Common Aviation Fuels, Mogas
Experimental aircraft commonly use engines which consume AVgas (Lycoming / Continental / Franklin and equivalent types) or engines running Mogas (Rotax, Subaru etc). Some engines are capable, or modified to run either fuel; although some with restrictions or maintenance limitations.
Adding bio-alcohol (or ethanol) oxygenates the fuel (adds oxygen to the fuel) resulting in a more complete fuel burn and reduces the smog forming and emissions harmful for the ozone layer. But the drawbacks are possible damage to rubber gaskets and composite fuel tanks as alcohol is a solvent.
Today, mogas can contain a certain percentage (3 - 5%, going up to 10% or more) of methanol or ethanol (bio-alcohol) and the possible effect of using that fuel in aviation engines must be considered by the pilot and that is the subject here.
Another thing is that the composition of mogas (even the amount of acohol) varies each and every day, whereas avgas always has the same specified composition.
Mogas in Aircraft Engines
Generally, mogas is cheaper than avgas (Europe) which is one of the reasons pilots of experimental aircraft wanting to use that fuel. Even some manufacturers advise users to mainly use mogas in their engines but that avgas can be used too. The Rotax four stroke models (912, 914) will run happily on avgas, but due to the higher lead content oil changes must be done every 50 hours and oil must be mineral or semi synthetic.
Nowadays, Lycoming is developing engines capable of running avgas and mogas. Mainly because of the anticipated reduced availability of avgas.
Running an aircraft engine on mogas can introduce unwanted and unexpected side effects because of the possible blending with bio-alcohol. Gasoline pumps should be labeled as such if the fuel the pump dispenses contains bio-alcohol so that the buyer of the fuel is warned.
Adverse effects
Bio-alcohol attracts, carries and retains water, this has number of side effects in the engine: bio-alcohol is absorbed by the water and is difficult to detect (see below). After engine shutdown water can lead to corrosion on vital parts. Water will freeze in cold conditions and during carburetion. Water also causes vapor lock and lowers the vaporization point of fuel. If the aircraft is then flown at higher altitudes (also favouring vapor lock) this could lead to an engine failure at the most unexpected time.
Other issues with bio-alcohol blended fuels are: lack of lubricity; alcohol is a solvent and could clean deposits in the fuel system and carry them to the filters clogging them. Bio-alcohol also burns leaner and may cause an increase in exhaust gas temperatures and possibly exhaust valve problems.
Composite fuel tanks
Some aircraft have composite tanks and the alcohol component in mogas slowly attacks these composites thereby dissolving the tank structure resulting in leaks. The solution is to either use a coating to protect the inside of the tank or use metal fuel tanks altogether. Make sure that all seal are resistant against alcohol. Vinylester resins have a much better resistance against chemical products.
Some aircraft manufacturers are advising against mogas and recommend avgas. But using avgas results in more maintenance for the Rotax engine due to the higher lead content and higher fuel cost for the aircraft owner.
Documentation
Before using an alternate grade for AVgas 80/87 or even using automotive fuel gasoline in your aircraft read Advisory Circular 91-33A or SAIB 07-06 from the FAA.
AVweb wrote an article on the use of Mogas in aircraft engines. The EAA has dedicated a website on aviation fuels.
The Dutch Aviation Authorities (IVW/DL) published a document about the use of Mogas containing bio-alcohol, very interesting for those running mogas with Rotax or any other aviation engines which may be run on mogas.
Mogas types and properties
Chevron has published an indepth technical document about Motor Gasolines engines and mogas properties. A must read if you want to know it all about mogas.
Testing for Alcohol in Mogas
The following steps describe how to test to see if there is any alcohol in your mogas:
- Using a glass or chemical resistant plastic (such as TPX) container, mark ten equally spaced volumes. A graduated cylinder is ideal; however, a non tapered glass jar, such as a large (quart) olive bottle, will work.
- Add one part of water (approximately 100 ml) into the container, fill to the first mark, and then add nine parts (approximately 900 ml) of automotive gasoline, fill to the top mark. Shake thoroughly, let stand for 10 minutes or until automotive gasoline is again bright and clear. Record the apparent level of the line between the automotive gasoline and water.
Interpreting the above test:
- If alcohol is present in the automotive gasoline, the water will absorb it, and the amount of water will appear to increase, indicating the automotive gasoline should not be used in the aircraft.
- However, if the water level remains the same, no alcohol is present in the automotive gasoline and it can be used in the aircraft.
Operators of aircraft approved for operation with Mogas containing methanol or ethanol shall consider the lower energy content of such a fuel (which could result in a lower performance or higher fuel consumption).
