Opposition to Autogas

Opposition to Autogas

The autogas debate is nearly 20 years old and there is still plenty of fuel for the fire. With over 50,000 STCs on the market autogas is definitely in for the long haul, but engine manufacturers and major oil companies are still preaching the dangers of autogas.

Buyer beware

Textron Lycoming has issued several service publications regarding the subject including Service Bulletin 398, Service Instruction 1070, and Service Letter 199. They clearly state Lycoming's position on the subject, and some of their reasoning follows:

Although STC's now make the use of automotive fuel, which meets minimum specified standards, legal for use in some aircraft, reciprocating engine manufacturers and most major oil companies do not approve. While it is true that octane levels appear adequate, these organizations are of the opinion that the varying quality control standards applicable to automobile gasoline produce undue risk when it is used in aircraft. Several specific reasons are given for the non-approval of automobile fuel:

1. Its use reduces safety. Although an operator may find that the engine runs well on a specific grade of auto fuel, there is no assurance that fuel from the same tank will be of the same quality when purchased the next time. Risk is increased.

2. Its use can void warranty, or result in cancellation of the owner's insurance.

3. The storage characteristics of automotive fuel are less desirable in comparison with the good storage characteristics of aviation gasoline. After several months, stored automotive fuel may suffer loss of octane rating, and tends to deteriorate into hard starting, along with forming gum deposits that cause sticking exhaust and intake valves, and fuel metering problems, resulting in rough running engines. The turnover of automotive fuel is so fast that long-lasting storage characteristics are not required.

4. The additives in automotive fuels are chemically different from those designed for aviation, and contain auxiliary scavengers which are very corrosive, and under continued use can lead to exhaust valve failures. They also cause rust and corrosion in the internal parts of the engine. The allowable additives for aviation gasoline are rigidly tested and controlled. There is no uniform control of additives in automotive gasoline. Many different additives are used, depending on the fuel manufacturer. For example, one fuel company adds a detergent to clean carburetors. This additive creates a significant increase in the affinity of the gasoline for water which can cause fuel filter icing problems in flight if outside temperatures are cold enough.

5. Automotive fuels have higher vapor pressures than aviation fuel. This can lead to vapor lock during flight because the fuel companies advise that automotive fuels can have double the vapor lock pressures of aviation gasoline, depending on the seasons of the year, and the location because of climatic conditions. In addition, automotive fuel also increases the possibility of vapor lock on the ground with a warm engine on a hot day.

6. Although the fuel octane numbers shown on the pump of automotive and aircraft gasolines may be similar, the actual octane ratings are not comparable due to the different methods used to rate the two types of fuels. Furthermore, aviation gasolines have a lean and rich rating, i.e. 100/130, whereas motor gas is not tested for a rich rating.

7. Automotive fuel used in an aircraft engine may lead to destructive detonation or pre-ignition and potential engine failure at high power conditions.

8. The actual Mo-Gas fuel requirements range from meeting ASTM or government specifications only in six states, to few or no minimum requirements in the remaining 44 states, as opposed to the uniform strict requirements for aviation fuel.

Manufacturer warnings

Teledyne Continental Motors (TCM) is also warning their customers away from using autogas.

TCM Bulletin M77-3 reads:

TCM does not recommend or authorize the use of automotive fuels in any of their aircraft 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-5572E, if satisfactory engine service life is anticipated.

Automotive fuels can contain additives that act as corrosive agents, formulate gum deposits and , therefore, 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 fuels 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/failure.

Weighing the risks

Auto fuel is now being used as a substitute for Grade 80 aviation gasoline under STC's issued by the FAA. Operators should consider the risks of using auto fuel in aircraft when deciding whether or not to switch. After all, a pilot can't pull over to the side of the road when fuel creates a problem with the engine.