It can be frustrating to pull up to the fuel pump and see the prices for 100LL be significantly higher than Jet A fuel. Oh what it would be like to top off your Cessna at only a couple dollars per gallon! Unfortunately there are some valid reasons that 100LL will continue to be expensive, but it at least helps to understand the reasons behind it. Why does Jet A fuel cost less than 100LL (avgas) fuel?
Jet A fuel costs less than 100LL (avgas) fuel because it is less complicated and expensive to manufacture, less expensive to transport via pipelines, and used in significantly higher quantities leading to economies of scale.
As of this writing, according to AirNav.com’s fuel report theaverage fuel price for Jet A was $4.68 per gallon, with prices ranging from$1.99 to $8.88. The national average for 100LL was $5.07 per gallon, withprices ranging from $3.30 to $10.00.
I had always just accepted the fact that 100LL was going tobe expensive, but it wasn’t until taking a deeper dive into understanding thereasons why that I became less frustrated about it. In order to fly pistonengine aircraft today, it’s important to understand that there are just somehurdles to making the fuel more affordable. Maybe it will ease yourfrustration, and maybe it won’t, but knowledge is power so it’s best to atleast understanding the driving forces behind fuel pricing.
Why Does Jet A Fuel Cost Less Than 100LL (Av Gas)?
Lots of people will just tell you that Jet A fuel costs lessbecause of the quantities in which it is used and so it’s a simple supply anddemand situation. While that’s certainly part of it, it’s nowhere near the fullstory. There are a couple other important factors that make Jet A cheaper and100LL more expensive.
Jet A Fuel is Less Expensive to Manufacture
To understand how and why fuel is priced the way it is, weneed to understand something about how it is made. Both Jet A and 100LL fueloriginate in raw crude oil. The raw oil is sent to a refinery, and through adistillation and refining process (it’s more involved than that, butdistillation is the basic principle) it is separated into various types offuels that can be used for different things. The most easily captured fuelsthat distill first are propane gasses and other liquefied petroleum.
Some of the next fuels that separate from the distillation process are diesel and Jet A (the kerosene family). Mostly all that is needed is to capture the fuel and go through a purification process to remove any Sulphur content.
The below image is a mockup of what types of refiningprocess various fuels have to undergo to transform from raw crude oil into theend product. You’ll notice that jet fuel has a fairly direct line fromdistillation to final product, whereas aviation fuel (100LL) must undergoreforming, alkylation, and other processes before it is finished.
This very limited amount of refining needed for jet fuel is a large part of why it costs less than 100LL fuel. If you’re wondering why jet fuel can’t be run in piston engines, we’ll cover that later in this article.
Less Expensive to Transport
Jet fuel is transported from refineries by pipeline, which is an extremely affordable method to move large volumes across great distances.
For example, shipping a gallon of fuel across 1,000 miles can cost pennies or less. 100LL fuel cannot be transported via pipeline because of its lead content, so it must be shipped through other methods (rail, truck, etc.) which is orders of magnitude more expensive than via pipeline.
Remember that at the pump you are paying for all of the costs (plus profit margin) that it took to get the fuel there in the first place. So when jet fuel costs less to get it there, it is reflected in the price.
Economies of Scale
The last reason that jet fuel is cheaper than 100LL isbecause of the enormous volumes needed. Refineries have much more economies ofscale (which basically means you are able to spread your fixed costs across amuch larger pool of final product, thereby reducing the overall average cost ofyour final product) producing jet fuel than they do 100LL.
For context, Plane and Pilot Magazine reports that in 2016 there were 576 million barrels (approximately 24 billion gallons) of Jet A fuel produced, and only 4 million barrels (approximately 168 million gallons) of 100LL produced.
That means that for every 1 gallon of avgas made, 144 gallons of Jet A fuel were produced. Not only is Jet A easier to produce and transport, but there is much greater demand for it which drives the price down as suppliers try to remain competitive.
Why Can’t Piston Engines Use Jet A Fuel?
So if Jet A is that much cheaper, how come we can’t put it in piston engines? Jet A is very similar to diesel fuel and so you cannot put it into your airplane’s piston engine the same way you don’t want to put diesel into a regular gas car engine.
Jet A fuel requires high pressure, high temperature combustion and so it would flood your piston engine and eventually not ignite. Alternatively, putting 100LL in a jet engine would be catastrophic as the fuel would simply detonate instead of burning evenly like it’s designed to do in a piston engine.
Generally speaking, piston engines are designed to be lightweight and efficient, using fuel that provides the most power relative to its size. This is why aircraft piston engines use 100LL that is extremely flammable (it takes less energy in order to combust, as compared to jet fuel) and efficient.
Turbine and jet engines are designed in a completely different way to use much higher temperatures and pressurization to ignite fuel in a continuous stream. This is why these engines call for fuel that can burn evenly at hotter temperatures to produce much larger amounts of thrust.
It’s worth mentioning that there have been several differentattempts at creating mainstream Jet-A powered piston aircraft, such as theDiesel Cessna 172, Diesel Diamond DA-40NG, and Diesel Piper Archers. Among thebenefits are better fuel efficiency, lower fuel prices (as discussed above),and increased range.
Paying for an engine conversion to diesel is very expensivethough (for example, upwards of $70k for a Cessna 172 diesel conversion) andgenerally in the United States the pain point of 100LL isn’t felt as much aselsewhere in the world.
For those of us who fly in the United States, we are spoiled with the availability of 100LL, whereas in other parts of the world it is much more rare and sometimes even unavailable altogether. Since the United States has the most active general aviation community in the world, the buying market simply hasn’t made diesel pistons mainstream, yet.
Cessna announced in 2018 that they were abandoning thediesel Skyhawk to pursue other priorities. Perhaps in the future this willchange but right now there isn’t enough market force in the US driving dieselinnovation for the large manufacturers to be fully onboard.
Why Can’t Piston Aircraft Use Automotive Fuel
Avgas is often twice or more expensive than what you pay at the pump to fuel up your vehicle. Why is this? Well, there are two major differences in aviation fuel and automotive fuel: octane and lead content. Automotive fuel usually ranges from 87 to 93 octane, and does not contain lead. The most common aviation fuel (for piston engines) is 100 octane and has a small amount of lead in it. So what does this all mean?
Octane Ratings of Aviation vs. Automotive Fuel
To compare the octane ratings we first need to understand what an octane rating means. Measuring octane ratings gets pretty technical but the basic takeaway is that the higher an octane rating, the more energy it takes for the fuel to combust. Higher performance engines require higher octane fuel because they create more compression and more energy, therefore they require fuel that can withstand the increased energy and not experience pre-ignition or detonation.
Pre-Ignition
Pre-ignition can happen on its own or as a result of detonation, and occurs when the fuel mixture ignites prior to the spark plug firing. It can be caused by glowing carbon deposits within a cylinder, or other hot spots caused by detonation.
Regardless of the cause, pre-ignition will cause a severe power loss as the piston is working against itself, and the cylinder temperature can rise dramatically as the burned mixture is compressed. Since no power is extracted from the burn, all of the heat energy is absorbed by the cylinder components and can quickly cause a lot of damage.
Pre-ignition is the first reason you don’t want to use a lower rated fuel than what has been certified for the engine of your particular aircraft, and why you can’t use automotive fuel in your airplane (unless it’s built for that, discussed in a moment).
Detonation
Detonation is distinguished from pre-ignition as it does not occur before the spark plug ignites. In normal circ*mstances the fuel is supposed to burn evenly throughout the power stroke of the cylinder to providing pressure. If the entire mixture ignites all at once (i.e. detonates) it can cause overheating of the cylinder components and excessive wear.
Detonation is usually caused by using fuel with too low of an octane rating because it requires less energy to combust and therefore combusts too rapidly.
Aircraft engines are designed to be higher performance for their size and weight, increasing carrying capacity and overall engine performance. Using lower octane fuel jeopardizes the performance and safety of the engine and could lead to engine failure, which is why you can’t use automotive fuel in typical piston engines on aircraft.
Lead Content of Aviation Fuel
The second major difference of avgas is that it contains lead (the 100LL stands for 100 octane fuel with low lead).
Specifically, avgas uses a Tetraethyl Lead additive which is very effective in increasing the octane level of the fuel, thereby increasing the compression and temperature it can undergo before igniting (so that it burns evenly throughout the combustion process and does not pre-ignite or detonate).
Aircraft engines are expected to be reliable, high performance relative to their weight, and be able to perform in extremely diverse conditions. All of these requirements lend themselves towards the use of high octane fuel in high performance engines, which is why the FAA is still permitting the use of small amounts of lead in aviation fuel.
The FAA has created several different initiatives toinvestigate an unleaded replacement for aviation gasoline but has yet to find asustainable solution. As mentioned previously, diesel conversions have beenpursued but not yet found real traction in the US marketplace due to theexpense.
What About Autogas Conversions?
Tens of thousands of airplanes have been issued an STC (supplemental type certificate) for automotive fuel conversion engines that are able to use the fuel you would put in your car. These engines are specifically made to be able to handle automotive fuel and their popularity seems to be mixed in the flying community.
On the one hand, owners report similar engine performance and obviously cheaper operating costs, however automotive fuel is not yet commonplace at most FBO’s around the country.
Additionally, critics argue that the ethanol found in automotive gas both limits the life of the fuel (2-6 month shelf life vs. approximately 12 months with 100LL) and has detrimental (erosive) effects on various components within the engine.
Lastly, if and when you go to sell the airplane, some of your potential buyers might not want that kind of engine thereby limiting your buyer pool.
Conclusion
Jet A fuel consistently costs less than 100LL due to its simpler refining process, ease of transportation (via pipelines because it does not contain lead), and economies of scale from the volume of demand. And while 100LL still remains expensive, unfortunately that probably isn’t going to change anytime soon since the market hasn’t yet created a mainstream alternative to the engine or fuel itself. For now aviation remains an expensive hobby but it starts to make more sense when you dig into the reasons behind it.
While you’re on the subject… here are other articles on Airplane Academy related to fuel:
- This is Why Fuel Tanks are Located in Aircraft Wings
- The Differences Between AvGas, Jet Fuel, Auto Fuel, and Diesel
- How Does Aircraft Fuel Not Freeze During Flight?
- Why Do Aircraft Have an Unusable Amount of Fuel?
