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.

Today, mogas can contain a certain percentage (3 - 5%) 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.

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 an 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.

Documentation

Before using an alternate grade for AVgas 80/87 or even using automotive fuel gasoline in your aircraft read AC91-33A from the FAA. Opens in a new window.

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 (IWV/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 created a rather 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:

Interpreting the above test:

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).