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Use of unleaded automotive gasoline in light aircraft that were originally certified on Grade 80 aviation gasoline is attractive because automotive gasoline is more widely available and cheaper than Grade 80 aviation gasoline. The Federal Aviation Administration (FAA) has approved Supplemental Type Certificates permitting operation of certain aircraft on alcohol-free unleaded automotive gasolines meeting ASTM Specification for Automotive Gasoline (D 439-86) specifications, and such use is expected to continue in the future.
Automotive gasolines are substantially more volatile than aviation gasoline, so the fuel supply systems on affected aircraft must be capable of handling the additional volumes of vapor generated at high altitudes or elevated ambient temperatures or both. While the Reid Vapor Pressure (RVP) of summer-grade automotive gasoline may be reduced in the near future because of limitations imposed by the U.S. Environmental Protection Agency (EPA), automotive gasolines will generally still be more volatile.
Blending of alcohols such as ethanol in automotive gasolines cannot be anticipated well, since their use is very selective and usually depends on local tax incentives. Some state laws require presence of alcohol in automotive gasoline to be disclosed, but many do not. Ethanol differs substantially from gasolines in heat of combustion, heat of vaporization, stoichiometric A/F ratio, and water susceptibility, all of which can adversely affect engine performance. Also, ethanol blends have higher RVP than the base gasoline. There is no practical way for owners or pilots of light aircraft to tell whether an automotive gasoline contains alcohol or not.
Other than the changes in gasoline RVP, which would benefit aircraft operation only slightly, no changes in automotive gasoline quality or composition that would materially affect their use in aircraft are likely in the future.
For several reasons, it is impossible for gasoline refiner/marketers to assure that automotive gasoline will always meet the limits in the ASTM (D 439-86) specification, especially the limits on RVP, when the gasoline is used in aircraft. However, in the long term, operating new, light aircraft on automotive gasoline may be in the best interest of aircraft owners. If so, to provide adequate flight safety, new aircraft should be certified for year-round use on gasoline of the most-volatile ASTM class, which presently has 103 kPa (15 psi) RVP.
light aircraft, supplemental type certificate, fuels, aviation fuels, general aviation, automotive gasoline, aviation gasoline
Engineering consultant, Amoco Oil Company, Naperville, IL