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Incapacitated Due to Alcohol Ingestion?!?  No, Not the Pilot, the PLANE…

Mogas STCs and E10

Danielle Gibeault CFI, CFII, MEI

With pilots everywhere looking for ways to control the cost of getting aloft, this is a good time to consider whether or not that mogas STC you’ve heard about might be a good choice. For about the last two decades, pilots have been able to burn auto gas (also known as mogas) in certain aircraft with an STC. What’s an STC? It’s the type certificate issued when an applicant has received FAA approval to modify an aircraft from its original design.  You can get one through the Experimental Aircraft Association (EAA) or Peterson Aviation that allows pilots of some aircraft to utilize automotive grade gasoline. But before you get too excited, there’s something you should know…

Peterson Aviation, the EAA, and the FAA all expressly prohibit the use of alcohol in aviation fuel. So? Here’s the problem: most automotive gasoline in the United States is oxygenated. And most of the time, oxygenation is accomplished through the use of a 10% alcohol mixture commonly referred to as E10.  You read that right. That STC permits mogas only as long as it doesn’t contain alcohol, but you may find it quite difficult to get alcohol free auto gas in the US.

So why are we concerned about this? Let’s take a closer look at the problems E10 can cause. It’s well known that the ethanol in E10 seriously degrades certain types of rubbers and polymers.  Gaskets, fuel lines, and bladders may be susceptible, especially in many of the older aircraft with original equipment still installed.  Additionally, we find that soft metals like zinc, aluminum, and brass are also incompatible with ethanol.  This creates the potential for the carburetor needle valve or floats to become problematic with the introduction of E10, causing fuel starvation or flooding. And the degradation is not just limited to the fuel system. The FAA found heavy wear on the valve guides of their test engine running E85.

Vapor-locking is a problem encountered as the gas begins to boil in the fuel lines and hinders fuel flow, starving the engine of fuel. There are aircraft that were not approved for the auto gas STC because they failed the vapor-lock testing.  Why is this important? E10 has an even LOWER boiling point than standard automotive gasoline!  Since E10 increases the potential for vapor-lock even beyond what we’d already seen with mogas, we can assume that there may well have been many more aircraft that would have failed the vapor-lock testing, thus making them ineligible for the STC in the first place, had E10 been a part of the testing process.

The classic quip states that the second worst thing you can do to an aircraft engine is run it; the worst thing you can do is not to run it. There’s more truth to this than you may have realized. An aircraft may sit for weeks or even months between flights, particularly in the winter. The shelf life for avgas is generally considered to be one year.  Auto gas has the same shelf life.  E10, on the other hand, has a shelf life of only 100 days. Have you ever found yourself taking longer than 100 days to get to the bottom of a tank of gas in the coldest, foggiest, snowiest part of the winter? Exactly. This can lead to additional fuel related problems.

One of the other great dangers in using E10 is the ability of the ethanol to absorb water.  Water mixes readily with alcohol, but will not normally mix with petroleum products. Before each flight, pilots will sump the fuel tanks, gascolators, and other aircraft drains.  Since water is heavier than gasoline, it sinks to the bottom and is easily removed. When alcohol is introduced into the fuel, it will allow the mixture to absorb water into solution which will not be drained out when sumping the aircraft.  This creates two major potential problems.  Firstly, the octane rating drops. In your car, knocking is easily audible and the condition is typically only transient.  No so in an aircraft.

Secondly (and even more dangerously), we have phase separation. The amount of water E10 can hold in solution is dependent on temperature, similar to the atmosphere’s ability to hold water. The warmer it is, the more water the atmosphere and E10 can hold.  During the course of a normal flight, an aircraft can go through drastic temperature changes. If, in sumping the tanks of an aircraft with E10 in the tanks, you found and removed some water, you would likely assume that all of the water had been safely purged from the system.  In actuality, the fuel may still be saturated with water in solution, where it is invisible to the pilot.  Climbing 7000’ will cause 30 gallons of saturated E10 to squeeze 7 fluid ounces, ALMOST ONE CUP OF WATER, out of solution.  No responsible pilot would accept any water, much less this much water, in the fuel tanks.  But taking saturated E10 up to altitude will do just that.

Pilots are prohibited from flying for at least eight hours after consuming alcohol. Your airplane is also legally required to be alcohol free before taking to the skies.  But is it? Better be sure. Operating outside of the requirements of that STC (which is what you find yourself doing when using fuel that contains alcohol) sets you up to be a test pilot of the very scariest kind. Fly safe, guys, and for more information on autogas STCs, contact the EAA and Peterson Aviation.

For more on this subject, take a look at FAA Special Airworthiness Information Bulletin CE-07-06.  You may also want to check out Water Phase Separation in Oxygenated Gasoline by David Korotney, and Spark Ignition Aircraft Engine Endurance Test of Aviation-Grade Ethanol 85, FAA 2006, for detailed information.