CHAPTER 1
Introduction

The Airport Cooperative Research Program (ACRP), funded by the Federal Aviation Administration (FAA), initiated ACRP Project 03-73 to answer a broad, foundational question: What can be learned from airports that have transitioned or are planning to transition to unleaded aviation gasoline while continuing to provide 100-octane low-lead (100LL) aviation gasoline? This guide and the associated spreadsheet tools are based on information developed from the experiences of airports that have already made this transition or were in the process of doing so and is intended to share best practices and lessons learned from those experiences.

Overview

The need for high-octane fuels like 100LL in gasoline-fueled aircraft engines is the result of some manufacturers designing high-performance, high-compression ratio engines to maximize the work the engine can perform per volume of fuel consumed, and to optimize the power-to-weight ratio of the engine. However, other gasoline-fueled aircraft engine designs can safely operate on fuels with octane ratings as low as 80 while also being capable of utilizing 100LL fuels. To put these octane ratings into context, unleaded gasolines used in on-road vehicles generally have octane ratings ranging from 87 (regular) to 91 (premium). This research was initiated to answer a broad, foundational question: What can be learned from airports that have transitioned or are planning to transition to unleaded aviation fuel, while continuing to provide 100LL? This guide is organized around that intent, with the goal of sharing best practices and lessons learned to support other airports as they plan and implement their own transitions. As a result of the fact that some engines require 100-octane fuel and all engines can operate using that fuel, aviation gasoline producers and airports have historically elected to produce only one fuel that meets the needs of all aircraft, 100LL gasoline.

As is discussed in Understanding the Transition to Unleaded Aviation Gasoline: A Primer, public health concerns regarding lead emissions from the combustion of 100LL are driving a transition to unleaded aviation fuels in all gasoline-fueled aircraft by the end of 2030.

This guide is organized around that intent, with the goal of sharing best practices and lessons learned to support other airports as they plan and implement their own transitions. The guide is accompanied by five standalone tools that are intended to assist airports with different aspects of the transition process. These tools are described in Chapter 3 of the guide and are available at nationalacademies.org/publications by searching for ACRP Research Report 284: Transitioning to Unleaded Aviation Gasoline: A Guide and Tools and looking under “Additional materials.”

The purpose of this guide is to provide a summary of the general steps and best practices that airports seeking to supply unleaded aviation gasoline should follow, and to highlight important

sources of information that will assist in the process. It is strongly suggested that those interested in the transition to unleaded aviation gasoline first read the Primer, as it provides additional background information.

The guide is based on what is considered to be the best publicly available information at the time of its completion in January 2025, particularly the information available from the Eliminate Aviation Gasoline Lead Emissions (EAGLE) initiative website, which is frequently referred to throughout this document (EAGLE n.d.). As described in more detail in the Primer, EAGLE is a government-industry partnership, working toward the transition to lead-free aviation fuels for piston engine aircraft by the end of 2030 without compromising the safety or economic health of the general aviation industry.

In addition to information and data available from EAGLE and other publicly available sources, this guide also relies on findings from an analysis of information collected through case studies of seven airports that had already begun or were in the process of being able to supply unleaded gasoline along with 100LL gasoline during the spring and summer of 2024 when interviews were conducted. Summaries of these cases studies, as well as results of interviews related to the transition at Alaskan airports are provided in Appendix A of this guide. The case studies were performed using a set of standardized survey questions.

Beyond this guide, there are several other sources of information regarding transitioning to unleaded aviation gasoline that need to be considered, which are described as follows:

• Information and reports from the FAA, including Building an Unleaded Future by 2030 (FAA 2024b) as well as Frequently Asked Questions and Definitions (FAA 2024e).
• EAGLE Guidance on Transitioning a Flight School to Unleaded Avgas (FAA 2023), which covers many of the same steps to transition and issues presented and could serve as a companion to the guide.
• White papers prepared by the National Air Transportation Association (NATA), including Unleaded Avgas Conversion Considerations for Aviation Fuel Providers and Factors Affecting the Commercial Sale of Emerging Unleaded Aviation Fuels (NATA 2022, 2024a).

It is strongly suggested that those involved in planning and executing airport transitions to unleaded aviation gasoline obtain and review these materials in addition to the ACRP Project 03-73 deliverables.

Following this background section, the next section presents a summary of the latest information regarding the status of unleaded aviation gasoline availability and requirements for use in aircraft. The subsequent sections of this guide discuss in some detail the suggested general approach to transitioning. This approach is based on a 10-step process intended to allow airports to transition to supplying unleaded aviation gasoline along with 100LL fuel through 2030. The guidance is based on information available as of January 2025 but also attempts to address the possibility of new and perhaps unforeseen developments related to unleaded aviation gasoline between now and the end of 2030. At a high level, these 10 steps are:

1. Communicate with fuel suppliers and fixed-base operators (FBOs). Conduct initial communications with fuel suppliers and FBOs as appropriate to determine what types of unleaded aviation gasolines are available to the airport, which aircraft can use them, if there are applicable ASTM International standards for the fuel, and if there are any compatibility issues associated with mixing each unleaded fuel with 100LL or other unleaded aviation gasolines. At airports with sponsor-owned FBOs, communications would be directly with fuel suppliers.
2. Characterize the airport fleet for unleaded fuel compatibility. Identify the aircraft in the airport fleet that can use the different types of unleaded aviation gasolines expected to be available to the airport at the time of transition and determine if they will need supplemental type certificates (STCs).

3. Estimate unleaded fuel demand and evaluate refueling infrastructure. Analyze the expected volumes of the available unleaded aviation gasoline(s) under consideration and determine if they can be provided using existing airport fueling infrastructure while continuing to provide 100LL, or if new infrastructure and/or new fuel delivery methods are needed.
4. Identify potential transition funding sources. Determine whether funding is available from the FAA, state sources, sponsors, or other organizations to assist with infrastructure changes and/or to provide subsidies for the cost of the available unleaded aviation gasoline(s) and, if necessary, STCs.
5. Perform financial analysis of transition options. Use the information gathered in Steps 1 through 4 to assess the financial implications, such as potential revenues and costs associated with each unleaded aviation gasoline under consideration.
6. Decide on the transition to unleaded fuel and select the fuel type. Use the results of the analysis in Step 5 in combination with the airportʼs Primary Management Compliance Documents (PMCDs) that include minimum standards/rules and regulations for compliance as part of the airportʼs decision-making process to determine if the airport will begin the transition to supplying one or more unleaded aviation gasoline(s) while continuing to provide 100LL.
7. Implement transition plans to unleaded fuel. Make the necessary arrangements to develop the required refueling infrastructure, including securing grant and incentive funding, to provide the selected unleaded aviation gasoline(s) and to work with the fuel supplier that will provide the fuel(s).
8. Develop safety procedures. To prevent misfueling of aircraft, develop procedures to ensure that unleaded aviation gasoline(s) either have FAA fleet authorization or are only used in aircraft that are type certified or hold STCs for their use.
9. Conduct training, outreach, and education. Ensure all personnel involved in the supply and use of the selected unleaded aviation gasoline(s) are informed of proper procedures and safety protocols through targeted outreach, education, and training before fuel distribution begins.
10. Monitor the transition, identify issues, and make adjustments. Begin using the selected unleaded aviation gasoline(s), monitor the progress of the transition, and evaluate and adjust as needed to address any issues that arise.

While the 10 steps outlined above are intended to encompass all the activities needed for an airport to transition to providing one or more unleaded aviation gasoline alongside 100LL through 2030, the guide is not intended to be exhaustive and there will undoubtedly be airport-specific issues that arise and need to be addressed. In addition, airports will also need to ensure that all actions comply with PMCDs and/or other applicable rules, regulations, minimum standards, and policies that govern the airport.

To assist airports with tracking their progress through the transition process, an Excel spreadsheet tool titled “Transition Process Checklist Tool” has been developed and is available on the National Academies Press website (nationalacademies.org/publications) by searching for ACRP Research Report 284: Transitioning to Unleaded Aviation Gasoline: A Guide and Tools.

Available Unleaded Aviation Gasolines and Determining Aircraft Compatible with Their Use

Although the suggested steps for an airport to transition to providing unleaded aviation gasoline along with 100LL are generally the same regardless of the unleaded aviation gasoline (or potentially gasolines) selected, it is important for airports to understand the issues associated with each unleaded aviation gasoline fuel, how to determine the engines and aircraft that are authorized by FAA to use each fuel, the compatibility of fuels with 100LL, and the status of the fuel with respect to being covered by an ASTM International standard. In addition to this guide, NATAʼs white

paper, Unleaded Avgas Conversion Considerations for Aviation Fuel Providers, is suggested as a source of information on some of the key issues raised in this section as well as others raised in the discussion of the suggested transition steps (NATA 2022).

ASTM International standards are voluntary, consensus-based standards developed by technical committees of industry experts and intended to ensure, in the case of aviation gasoline fuels, that commercial fuels meet the requirements of gasoline-fueled aircraft engines. The status of the various unleaded aviation gasolines with respect to ASTM International standards is addressed here because that status may play a role in whether an aviation gasoline will be allowed to be dispensed by a particular entity or, potentially, if the use of an unleaded fuel is considered to be compatible with the warranties provided by engine and aircraft manufacturers.

There are four types of unleaded aviation gasolines that are or are becoming commercially available as of this reportʼs writing in January 2025. These are:

1. Non-oxygenated motor gasoline (mogas), to which the latest version of ASTM Inter national Standard D4814 “Standard Specification for Automotive Spark-Ignition Engine Fuel” applies.
2. Swift Fuels UL94, to which the latest version of ASTM International Standard D7547 “Standard Specification for Hydrocarbon Unleaded Aviation Gasoline” applies.
3. General Aviation Modifications, Inc. (GAMI) G100UL, for which no ASTM International standard currently exists. However, GAMI has developed its own proprietary fuel specification as part of the FAA STC approval process (GAMI 2024a).
4. Swift 100R, for which an ASTM International standard is in development (Swift Fuels 2024).

In addition, a fifth unleaded fuel (UL100E), is being developed by LyondellBasell and VP Racing Fuels, but it is still undergoing testing under the FAAʼs Piston Engine Aviation Fuels Initiative (PAFI) and, at present, would not be covered by an ASTM International standard, although an ASTM International standard is under development (FAA 2022b).

1. Through the FAAʼs Type Certification process by the original engine/aircraft manufacturers.
2. Through the FAAʼs STC process by fuel producers via an authorized models list.
3. Through the FAAʼs Fleet Authorization process under PAFI, which requires both testing and the development of an ASTM International standard for the fuel.

Currently, mogas, Swift UL94, Swift 100R, and GAMI G100UL can only be used in aircraft that have acquired STCs specific to each fuel. It should be noted that the STC process does not generally apply to experimental or amateur-built aircraft.

Mogas, without the oxygenate ethanol that is added to most automotive fuels in the United States, can be used in gasoline aircraft engines designed for use with octane ratings in the 80 to 87 range and has been in use for a considerable period. Use of mogas requires the aircraft owner to complete all steps required to obtain an appropriate STC, including having any mandatory inspections and modifications performed. Mogas or Auto Fuel STCs are available from the Auto Fuel STC website (Auto Fuel STC n.d.). Many gasoline-fueled aircraft in the United States can use mogas. At the time of this publication, according to the FAAʼs Airport Data and Information Portal (FAA n.d.), 172 airports in the country currently supply mogas. As noted above, mogas specifications are established through an ASTM International standard. In addition, provided the aircraft has a mogas STC, that fuel can be added to existing 100LL fuel that is already present in the aircraft during refueling, and vice versa.

Swift UL94 can be used in about two-thirds of gasoline-fueled aircraft in the United States [Swift Fuels n.d.(b)]. Again, this requires the aircraft owner to complete all steps required to obtain an appropriate STC, including having any mandatory inspections and modifications performed.

STCs can be obtained for authorized aircraft from Swift Fuels on their website [Swift Fuels n.d.(a)]. According to FAAʼs Airport Data and Information Portal, Swift UL94 is currently being supplied by 36 airports as of April 30, 2025 (FAA n.d.). As noted earlier, Swift UL94 specifications are established through an ASTM International standard. Again, provided the aircraft has a UL94 STC, that fuel can be added to the existing 100LL fuel that is present in the aircraft during refueling and vice versa.

Swift 100R is just becoming available, and at present, STC coverage remains limited. 100R is available in three airports in California, San Carlos, Half Moon Bay, and Santa Monica [County of San Mateo 2024, National Business Aviation Association (NBAA) 2024]. In addition, Swift has indicated that it is working to secure the development of an ASTM International standard for the 100R fuel. Provided the aircraft has a 100R STC, that fuel can be added to existing 100LL fuel present in the aircraft during refueling, and 100LL can also be added to Swift 100R in a similar situation. At present, Swift 100R STCs are available only for Cessna 172R/S airplanes with Lycoming IO-360-L2A engines (Swift Fuels 2024).

GAMI G100UL, like Swift 100R, is intended ultimately to be used in any gasoline-fueled aircraft after the aircraft owner completes all steps required to obtain an appropriate STC, including having any mandatory inspections and modifications performed. STCs can be obtained for authorized aircraft from GAMI on their website (GAMI 2024b). According to FAAʼs Airport Data and Information Portal, GAMI G100UL is currently being supplied by two airports in the country (FAA n.d.). At present, there is no ASTM International standard that applies to G100UL, and it does not appear that GAMI is pursuing the development of such a standard. Provided the aircraft has a G100UL STC, that fuel can be added to existing 100LL fuel that is present in the aircraft, and 100LL can also be added to G100UL in a similar situation.

Given that both Swift 100R and GAMI G100UL are not covered by applicable ASTM Inter national standards, they have at present been authorized by FAA as “Company Specific Products” and the fuel producers are responsible for ensuring that commercial fuel conforms to each companyʼs specifications for its product.

More information regarding the differences between ASTM International standards and Company Specific Product specifications is available from the EAGLE website, including through an EAGLE fact sheet (https://flyeagle.org/wp-content/uploads/2024/11/Fact-Sheet-The-Value-of-Industry-Voluntary-Consensus-Standards.pdf) available on the website (EAGLE 2024b).

It is also very important to note that Swift has published guidance stating that G100UL must not be used in aircraft authorized for a Swift Fuels STC [Swift Fuels n.d.(b)]. However, as stated above, fuels from both producers can be mixed with 100LL (EAGLE 2024a).

Finally, as noted earlier, a third 100-octane unleaded aviation gasoline ultimately intended for use in all gasoline-fueled aircraft, UL100E, is under development by LyondellBasell and VP Racing Fuels (VP Racing Fuels n.d.). This fuel is still undergoing testing under the FAA PAFI program and is expected to be allowed for use under an FAA Fleet Authorization rather than the STC process. The FAA Fleet Authorization process requires that the authorized fuel be subject to an applicable ASTM International standard, which would have to be developed for the 100E fuel. While 100E is compatible with 100LL, it is not known at this time if 100E will be compatible with other unleaded aviation gasolines.