Learning Center Library Contents

The introduction of turbine-powered aircraft into the civil aircraft fleet during the 1950s caused many changes in the marketing of aircraft fuels. As the air carrier and military fleets were converted to turbine-powered aircraft, the demand for aviation gasoline (avgas) decreased drastically. Aviation fuels now represent a relatively small portion of the petroleum industry's products and, therefore, the production of avgas in multi grades is no longer economically feasible. During the past few years, we have seen 91/96, and 115/145 octane fuels disappear from the market.

In 1971, the oil companies began development of a single grade avgas that would meet the needs of all reciprocating powered aircraft. When 80/87 began to disappear from the avgas market and 100LL was introduced to take its place, operators expressed concern about the service life expectancy of their low compression engines. Some operators experienced accelerated exhaust valve erosion and valve guide wear from the use of highly leaded 100/130 (green) avgas in engines that were rated to use a minimum grade 80 octane fuel. The engine manufacturers were quick to provide aircraft owners with amended operating procedures and maintenance schedules which helped minimize the engine malfunctions resulting from the use of high lead 100/130 avgas. Experience of the past ten years has proven that low compression aircraft engines can be operated safely on 100 low lead (LL) avgas without difficulty, providing they are operated and serviced in accordance with the approved aircraft owner's manual or other officially approved document.

Leaded automotive gasoline is not recommended as a substitute for aviation gasoline because of the differences in properties and composition of the two types of fuel. Regular leaded automotive gasoline may cause preignition and detonation, vapor lock, and sticking or burned valves when used in aircraft engines. Lead-free automotive gasoline, however, has been extensively tested by the Experimental Aircraft Association (EAA) and other organizations in aircraft equipped with low compression engines that use low octane fuel . The Federal Aviation Administration (FAA) has issued supplemental type certificates (STC) to these organizations, thus permitting the use of unleaded automotive gasoline of 87 minimum antiknock index per ASTM specification D-439. Each make/model aircraft must be modified and operated in accordance with the instructions, limitations, and procedures contained in the STC when unleaded automotive gasoline is used.

Be sure you get the type of fuel that is specified for your aircraft. Federal regulations require that all aircraft filler openings must be marked with the word "fuel" and the minimum fuel grade for reciprocating powered aircraft, or the permissible fuel designation for turbine-powered aircraft. Even these requirements do not rule out the possibility of being serviced with the wrong type of fuel. Pilots should be particularly cautious when being serviced at facilities that provide turbine fuel as well as avgas. Turbine or jet fuel is detrimental to the reciprocating engine and extended use of avgas can damage turbine engines. Therefore, it is imperative for flight crews to double check when their aircraft is serviced to assure that they receive the proper type and grade of fuel. Although this responsibility is placed upon the pilot by regulation it just makes good sense to be sure. The fuel system sumps should always be drained and checked for contaminants after each fueling of the aircraft and during preflight inspection.

The FAA and several engine manufacturers have approved the use of certain carburetor anti-icing fuel additives in aviation gasoline. However, such additives should not be used without consulting the airframe manufacturer because their chemical content may not be compatible with the aircraft fuel system cells, seals, etc.

The same is true with lead scavenging additives such as Tricresyl Phosphate (TCP). TCP, for example, has been used successfully to reduce lead fouling of spark plugs in normally aspirated reciprocating engines for several years. However, TCP should not be used in turbocharged or supercharged engines without approval of the airframe manufacturer. TCP must be mixed according to the instructions provided by the manufacturer (Alcor Inc.) for maximum effectiveness.

In most cases spark plug fouling can be reduced or eliminated by simply applying proven operating techniques. For example, low operating temperatures coupled with rich fuel mixtures result in incomplete vaporization of the tetraethyl lead in the combustion chamber, causing lead fouling of the spark plugs. Maintaining proper cylinder head temperatures will minimize plug fouling problems. In addition:

• Be certain that maintenance personnel have installed the spark plugs recommended for the particular engine installation.
• Have the carburetor idle mixture checked and adjusted.
• Use recommended leaning techniques in cruise condition at all altitudes.
• Avoid low power letdowns, descend with power, and avoid over-rich conditions. Carburetors and fuel injectors are normally set slightly rich in the closed throttle position, so it is best to carry a slight amount of power on landing approaches rather than approach with closed throttle.
• Keep the cylinder temperatures in the normal range during operation.
• After flight or ground operations, before shutdown, advance the throttle to about 1800 RPM for 15 to 20 seconds to clear the plugs and combustion chambers, retard the throttle to about 1200 RPM and shut the engine off immediately with the mixture control. You should not have plug fouling or misfiring on your next startup.

As long as you make sure the aircraft is serviced with the proper fuel, check the sumps for contaminants, operate the engine according to the aircraft owners manual and have the spark plugs serviced as recommended, you should not have plug fouling problems.

Occasionally, aircraft are inadvertently serviced with the wrong type of fuel and, in most instances, it is because of misleading signs. For example, certain turbo-supercharged reciprocating powered aircraft have paint designs with the word "turbo" conspicuously displayed on the vertical stabilizer or on the engine nacelle. Line service personnel have assumed this wording to mean "turbojet" and have filled the tanks with jet fuel instead of avgas. Another incident involved an air carrier type aircraft that was originally equipped with reciprocating engines, which most operators had converted to turbo-props. The service personnel assumed that the aircraft was a converted model, when in fact it was not.

Reciprocating engines may run briefly on jet fuel, but detonation and overheating will soon cause power failure. So beware of getting jet fuel when you need avgas. Similarly, avgas is no substitute for jet fuel. The engine failure caused by running a turbine engine on the wrong fuel may not be as sudden, but prolonged operation on gasoline will severely damage the engine due to the lead content and the different combustion ranges of the fuel. Time limitations for use of avgas in turbine engines are listed in the airplane or rotorcraft flight manual.

As pilot in command, you have the responsibility to determine that your aircraft is properly serviced. Check your aircraft before each flight and be sure you have the correct type of fuel. This practice may save your life! Take the time to inspect your aircraft thoroughly. Good preflight procedures:

• Be sure all of the fuel and oil tank caps and covers are installed and secured properly after you visually check the fluid level.
• Observe the color and odor of the fuel as you check the tank.
• Draw a generous sample of fuel from each sump and screen drain into a transparent container. Check for the presence of water, dirt, rust or other contaminants. Unless you have the proper kind of fuel tester (e.g., GATS jar with intact fuel strainer), do not attempt to save the fuel drained from the sumps by pouring it back into the tank. Note: If you are not equipped to safely pour the drained fuel back into the tank, please be sure to dispose of it in accordance with EPA regulation.
• Check that each fuel tank vent is clear of restrictions; i.e., dirt, ice, snow, bent or pinched tubes, etc.

The purpose of this series of Federal Aviation Administration (FAA) Aviation Safety Program publications is to provide the aviation community with safety information that is informative, handy, and easy to review. Many of the publications in this series summarize material published in various FAA advisory circulars, handbooks, other publications, and various audiovisual products developed by the FAA and used in its Aviation Safety Program.

Some of the ideas an materials in this series were developed by the aviation industry. The FAA acknowledges the support of the aviation industry and its various trade and membership groups in the production of this series.

Comments regarding these publications should be directed to the National Aviation Safety Program Manager, Federal Aviation Administration, Flight Standards Service.