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One of the main reasons the common avgas grades are still full of 3-plumba-3,3-diethylpentane is the incompatibility of many aircraft fuel systems with the oxygenates (primarily ethanol, with some ETBE) used as octane boosters in mogas, as mentioned (for instance) here:

[...] 87UL/E0 (similar fuels are called 82UL or 85UL) -- This is a tightly specified form of motor gasoline (mogas) -- without any ethanol or oxygenates in it, as many aircraft fuel systems aren't compatible with ethanol -- that is used in low-compression aviation piston engines mounted in airframes certified or STCed for operation on motor gasoline. [...]

Bolding in original; emphatic italics added.

But the automotive industry had the exact same problem, and, yet, adapted easily to the advent of alcoholic unleaded gasoline (namely, by replacing incompatible fuel lines and seals and whatnot with ones not dissolved by ethanol). So why are oxygenates so much more of a stumbling block for aircraft fuel systems and engines?

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The modifications required are just not economically viable.

The main issue is vapor lock, because ethanol has a lower vapor pressure (i.e. more likely to vaporize) than gasoline. Worse, mogas ethanol levels are varied to achieve ideal vapor pressure for different temperatures. This is fine for cars that go through a tank of gas every week or two, over a relatively small temperature range, but a serious problem for planes where the gas may sit for months and then experience a wide temperature range at different altitudes in a single flight.

Vapor pressure matters because engine-driven fuel pumps, which are the norm on planes, pull fuel through the fuel lines. Especially when combined with low wing tanks, ethanol may vaporize in the lines and block any further fuel flow, which is a far more serious problem for a plane than for a car. Pure gasoline has a high enough vapor pressure that doesn't happen. When ethanol was first added to mogas, carmakers solved this by moving the fuel pumps to the tank so the fuel is instead pushed through the fuel lines and therefore won't vaporize regardless of its vapor pressure, and given normal fleet turnover, that was the norm within a few years. Some planes do have tank boost pumps like cars, but they are usually not intended to be run full-time; ethanol would require tank boost pumps that can run full time, which means either replacing the existing ones or adding new ones.

Ethanol is also more corrosive than gasoline. Using ethanol regularly may mean replacing fuel pumps (both engine and, if applicable, tank), fuel lines, seals, injectors or carbs, etc. To make such modifications, you'll need an STC for each. Developing an STC is expensive and the market is quite limited, so the companies selling them need to charge a lot to recover their costs--on top of the high cost of the parts themselves, which have the same issues. Also, while many engines were retroactively rated for mogas (even with ethanol in some cases), you still need an expensive STC to change the old placards that require avgas.

Also, while not directly related to ethanol, mogas doesn't contain lead. Older airplane engines still require a tank of leaded avgas occasionally to lubricate valve seats and guides. That can usually be fixed by replacing certain parts with newer ones made from newer materials, but that's not cheap either and usually best delayed until the next overhaul. Until then, you can't run mogas full-time, regardless of the ethanol content.

Once you add up all these costs, retrofitting most GA aircraft for ethanol could easily end up being more than the planes are worth.

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  • $\begingroup$ Don't we have to worry about alchohol attracting water also? Engines don't run well when they get a slug of water going thru the system. en.wikipedia.org/wiki/Dry_gas $\endgroup$ – CrossRoads Apr 13 at 14:29
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    $\begingroup$ Water is a problem with pure gasoline because they separate, so the engine receives a slug of pure water and dies suddenly. Adding ethanol makes gasoline and water miscible, so the engine is still getting mostly fuel and keeps running, though a bit rough like it's too far lean of peak. That's not desirable from a maintenance perspective, of course, but it's less likely to kill you. $\endgroup$ – StephenS Apr 13 at 14:53
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    $\begingroup$ Cars are neither replaced as quick, nor use up the fuel as quick. Faster than GA aircraft probably yes, but there are many people who only burn through a couple of tanks of gas a year, and plenty of twenty year old cars around. $\endgroup$ – Jan Hudec Apr 13 at 18:55
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    $\begingroup$ @ZeissIkon the car industry "adapted readily" because they were given no choice in the matter. Adapt or die was basically the message. With light aircraft (now the only gasoline powered ones in common use) it'd mean death for general aviation as replacing the entire fleet over the span of a few years would be impossible. $\endgroup$ – jwenting Apr 15 at 4:32
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    $\begingroup$ @ZeissIkon, I mean vs. what you claimed in the comment. $\endgroup$ – Jan Hudec Apr 15 at 16:32
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The main issue, the one that bans ethanol blend fuel for use under the autofuel STC, is the effects of ethanol on rubber components in the fuel system. Ethanol is also mildly corrosive to aluminum but nowhere near as bad as methanol, which will dissolve a fuel tank in days (methanol powered race cars race and then purge their fuel systems right away).

10% ethanol in a fuel tank won't generally hurt it. If you install ethanol compatible hoses and seals (especially the carburetor float seal) you can run ethanol blend fuel mogas and the airplane won't care, notwithstanding the other issues like the water absorption tendency. This is an option for homebuilts that aren't restricted in materials used in the fuel system, and can be a good safety measure against accidental adding of mogas with ethanol.

There is a quickie test for finding ethanol in gas. Add fuel to a test tube like container, add some water, which goes to the bottom, mark the water line, then shake the tube. After the mix settles out, if the amount of water has magically increased, that's water plus ethanol mixed. I do that test if I'm using mogas from an unconfirmed source.

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  • $\begingroup$ So why can't the compatible-materials solution be used for all piston aircraft, instead of just homebuilts? $\endgroup$ – Sean Apr 14 at 4:18
  • $\begingroup$ Also, where did you find mogas that doesn't have ethanol in it? $\endgroup$ – Sean Apr 14 at 4:19
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    $\begingroup$ @sean that depends on where you are. In my country mogas without ethanol is available at twice the price of mogas with ethanol. $\endgroup$ – vasin1987 Apr 14 at 14:10
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    $\begingroup$ You can only use approved materials so it depends on if you can find them. For example, the MIL-SPEC that hoses are made to doesn't make alcohol resistance a requirement because they assume the use of avgas. Traditionally, premium fuel is ethanol free, at least in Canada. $\endgroup$ – John K Apr 14 at 15:20
  • $\begingroup$ @vasin1987: Which country is that? $\endgroup$ – Sean Apr 16 at 19:22
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It's not just big twins that have "high compression" or use turbos. Many four and six seat engines have higher compression engines also without turbos. My Lycoming O-360-A1F6D has no turbo. has 8.5:1 compression ratio cylinders and needs 100LL to prevent early detonation.

https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO%20%26%20TIO-360%20Oper%20Manual%2060297-12.pdf

O-360-A series

FAA Type Certificate ...................................... 286

Rated horsepower ......................................... 180

Rated speed, RPM.......................................... 2700

Bore, inches.................................................... 5.125

Stroke, inches.................................................. 4.375

Displacement, cubic inches............................. 361.0

Compression ratio .......................................... 8.5:1

(The -A1F6D indicates things like hollow shaft for oil pressure to drive the constant speed propeller via governor, counterweights on the crankcase to balance the cylinders, dual magneto vs 2 separate magnetos).

And if you look at page pdf page 42 (marked as 3-10), almost all the O-360 engines need higher octane fuel:

FUEL AND OIL *Aviation Grade Fuel

Model Series Minimum Grade

O-360-B, -D 80/87

O-360-A1P, -C1F, -C4F; HO-360-C1A 91/96

O-360-C, -F; HO-360-A, -B; IO-360-B, -E; HIO-360-B 91/96 or 100/130

O-360-J2A 91/96 or 100/100LL

IO-360-L2A, -M1A, -M1B 91/96 or 100LL

HIO-360-G1A 91/96 or 100LL

O-360-A, -C1G, -C4P, -A1H6; TIO-360-C1A6D 100/100LL

IO-360-B1G6, -C1G6, -J, -K2A, -A1D6D, -A3B6, -A3D6D;

HIO-360-A1B 100/100LL

AIO-360-A, -B; IO-360-A, -C, -D, -F 100/130

HIO-360-A, -C, -D, -E, -F 100/130

TIO-360-A 100/130

The fuel injected version used in the retractable gear version of my plane. IO-360Axxx, has higher ratio cylinders, 8.7:1 and develops 15-20 more horsepower.

Here's a little more info on Leaded Aviation fuel, from https://www.faa.gov/about/initiatives/avgas/

FAA Home ▸ About FAA ▸ Programs & Initiatives ▸ Aviation Gasoline Aviation Gasoline About Aviation Gasoline Subscribe Share Print The Federal Aviation Administration (FAA) shares the Environmental Protection Agency's (EPA) concerns about lead emissions from small aircraft. Owners and operators of more than 167,000 piston-engine aircraft operating in the United States rely on aviation gasoline (avgas) to power their aircraft. Avgas is the only remaining lead-containing transportation fuel. Lead in avgas prevents damaging engine knock, or detonation, that can result in a sudden engine failure. Lead is a toxic substance that can be inhaled or absorbed in the bloodstream, and the FAA and EPA and industry are partnering to remove it from avgas. Avgas emissions have become the largest contributor to the relatively low levels of lead emissions produced in this country.

To help "get the lead out," FAA is supporting the research of alternate fuels at our William J. Hughes Technical Center in Atlantic City. We are working with the aircraft and engine manufacturers, fuel producers, the EPA and industry associations to overcome technical and logistical challenges to developing and deploying a new, unleaded fuel.

The FAA continues to work with EPA to make this a smooth transition and to ensure the supply of aviation gasoline is not interrupted, and that all aircraft can continue to fly.

This follow-up article has more info:

The unleaded fuel disaster - what it means for pilots | Air Facts Journal https://airfactsjournal.com/2018/11/the-unleaded-fuel-disaster-what-it-means-for-pilots/

The new timeline focuses on testing of an unleaded fuel being developed by Shell—now the lone participant in PAFI following the elimination of Swift Fuels’ candidate fuel.

Another article, from AOPA https://www.aopa.org/news-and-media/all-news/2018/october/03/faa-sees-mid-2020-completion-of-unleaded-avgas-project

The FAA had halted testing last spring, calling for Shell and Swift to address concerns that emerged in data gathered in early phase tests. In August, following a meeting of the PAFI Steering Group at AOPA Headquarters, AOPA reported on PAFI’s continued progress and plans going forward.

Before flight testing under PAFI resumes, preliminary work “will include clearing material compatibility, durability, detonation, and performance issues,” the FAA said.

Addressing the extension of testing into 2020, the FAA reiterated that “the PAFI mission endures,” noting, “although it will take additional time to realize this goal, it is essential to ensure a viable, safe, and economical fuel is ultimately authorized.”

“AOPA is encouraged to see Shell’s continued commitment and efforts to work on issues and come up with mitigations,” said David Oord, AOPA senior director of regulatory affairs. “We and the other members of the PAFI steering group were pleased to see that early results from their efforts look promising.”

Oord reiterated continued support of the government-industry efforts to identify, test, and authorize a general aviation fleetwide unleaded replacement for 100LL avgas, noting AOPA’s longstanding position that the resulting fuel must not require extensive changes to engines, aircraft systems, or fuel delivery systems.

“Ultimately, we are looking for the best fuel, whether it results from PAFI or other means.The goal is the same, regardless of the path we take to get there.”

I recall seeing a (or some) articles during the Feb 2019 Gov't shutdown saying testing was suspended for an unknown time, so unleaded fuel will not be available for a while.

The higher horse power engines, like the O-540 and IO-540, are 6-cylinder versions of my 4 cylinder engine. So if mine has 180HP/4 cylinders, ~ 45 HP/cylinder, than an O-540 is generating ~ 6x45 = 270HP, and and IO-540 ~ 50 HP/cyl = 300 HP. Those can be found on 6-seat planes, and the bigger twins. We all have the same need for 100LL fuel. My home airport has stopped carrying 87 octave mogas, I presume that demand has dropped, and only carries 100 LL now. I can't even recall the last airport I was at that had 87 mogas.

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    $\begingroup$ Lots of good detail about octane/AKI, compression ratios and lead, but this question is about the italicized part of the quote saying airplanes aren't compatible with ethanol. $\endgroup$ – StephenS Apr 15 at 17:20
  • $\begingroup$ Yes, but the alcohol/ethanol had been hashed out already. And this didn't fit in a comment. $\endgroup$ – CrossRoads Apr 15 at 17:25
  • $\begingroup$ No turbo high compression 100LL engines run fine on 92 or 94 octane mogas. A glider club I used to be in has many thousands of hours on premium mogas in O-360s. $\endgroup$ – John K Apr 16 at 19:40
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There are actually several different things affecting this.

The usage of aviation gasoline

When it comes to aviation gasoline, most volume is used by high-performance engines having turbos. Examples are twin engine airplanes used in line operations. These are made for the higher Octane number and cannot run on lower Octane numbers. (As far as I know, it is currently not possible to create gasoline with high enough Octane numbers for these Engines without using lead, but I might be wrong there).

As these engines are the main volume users and they require high octane fuel, you will find that on "larger" airports Avgas 100LL is readily available. Most smaller planes are certified to use this fuel and, well, as it is available they do. Some airports may offer additional fuel choices, but not necessarily at a lower prices.

In order to run on different fuels you need a STC

A STC (Supplemental type certificate) will cost money to create (sometimes payed for by the aircraft manufacturer) and might require extra costs for the airplane owner. You might need to change some parts in the engine, fuel lines, fuel tanks and so on. World-wide avgas or non-leaded fuel is not necessarily less expensive then 100LL so you might not recoup your investment.

Temperature and elevation

Gasoline is a mixture of several different components. Aviation gasoline is tightly specified to work in low temperatures and at high elevations. Normal domestic fuel is less tightly specified and might evaporate at high elevation and/or freeze at low temperatures. The mix often varies between summer and Winter to keep production costs low.

The way forward Of course, in due time this might change. Sometimes you can get an Aviation fuel supplier to complement with one of the non-leaded fuel specifications. We did this several years ago at the local glider field and as well as the local municipal Airport (ESSB). The longer time hope is to go for electrical airplanes, but when that happens we will see.

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  • $\begingroup$ This doesn't really answer the question of why Alcohol fuels aren't used in aviation. $\endgroup$ – OSUZorba Apr 16 at 3:31
  • $\begingroup$ Sorry. I thought it was clear: 1) it is not currently available 2) to make it available has a cost 3) it might not work -- additionally even if was available it might not save any money for the owner. $\endgroup$ – ghellquist Apr 16 at 10:30

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