Aviation From My Rocking Chair: Auto Fuel and Octanes. Is It Smoke and Mirrors? Part 2

As promised last month, this time we will let two of the major engine makers, Lycoming and Continental, weigh in on the auto fuel controversy. I originally wrote this article in 2002 and I am stunned by it’s relevance 19 years later. As far as I can tell, very little has been changed. In 2002 there were 50,000 auto-gas STC’d aircraft flying (many more today), but reciprocating engine manufacturers and most major oil companies do not approve the use of auto gas in airplanes. The problem is not so much Octane rating (which appears to be adequate) but automotive fuel quality control standards which may vary enough to put aircraft at risk (read LIABILITY)

Lycoming Service Bulletin 398, Service Instruction 1070, and Service Letter 199 all refer to the company’s position on using auto fuel in their products. Specific reasons for non-approval are:

• (1) Its use reduces safety. Fuel quality may not be the same from supplier to supplier, or from day to day, even though the same specific grade is purchased.
• (2) It’s use can void warranty or result in cancellation of the owners insurance.
• (3) Storage characteristics of auto fuel are less desirable than Avgas. After several months, auto fuel may lose octane rating, cause hard starting, and form gum deposits that cause sticking valves.
• (4) Additives in auto fuels are chemically different from those in Avgas. They contain scavengers which are very corrosive and under continued use can lead to exhaust valve failures. They can also cause rust and corrosion on internal engine parts. Note: One fuel company adds a detergent to clean carburetors which creates an affinity for water in gasoline. This can cause fuel filter icing problems at low temperatures.
• (5) Auto fuels have higher vapor pressure (sometimes double!) than Avgas, which can cause vapor lock during flight.
• (6) Even though octane ratings on Auto gas and Avgas appear to be the same, ACTUAL octane is not due to different rating methods for the two fuel types. Avgas has a Rich and Lean rating (i.e. 100/130) while Auto gas is not tested for a Rich rating.
• (7) Auto gas used in an aircraft engine may lead to pre-ignition and destructive detonation.
• (8) Auto gas fuel requirements range from meeting ASTM or government specifications in some states to few or none in others. Avgas has uniform requirements in all states. Whew! That was Lycoming’s take on the Mogas v Avgas problem. Here is what Teledyne Continental TCM bulletin M77-9 has to say: “TCM does not recommend or authorize the use of Automotive fuel in any of their engines. The engine warranty and Pro-Rata policy will be voided if such fuels are utilized. Fuels must conform to ASTM-D910 or MIL-G-5572F if satisfactory engine service life is anticipated. Automotive fuel can contain additives that act as corrosive agents, formulate gum deposits, and increase combustion chamber deposits. Continued operation on automotive fuel can lead to detonation, pre-ignition, and sticking or eroded valves. The vapor pressure of automotive fuel exceeds that allowable for aviation fuels. This increased vapor pressure increases the tendency to vapor lock at higher altitudes. A vapor lock condition can cause complete power loss. The use of any fuel that does not conform to the above specifications may abuse cylinder assembly, valve, piston, and/or piston ring damage/failures.” As can be seen, the engine people are quite adamant about not using auto fuel for anything except cleaning parts! O.K. Troopers, next month I’ll attempt to show the other side of this argument. Until then, blue side up and brown side down. Rick Vaux TC 4130