Aviation From My Rocking Chair: Auto Fuel and Octanes. Is It Smoke and Mirrors? Part 3

Hello again troopers, I’ll try to finish the discussion about auto-fuel in airplanes this month. I would remind you this original article was written in 2002, so the numbers will not be the same. At the end of our exercise, I’ll bring it up to date. I think a good news/bad news format might work so…first the bad news (everyone likes to end on a good note.)

(1) Auto-fuels often contain oxygenators during winter months (So-Cal has them all year). These chemicals are often alcohol, although MTBE or ETBE (ethers) are also used. Alcohol has a great affinity for water and can hold amounts in solution sufficient to cause pre-ignition and detonation due to degraded octane ratings. Corrosion on internal engine parts can also be a problem. MTBE and ETBE do not have the same problems, but they cause pollution to ground water and may not be available in auto-fuel for the future. (2) Alcohol and Aromatics (Ether) tend to dry rubber O rings and Neoprene seals or components. This drying causes seals to swell and deteriorate faster with auto-fuel. O rings and rubber parts must be changed on a regular basis. No more “ Wait until it fails.”

(3) With older engines (especially radials) lack of Tetraethyl Lead cushion causes increased wear to exhaust valves and valve seats.

(4) Unless auto-fuel is available on the airport, you must transport it to your aircraft. (5) Fuel quality is the direct responsibility of the pilot/owner. This may require testing for Octane, Water in fuel, and Vapor Pressure. Note: If alcohol is present in fuel, water will be in solution and not show up in a sample as it would with Avgas.

(6) Manufacturers warranties may be invalid if using auto-fuel.

(7) On a certified aircraft an STC must be purchased in order to legally use auto-fuels. Not all aircraft are approved.

(8) Range when using oxygenated fuels is reduced by 3-5 % due to BTU output levels lower than Avgas. Are there enough advantages to outweigh these problems? You must be the judge. Some advantages are: (1) Cost! On 9/6/01 fillupflyer.com reported auto-fuel sold on airports for $1.88 per gal And the average cost of 100LL was $ 2.53 per. Gal. A difference of $0.65 per gal. The FAA finds the average single engine piston aircraft consumes 10.8 gal/hr of fuel. They also report the average yearly hours flown is 70. The EAA, using a more conservative 50 hours a year, has determined the average amount of money saved per aircraft/per year is $325.00 (50 hrs x10gal = 500gal x .65 = $325.00).

(2) Fuel is available everywhere, even at “No Service” airports.

(3) Once modified for no lead fuels, both the aircraft and the engine will be ready when 100LL goes away. Let me update the above information as of 3/1/21.

Avgas (100LL) at Sequim Valley was $5.09 per gal. The Sequim CO-OP was selling 93 octane no alcohol for $3.80 per gal. A difference of $1.29 per gal. 100LL cost ($5.09 x 10 x50 = $2545). 93 octane auto-gas cost ($3.80 x 10 x50 = $1900). The difference is $645 per year. Also note that unlike #2 above, fuel is not available everywhere. There are only two places that sell no alcohol premium in Sequim. Personally, I did not believe in 2002, nor do I believe in 2021, that the cost difference between 100LL and auto-fuel is sufficient to outweigh the increased maintenance and fuel quality problems. That said, I think the EAA deserves a round of applause for giving us the choice.

Now, before I check out for another month, pleases read Ben Vissor’s column in the February 18, 2021, issue of General Aviation News titled “Still no answer on unleaded avgas.”

Regards to all and remember brown side down and blue side up.

Rick Vaux. TC4130