Can You Put Automobile Gas in Your Aircraft?

Every student pilot hears this dire warning about using automobile gas in an aircraft engine that was not certified for it. These warnings are usually accompanied by a story about engine failure and an unscheduled off-airport landing, which often occurs shortly after takeoff.

While it is true that the use of automobile fuel in an airplane can be a tricky business, it is unlikely that engine failure will happen so quickly, says Richard Scarbrough, an airframe and powerplant mechanic and aircraft maintenance columnist for FLYING. 

"That's not to say that the use of auto gas couldn't create issues down the road for the aircraft," he says, explaining, "100LL supplies the engine with lubrication through the lead, which auto gas doesn't." 

But with the high price of avgas, is it worth the risk? If it is, how would you do it?

Auto fuel is usually less expensive than aviation fuel. But, because it usually comes in at a lower octane, (80 versus 87), the consequence will be lower compression, as engines designed with higher compression ratios usually need higher octane fuel to avoid detonation. To mitigate these challenges, many pilots run a mixture of 100LL and auto gas in their aircraft.

Scarbrough notes that in order to legally use auto gas in an aircraft, the aircraft must have a supplemental type certificate (STC), which is a kind of type certificate issued by the FAA that gives the applicant approval to modify an aeronautical product from its original design.

"If you are flying without an STC and the feds see you have auto gas in an airplane not certified for it, it is a violation," he says.  

Such violations depend on who catches you—it could be as little as “education” on the spot or a warning notice or letter of correction, which indicates the corrective action a pilot must take within a certain timeframe.

The STC is obtained from a provider—a business that specializes in the STC—usually for a few hundred dollars.

"Most aircraft owners are glad to pay that for the fuel STC because they will make up the cost of the STC with one or two fill-ups," Scarbrough explains.

Aircraft owners who wish to take advantage of the STC must file a Form 337 (major repair or alteration) with the FAA and in addition, placard the aircraft appropriately.

Obtaining an STC can be both time-consuming and expensive, but for some aircraft operators, the need to modify the aircraft comes out of a sense of necessity. Such was the case for Petersen Aviation in Minden, Nebraska.

Since 1983, Petersen Aviation has been supplying an auto fuel STC for 48 engine types. According to Todd Petersen, before 1983, the family business sprayed crops using AgCats, and the high cost of 100LL and difficulty getting a reliable source inspired them to look into the idea of running the aircraft on auto fuel. They realized they were not the only pilots facing this dilemma, and the STC business was born.

According to Petersen, the STC process required them to demonstrate to the FAA that auto fuel would not adversely impact either the aircraft or the engine itself. It is a time-consuming process, he notes, adding that the FAA sees many applications for STCs every year that aren’t completed for the amount of research and testing and paperwork the applicant has to do.

"To get a supplemental type certificate to run an aircraft on auto fuel, you actually need two supplemental type certificates—one for the airplane, the other for the engine," Petersen says.

The testing for the fuel STC involves running the particular engine on auto fuel and determining if there are any adverse reactions, such as detonation or vapor lock. Sometimes, the aircraft requires alterations to the fuel system, such as different seals or a different fuel pump, to make the STC viable.

When an aircraft owner obtains the STC, the airplane gets the 337 form and any parts that are necessary to make it work, along with placards stating that the aircraft is operating on automobile fuel. 

"These are placards that go on the engine and on the wings [by the fuel cap], and an IA [airframe and powerplant technician with inspection authority] has to install them," Petersen says. In addition, documentation of the STC is made in the aircraft's logbooks by an IA.

Once the STC is in place, the aircraft can use auto fuel, but you must be careful about the type you select, Petersen warns. Auto gas that contains ethanol—a form of alcohol often added to automobile fuel to reduce carbon emissions—can have a damaging effect on the aircraft unless the engine contains ethanol-tolerant parts.

"If the aircraft is filled with auto gas containing ethanol and the airplane just sits, you are setting yourself up for engine failure," Petersen explains. "The seals and gaskets can be damaged when the aircraft sits past 10 days. Also, ethanol draws moisture out of the air and pulls it into the gas, so you have to be careful in a humid environment. The engine may not want to run."

Petersen suggests that aircraft owners who want to run auto fuel search for a filling station that doesn't add ethanol to the product, noting that marinas are often a good option because most boats will not take ethanol enhanced fuel either.

One of the common concerns when using auto fuel in an aircraft engine is that doing so may void the manufacturer's warranty. Lycoming Engines provides guidance on this. 

According to Brandon Dildine, field service technical representative for Lycoming, automotive fuels may be used in some Lycoming engines provided they meet the requirements listed in Service Instruction SI1070AB. 

"Use of fuels that do not meet the requirements listed in this publication may result in engine damage or failure and will void the engine warranty,” Dildine says. He adds that unapproved fuels containing additives, such as ethanol or oxygenates:

Dildine advises people to consult the airframe manufacturer before using automobile fuel to verify that the fuel approved for the engine is also compatible with all airframe components.

Dildine further warns, "All automotive fuels used should be tested to verify that alcohol is not present, and a fuel data sheet should be obtained from the distributor for the specific batch to verify that all specifications have been met. 

“Due to federal regulations and changes in seasonal fuel blends, the formula may change from batch to batch or could become contaminated during distribution when transferred in tanks or pipes that had blended fuel in them."