So, if you have a low-compression engine, can you just fill it up with autogas and take off? Nope, you've got to get an appropriate STC -- and despite what you may have heard elsewhere, it is very important to get that STC, even though it usually will consist of one or two pieces of paper, plus new decals for your fuel ports.
Why is the STC important? While unleaded autogas provides sufficient octane to substitute for 80/87 avgas in low-compression engines, there are other differences that can cause problems when using autogas in some engine installations. The two most significant are lower vapor pressure -- which can lead to vapor lock -- and incompatibility between some of the additives in autogas and some components (particularly seals) in some aircraft fuel systems.
In order to qualify for an STC, a particular airframe/engine combination has to be rigorously tested, to include either a 150 hour engine endurance test or 500 hour flight test, under controlled conditions. The tests also include checking operation at high ambient temperatures, which can create vapor lock. Some aircraft don't pass -- the Piper Apache and Comanche-250, and Cessna Skyhawk with Avcon's 180HP conversion all failed testing, and cannot legally run autogas.
In a nutshell, by buying the STC you are paying for a bunch of research and testing to verify that it really is safe to use autogas in the airframe/engine combination you have. In a few cases, you may be required to have modifications made or the STC may authorize only premium (91 octane or higher) autogas. For example, Petersen Aviation's STC for Piper PA-28-160, -161, -180, and -181 models requires replacing the electric boost pump and running premium gas.
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So, if you have a low-compression engine, can you just fill it up with autogas and take off? Nope, you've got to get an appropriate STC -- and despite what you may have heard elsewhere, it is very important to get that STC, even though it usually will consist of one or two pieces of paper, plus new decals for your fuel ports.
Why is the STC important? While unleaded autogas provides sufficient octane to substitute for 80/87 avgas in low-compression engines, there are other differences that can cause problems when using autogas in some engine installations. The two most significant are lower vapor pressure -- which can lead to vapor lock -- and incompatibility between some of the additives in autogas and some components (particularly seals) in some aircraft fuel systems.
In order to qualify for an STC, a particular airframe/engine combination has to be rigorously tested, to include either a 150 hour engine endurance test or 500 hour flight test, under controlled conditions. The tests also include checking operation at high ambient temperatures, which can create vapor lock. Some aircraft don't pass -- the Piper Apache and Comanche-250, and Cessna Skyhawk with Avcon's 180HP conversion all failed testing, and cannot legally run autogas.
In a nutshell, by buying the STC you are paying for a bunch of research and testing to verify that it really is safe to use autogas in the airframe/engine combination you have. In a few cases, you may be required to have modifications made or the STC may authorize only premium (91 octane or higher) autogas. For example, Petersen Aviation's STC for Piper PA-28-160, -161, -180, and -181 models requires replacing the electric boost pump and running premium gas.