubrication. Avgas was even used in cars at the local gas station.

However, Avgas revealed toxic components when inhaled or entered into the bloodstream over time. This means that even touching it can cause bodily damage. Furthermore, it was found to cause significant damage to the environment through air and water pollution. By the 1990s, Avgas was banned by the U.S. Environmental Protection Agency (EPA) for auto use. It’s currently used only in aviation but is still extremely toxic to the environment and people.

Aircraft Fuel Key Characteristics

Take a look at what makes each fuel unique.

Jet A

Most common in the U.S., this type of fuel has a higher freezing point than Jet A1 at -40°C, making it ideal for shorter flights. It also rarely contains static dissipater additives to decrease its static charge.

Jet A1

Jet A1 is more widely used worldwide and has a freezing point of -47°C. This feature makes it better suited for longer flights, especially those facing extremely cold weather conditions. Jet A1 does contain static dissipater additives to help reduce its static charge.

Avgas 100LL

Each number attached to Avgas represents its octane rating or how much lead is present. Avgas 100LL means “low lead” and is dyed blue for quick identification.

Avgas 100

This fuel contains a high lead level and is dyed green for distinction. It’s extremely important to be careful with this type of fuel as it may cause:

  • Skin irritation
  • Poisoning if swallowed
  • Drowsiness or dizziness
  • Cancer
  • Fertility problems
  • Harm to aquatic life with long-lasting effects
  • Death

The Federal Aviation Administration (FAA) is working with the EPA to remove tetraethyl lead from fuel completely.


SAF stands for sustainable aviation fuel. It’s produced from sustainable feedstocks and is very similar in its chemistry to traditional fossil jet fuel. Using SAF results in a reduction in carbon emissions compared to the traditional jet fuel it replaces over the fuel’s lifecycle. Typical feedstocks used are cooking oil and other non-palm waste oils from animals or plants; solid waste from homes and businesses, such as packaging, paper, textiles, and food scraps that would otherwise go to landfill or incineration. Other potential sources include forestry waste, such as waste wood, and energy crops, including fast-growing plants and algae. As an example, Air BP’s SAF is currently made from used cooking oil and animal waste fat.

Jet fuel packs a lot of energy for its weight, and it is this energy density has enabled commercial flight. A return flight between London and San Francisco has a carbon footprint per economy ticket of nearly 1 tonne of CO2e. With the aviation industry expected to double to over 8 billion passengers by 2050, we must act to reduce aviation’s carbon emissions.

Furthermore, all aircraft certified for Jet Fuel can use SAF, and it is safe as you are really pumping a blended Jet A or Jet A 1 fuel which has been deemed safe.

The issue is cost, as SAF is more expensive. SAF is currently more costly than traditional fossil jet fuel. That’s down to a combination of the current availability of sustainable feedstocks and the continuing development of new production technologies. As the technology matures, it will become more efficient, and so the expectation is that it will become less costly for customers.

Fuel Grade

Jet fuel is graded based on its main compound, which is kerosene. It can also be separated into civil or military jet fuel ratings. As mentioned before, Avgas features its octane amount within each type, of which there are five current grades: 100, 100LL, 115, UL91, and UL94. Avgas 115 is for military use, and the final two are unleaded versions.

Curious about diesel engine aircraft? Browse VREF Online to find out more.

Why Do Aviation Piston Engines Need High Octane Fuel?

Consider your car’s engine briefly. When you drive, you spend little time with the throttle wide open, developing maximum power, especially with the advent of the 7, 8, and 9-speed transmission. Additionally, your car’s ECU (engine control unit) is effectively a FADEC (full authority digital engine control), carefully controlling timing, fuel mixture, fuel delivery, and spark to eliminate the possibility of detonation.

Now consider the typical aviation engine, which is designed to operate at or above 75% maximum continuous power for the entirety of its lifecycle without the aid of electronic controls. If typical automotive engines were operated similarly to aviation piston engines, higher octane values would be required to maintain reliability, as was the case before the early 1980s. If you are old enough, you will remember leaded gas being available at the pump for cars. The lead additive makes the engine run smoother at the expense of the population’s health.

The high octane rating requirements of aviation piston engines stem from the pinnacle of piston engine development in the 1940s into the 1950s. Supercharging and turbocharging drove the outputs of piston engines to higher and higher power to maximize engine performance for military and transport aircraft.

The extreme operating pressures and temperatures required to fuel would resist detonation under high engine loads for hours.

The requirement for aviation piston engines to maximize power output under continuous high load is the driving design requirement for the use of high octane rating fuels. While some aircraft engines can operate on lower octanes, many aircraft that utilize high-performance or turbocharged engines are still limited to using high octane fuels to avoid fuel detonation or premature overhaul. Longevity, dependability and high power ratings are dependent on 100LL.

Why Are There So Few Diesel Engines Used in Aviation?

Piston engine-powered aircraft continue to rely on AvGas as a fuel source because the design requirements of diesel engines incur an inherent weight penalty, and new engine development is expensive and prolonged. Instead of a TBO (time between overhaul), there is a TBR (time between replacement), and for $90,000 for the engine, most people are not financially prepared for the switch to diesel.

Diesel engines are typically heavier than their gasoline equivalents to meet the strength requirements associated with compression engine overhaul limits. Automotive and aviation diesel engines alike nearly exclusively require turbocharging to be effective power plants as throttle response is a primary design consideration. If you think about the old diesel engines from the 70s and 80s, you’ll remember how awful they were to drive in traffic, taking two to three times longer to reach optimal RPM to generate power.

Additionally, aviation diesel is primarily derived from automotive diesel and thus must be geared down to drive a propeller. These gearboxes are failure-prone and often result in expensive replacement costs over dramatically shorter time intervals than is typical for gasoline engines. These additional mechanical systems drive up the weight and complexity of the engine, a serious detriment to the implementation of diesel engines as a primary powerplant for piston-powered aircraft.

As with all modern vehicles, weight gain with less power is the opposite direction manufacturers are going.

Lower performance is not a detriment for certain segments of the aviation industry, particularly in the training sector, but sufficient sales have not yet materialized to justify the development costs and timelines necessary to certify new piston aircraft under current regulatory burdens.

The automotive roots of these engines do provide some benefits; however, much of the research and development costs of the engine itself have been amortized by the automotive industry, and the engine control systems are typically electronic (i.e., FADEC controlled just like many turbine engines).

Electronic controls simplify engine operation and allow for extremely efficient operations. However, there is not sufficient net operating cost-benefit for these engines to make significant sales gains within the piston aircraft fleet, either with OEMs or aftermarket STCs.

It is worth mentioning that the word diesel in the United States carries a negative connotation; thank Volkswagon! With all the research and development that has gone into the new line of Continental Diesel engines, it will take new marketing and a new name to get Americans to stop thinking about the black smog, environmentally cheating diesel of yesterday to the ones built for today. Would people be more inclined to purchase a Jet-A engine? I think so, and it is going to take a long time and lots of marketing dollars to accomplish the transition to diesel.

Preparing For The Inevitable End of 100LL in Aviation?

With the support of FAST ( Future Avgas Strategy & Transition Plan), the EPA investigation studied the viability of potential fuels, including low-octane 87-89 mogas, which were determined to be unacceptable and unable to support many of the high-performance engines. The mid-octane UL94 would require a lot of engine modifications and result in a loss of performance and high octane synthetic or biofuels, or SAF.

UL94 would be the easiest replacement for 100LL as the distribution structure is in place, and it can be “more easily certified, which is an important point. There are some engines. However, that will require substantial modifications ranging between $10,000 and $35,000.

Fuel Cost

Following labor, jet fuel is one of the most expensive operating costs for airlines and aircraft owners. To clarify, the cost of jet fuel has increased significantly post-pandemic. Compared to last year, it is up over 82% from this time last year.

Jet A is cheaper than Avgas, but it does not perform well. Because of its incompatibility between its structure and most plane engines, the expenses associated with this fuel add up over time. Owners and operators must overhaul the engine more frequently, completely replace certain parts, and/or make up for less performance in some ways.

On the other hand, Avgas is known to be deadly and is being phased out over time. Ultimately, there are no other comprehensive, sustainable alternatives, which means aircraft owners and commercial operators will continue to pay for Avgas as long as it’s available. Furthermore, far too many existing operating aircraft used globally rely on Avgas, which means it will take much longer to see it eliminated from aviation completely. From vendors and all the airports that would need to make the switch to processes like refining and delivery, the whole support system for Avgas would need to adjust. And this change simply cannot be done quickly.

Other Types Of Aviation Fuel

Companies everywhere are starting to commit to more sustainable fuel solutions. The goal is to lower carbon emissions and reduce their impact on the environment and its resources. Many businesses invested in oil and gas are working to find alternatives, experimenting with derivatives like electric solutions and odd options like cooking oil.

Having less impact on the world we live in is becoming more of a target moving forward. We just aren’t quite there yet.

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