Can you put automotive gasoline on your plane?

“Automotive fuel is not certified for aircraft.”

Every student pilot hears this terrible warning about using automotive gasoline in an aircraft engine that has not been certified for it. These warnings are usually accompanied by a history of engine failure and an unscheduled landing outside the airport, which often occurs shortly after takeoff.

“If you’re flying without STC and authorities see you have auto fuel on an aircraft that isn’t certified for it, that’s a violation.”

Richard Scarbrough, airframe and powerplant mechanic and aircraft maintenance columnist for FLYING.

While it’s true that using auto fuel on a plane can be a tricky business, engine failure is unlikely to happen so quickly, says Richard Scarbrough, airframe and powertrain mechanic and columnist in aircraft maintenance for FLYING.

“That’s not to say that using autogas couldn’t create problems for the plane,” he says, explaining, “100LL provides the engine with lead lubrication, which autogasoline does not.”

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

Why would you?

Automotive fuel is generally less expensive than jet fuel. But, as usually happens at lower octane (80 versus 87), the consequence will be lower compression, since engines designed with higher compression ratios usually need lower octane fuel higher to avoid detonation. To alleviate these challenges, many pilots use a mixture of 100LL and auto gas in their aircraft.

Scarbrough notes that to legally use autogas in an aircraft, the aircraft must have a Supplemental Type Certificate (STC), which is a type certificate issued by the FAA that gives the applicant permission to modify a product. aeronautics from its original. pattern.

“If you’re flying without STC and authorities see you have auto fuel on an aircraft not certified for it, that’s a violation,” he says.

Such violations depend on who catches you – it can be as little as an on-the-spot “education” or a warning or correction letter, which outlines the corrective action a pilot should take. take within a certain time.

STC is obtained from a vendor – a company that specializes in STC – usually for a few hundred dollars.

“Most aircraft owners are happy to pay that for the fuel STC because they’ll offset the cost of the STC with one or two fill-ups,” says Scarbrough.

Aircraft owners who wish to take advantage of STC must file a Form 337 (Major Repair or Modification) with the FAA and additionally plate the aircraft appropriately.

Who did that ?

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

Since 1983, Petersen Aviation has supplied an automatic 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 the difficulty of obtaining a reliable source prompted them to consider the idea of ​​running the auto-fuel aircraft. They realized they weren’t the only pilots facing this dilemma, and the company STC was born.

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

“To get an additional type certificate to run an aircraft on auto fuel, you actually need two additional type certificates, one for the aircraft, one for the engine,” Petersen explains.

Testing for STC fuel 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 modifications 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 aircraft receives the Form 337 and all the parts needed to operate it, along with placards indicating that the aircraft is running on automotive fuel.

“These are signs that go on the engine and on the fenders [by the fuel cap]and an AI [airframe and powerplant technician with inspection authority] has to install them,” says Petersen. In addition, STC documentation is recorded in the aircraft’s logbooks by an IA.

Once the STC is in place, the plane can use auto fuel, but you have to be careful which type you select, warns Petersen. Automotive gasoline containing ethanol – a form of alcohol often added to automotive fuel to reduce carbon emissions – can have a detrimental effect on aircraft unless the engine contains ethanol-tolerant parts .

“If the plane is filled with automotive gasoline that contains ethanol and the plane sits still, you risk engine failure,” says Petersen. “Seals and gaskets can be damaged when the plane sits for more than 10 days. Also, ethanol pulls moisture out of the air and sucks it into the gas, so you have to be careful in a humid environment The motor may not want to run.

Petersen suggests that aircraft owners who want to use auto fuel look for a gas station that doesn’t add ethanol to the product, noting that marinas are often a good option because most boats won’t take fuel either. improved ethanol fuel.

Lycoming engines take automatic fuel

One of the common concerns when using auto fuel in an aircraft engine is that it can void the manufacturer’s warranty. Lycoming Engines provides advice on this.

According to Brandon Dildine, Field Service Technical Representative for Lycoming, automotive fuels can 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,” says Dildine. He adds that unapproved fuels containing additives, such as ethanol or oxygenates:

  • may be more likely to vaporize in fuel lines, causing loss of power;
  • may cause detonation in normal operation;
  • are less stable and can degrade over short periods of time, resulting in fuel that does not meet its old specifications, often resulting in varnish buildup in the fuel system;
  • can more easily collect moisture from the air;
  • may cause corrosion of airframe/engine fuel system components;
  • and can lead to degradation of gaskets, hoses, diaphragms, sealants and bladders used in the fuel cell/system that are not compatible.

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

Dildine further warns: “All automotive fuels used should be tested for the absence of alcohol, 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 become contaminated during dispensing when transferred to tanks or pipes containing mixed fuel. “

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