Aviation gasoline (Avgas) and automobile gasoline (Mogas) are different in many ways. Aside from the octane rating systems, which are very different, they also can have a radically different fuel vapor pressure. Most people have heard of a car engine stalling due to “vapor lock.” This occurs when the fuel vaporizes — turns from a liquid to a vapor — in the fuel lines or the carburetor. This bubble of gas vapor prevents the fuel pump from pumping fuel to the engine, so it quits.
Aircraft operate in extreme environments. You can take off from Palm Springs when it is 110 degrees on the ground, and in five minutes be up where the outside temperature is below freezing. The fuel has to flow in a reliable manner. Avgas is manufactured to have a vapor pressure of 6.5 pounds per square inch (psi), regardless of the time of year or the area of the country. Mogas vapor pressures can vary between 7 and 15 psi, depending upon the season and location.
In order to use mogas in an aircraft, one has to first obtain a supplemental type certificate (STC). To obtain an STC for mogas, the operator has to demonstrate that the airplane will operate safely on mogas. The fuel vapor pressure is not the only issue. Additives in the mogas must be compatible with the engine and other components. For instance some mogas has alcohol added, and that can damage fuel bladders and gaskets. Some aircraft engines rely upon the higher lead content in avgas to lubricate exhaust valves. A particular aircraft engine can be approved for use of mogas, while a particular aircraft with that engine can not be approved. This is usually due to vapor lock problems. Low wing aircraft, like the Beech 19, which rely upon a fuel pump to provide fuel to the engine, have difficulties obtaining a mogas STC.
Charles Chambers operated a Beech 19 Musketeer. There are no STC’s to use mogas in that airplane. This is not from a lack of trying to obtain a mogas STC, it is because testing showed that particular aircraft cannot operate safely on mogas. With that in mind, here is what Mr, Chamber admitted to doing on a post on BajaNomads, one year prior to the fatal accident which killed him and two others.
|“Actually I do not have an stc for mo-gas…”
Click on image for enlarge.
Here is a photo of Mr. Chambers Beech 19 being refueled with mogas at Scorpion Bay, Baja California immediately prior to his departure on what turned out to be his last flight.
|Refueling N6064N from a gas can with automobile fuel|