Options for Reducing Lead Emissions from Piston-Engine Aircraft (2021)

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5 Existing Fuel Options for Piston- Engine Aircraft to Reduce Lead This chapter reviews existing fuel options for piston-engine aircraft and considers their potential to reduce lead use by the general aviation (GA) fleet. The chapter begins with an explanation of the reasons why lead is added to aviation gasoline (avgas), including the almost universally used grade, 100 octane low lead aviation gasoline (100LL). This discussion is followed by a review of other grades of avgas that have lower (100 octane very low lead aviation gasoline [100VLL]) or no added lead (UL94) and that are available for purchase at some airports or approved in current fuel specifications. Consideration is then given to the applicability of unleaded automotive or “motor” gasoline supplies—sometimes referred to as MOGAS—to the piston-engine aircraft. The prospects for more widespread use of these fuels are assessed and each fuel’s potential impact on GA emissions of lead is estimated. The chapter concludes with findings and recommendations applicable to each fuel type. ROLE OF LEAD IN AVGAS Aviation has always had a great need for engines with high power-to-weight ratios because of the importance of minimizing aircraft weight to achieve high levels of performance. In reciprocating engines, high power to weight can be achieved by increasing the brake mean effective pressure (BMEP). For a four-cycle engine, this relationship is 87 

88 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT (R PM ) Vd pme P= 120 where is the engine displacement (the cumulative cylinder volume) and is the BMEP. BMEP can be increased by either (a) increasing the compression ratio within the cylinders/pistons of the engine or (b) supercharging or turbocharging the inlet air. However, increasing BMEP in an engine de- sign requires selecting fuels that are able to burn at higher pressures and temperatures without detonation. Detonation is rapid combustion, simi- lar to an explosion involving a supersonic flame front and shock wave. Detonation in a reciprocating engine is often called “knock” because of its characteristic sound. Knocking can lead to failure of critical engine components in flight and must be avoided. A primary measure of a fuel’s resistance to knock is the octane number. Fuels with higher octane can be operated in higher compression engines (i.e., engines with higher BMEP). Chemicals are typically added to avgas to achieve the higher octane. The most used additive is tetraethyl lead (TEL),1 which was developed and marketed beginning in the 1920s. Until that time, avgas did not contain lead. In addition to its role in increasing octane, TEL (1.0 gram of which contains 0.64 grams of lead) helps reduce engine valve wear by lubricating valve seats and guides. Adding lead to avgas also has many disadvantages. In addition to be- ing toxic to humans (as discussed in Chapter 3), lead deposits foul spark plugs and other engine components. To prevent unacceptable lead buildup in engine combustion chambers and on components, the lead scavenger eth- ylene dibromide is added to avgas. Nevertheless, lead deposits still require periodic cleaning, and the scavenger itself causes the creation and emission of lead dibromide as well as the formation of hydrobromic acid. Because this acid can cause internal corrosion in engine components, more frequent oil changes may be required to minimize engine damage. Because of the need for a fuel’s octane number to be commensurate with an engine’s BMEP, fuel selection and engines are inextricably linked. Aviation engine types are certified (type certificate [TC]) for airworthiness by the Federal Aviation Administration (FAA) to operate with a fuel meet- ing specifications identified by the TC applicant (the engine or aircraft manufacturer). While FAA does not approve specific fuels for use in aircraft engines, it certifies aircraft and engine types based on the fuels identified by the aircraft and engine manufacturer in the TC application. The fuel is then regulated as an operating limitation (FAA, 2018). Once a particular grade 1  (CH 3CH2)4Pb. 

EXISTING FUEL OPTIONS FOR PISTON-ENGINE AIRCRAFT 89 of avgas has been defined as an operating limitation for an aircraft, it may only be changed by a TC amended by the manufacturer for the aircraft type or a supplemental type certificate (STC) obtained from FAA by the owner of an individual aircraft. FAA, however, can issue a Special Airworthiness Information Bulletin (SAIB) indicating that a grade of avgas is acceptable for use by designated aircraft and engine types that were certified for opera- tion with other specified fuels. Fuel operating limitations are defined in the aircraft’s TC data sheet and flight manual. The operator is required to use only the fuels listed in those documents. The fuel operating limitations, therefore, must be precise to help the operator ensure that the engine and aircraft continue to con- form to the operating limitations of their certification. In turn, fuel refiners and suppliers need this precision to produce and distribute fuels suited to the mix of aircraft in the fleet. While some engines—typically older, lower compression types—are certified to operate with a lower octane fuel, they can also operate safely with higher octane fuels. Other engines are certified only for higher octane fuels. Accordingly, a fuel with an octane value that can satisfy both segments of the fleet may be preferred by fuel refiners and suppliers to allow for the efficiencies of higher production, distribution, and dispensing volumes. HISTORICAL AND CURRENT USE OF LEADED AND UNLEADED AVGAS The specific properties of avgas are defined in ASTM International specifi- cations for the supply and purchase of avgas. Until the 1960s, avgas grades were commonly expressed as two successive octane numbers, such as 80/87 and 100/130. The first number is the lean motor octane number (MON), which is typically used in referring to avgas grades, and the second number is the rich rating, which is seldom used. The most prevalent avgas grade is 100LL, where the “100” refers to the MON rating and “LL” stands for “low lead.” According to ASTM International standard D910, which contains specifications for leaded avgas, 100LL cannot have a lead content that is less than 0.28 grams per liter or greater than 0.56 grams per liter. Prior to the introduction of 100LL, lead content could be as high as 1.12 grams per liter. While 100LL dominates the avgas market and can be used by all piston-engine aircraft, ASTM International has specified other avgas grades for use in all or a large share of the piston-engine fleet, including the “very low lead” (VLL) 100 MON grade and an unleaded (UL) 94 MON grade. According to ASTM D910, 100VLL cannot have a lead content less than 0.28 grams per liter or greater than 0.45 grams per liter. Thus, the mini- mum allowable lead content is the same for 100LL and 100VLL, but the 

90 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT maximum allowable lead content is 19.6 percent lower for 100VLL. With SAIB NE-11-55 issued on September 14, 2011, FAA indicated that 100VLL is acceptable for use by all aircraft that require 100LL (or a lower MON), but as explained below, this grade of avgas is not being produced. While 100VLL is not currently available for purchase, the fact that it is specified by ASTM International and has been approved for use by all piston-engine aircraft warrants its consideration as an existing fuel option. A different ASTM International standard, D7547, governs unleaded avgas, but it applies only to fuel grades with a MON rating of 94 or lower. Simply removing TEL from 100LL would result in a fuel with approximately 94 MON although some reformulation would be required to comply with all of the specifications of ASTM D7547. Currently the single grade of avgas that meets this standard, a proprietary UL94, is only available for sale in a select number of airports, mainly in the Midwest. There is currently no ASTM International specification for an unleaded fuel having a MON higher than 94, nor has FAA approved such a fuel as an operating limitation of any engine or aircraft TC or STC. Following a short primer on the history of leaded avgas and how 100LL became prevalent, the status of these two other lower-lead and unleaded grades (100VLL and UL94) is discussed. Convergence to 100LL Avgas As noted above, TEL was found to be beneficial as an anti-knock agent in avgas during the 1920s. Indeed, by 1930 the U.S. Army Specification 3 referenced TEL in 80 MON fuel. At the time, however, virtually all piston- engine aircraft were satisfied with 80 MON avgas, with or without TEL. The development of high-performance combat aircraft in the 1930s drove the development of much higher octane avgas through the addition of TEL. 100 MON and even 115 MON grades were developed to satisfy engines with higher compression ratios and forced induction through turbocharging and supercharging. Although initially introduced for military aircraft, higher performance engines were quickly adopted for civilian passenger and cargo aircraft to provide the higher payloads, greater range, and higher ceilings enabled by the 100 and 115 MON fuels. In 1947, the first ASTM International speci- fication (ASTM D910) for leaded avgas was introduced, covering 91, 100, and 115 MON grades. By 1954, most avgas contained lead. In 1960, ASTM D910 included 80, 91, 100, 108, and 115 MON grades, all which were leaded, with a footnote referencing unleaded 80 MON at least until 1995. During the 1960s, most of the military and commercial air transport fleet had changed over from large piston-engines to turboprops and tur- bojets burning jet fuel, eliminating most of the demand for avgas grades with MON ratings exceeding 100 and substantially reducing demand for 

EXISTING FUEL OPTIONS FOR PISTON-ENGINE AIRCRAFT 91 100 MON by limiting the avgas market to mainly GA. By the late 1960s, the reduced consumption of avgas could no longer support the commercial production of multiple grades, each associated with separate requirements and infrastructure for production, distribution, storage, and dispensing. By 1972, the four existing grades of leaded avgas covered by ASTM D910 converged on one grade, 100LL, which became the de facto standard. No regulatory action was required because 100LL satisfied requirements for legacy aircraft, which were typically certified for 80 or 91 MON as a minimum. This convergence permitted the use of a common avgas distri- bution system and one type of avgas storage tank and dispensing system at an airport to serve all gasoline-engine aircraft. Thus, even though many aircraft in the piston-engine fleet are able to use avgas grades with a MON lower than 100 (and thus with lower levels of TEL or no TEL) based on their applicable TCs or STCs, the grade of avgas offered for sale is generally limited to the universally usable 100LL. Even with this convergence, 100LL avgas can be described as a bou- tique fuel when considered in terms of the total market for gasoline. For example, according to the U.S. Energy Information Administration (EIA), national automobile gasoline consumption averaged about 400 million gal- lons per day in 2019, while avgas consumption was less than 200 million gallons for the entire year (about 525,000 gallons per day), or the yearly av- gas consumption was less than half of the daily consumption of automobile gasoline.2 According to EIA, from 1981 to 2019 the demand for avgas (as measured by the amount of product supplied by the refineries) had dropped by nearly 60 percent (468 million gallons in 1981 to 197 million gallons in 2019).3 Because of this decreased demand, fewer than 10 percent of some 120 North American gasoline refineries currently produce 100LL (NASEM, 2019), and only one chemical manufacturer supplies TEL, as the demand for this additive declined dramatically following the removal of lead from automotive gasoline. 100VLL Avgas Concerns over the toxicity of TEL led to the addition to ASTM D910 of a second grade of 100 MON avgas, 100VLL, in 2011. As noted above, 100VLL has the same minimum allowable lead content as 100LL, but 100VLL has a maximum allowable lead content that is reduced by 19.6 percent. Undoubtedly, some 100LL fuel batches will meet the 100VLL stan- dard because they do not exceed the upper limit, but they are not marketed as such. Indeed, samples of 100LL tested by the Coordinating Research 2  See https://www.eia.gov/petroleum/data.php. 3  See https://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=pet&s=mgaupus1&f=a. 

92 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT Council (CRC) had an average lead content of 0.47 grams per liter (CRC, 2010), which is slightly higher than the maximum allowable lead content (0.45 grams per liter) for 100VLL. 100VLL fuel would satisfy every aircraft that currently operates on 100LL, as the minimum allowable lead content (0.28 grams per liter) of the two grades is the same. Therefore, total replacement of 100LL by 100VLL appears to offer an opportunity to reduce overall lead consump- tion in piston-engine aircraft, provided that fuel producers are able to meet the D910 specifications together with 100VLL’s upper lead limit. However, 100VLL has not taken hold in the marketplace for reasons that are not entirely clear, but perhaps it is because there are no strong incentives to use the more expensive hydrocarbon blending components and meet the tighter tolerances needed to achieve 100 MON with less added lead.4 Unleaded Gasoline Alternatives The drawbacks to the use of TEL in avgas, particularly its toxicity, have led to interest in lead-free fuels for aviation use. The two specific types of unleaded gasolines that are currently or potentially available for purchase today and permitted as operating limitations for some aircraft are a propri- etary UL94 avgas and an appropriately formulated automotive gasoline, or MOGAS. This section starts with a discussion of UL94 and follows with a discussion of MOGAS. UL94 Avgas Whereas the specifications for leaded avgas are contained in ASTM D910, the specifications for unleaded avgas are contained in ASTM D7547. Sim- ply removing TEL from 100LL would result in a fuel with approximately 94 MON, although some reformulation would be required to comply with all of the specifications of ASTM D7547. In 2011, FAA examined nearly 2,700 TC data sheets for every aircraft and engine type in the 2010 U.S. Aircraft Registry of 189,415 aircraft with piston engines to determine the mix of fuel specifications in the operating limitations (FAA, 2011). These aircraft, some dating from the 1930s, were produced by more than 2,600 manufacturers. While a review of about 500 of the TC data sheets indicated that they covered nearly 85 percent of the fleet, the remaining 15 percent required a review of more than 2,000 data sheets. From this analysis, FAA determined that approximately 43 percent of the piston-engine fleet requires a 100 MON grade avgas while approximately 57 percent could be satisfied 4  According to an FAA presentation to the committee on February 25, 2020, a higher-quality alkylate may be needed, for example. 

EXISTING FUEL OPTIONS FOR PISTON-ENGINE AIRCRAFT 93 with a 94 MON grade. This determination is sometimes used to support the notion that even if an unleaded lower octane avgas were to become widely available and used by a significant portion of the fleet, a 100 MON avgas would still need to be available to satisfy a large share (43 percent) of the fleet. Several factors warrant consideration when assessing the potential for an unleaded, lower octane avgas to have an appreciable effect on aviation lead emissions. First, it should be noted that FAA’s 2011 estimate of 57 percent of piston-engine aircraft being able to safely use a 94 MON avgas is based on fleet data that are now 10 years old and include aircraft that are no longer active. This time lag may be a minor consideration, however, because GA aircraft have long service lives and fewer than 1,000 new piston-engine aircraft enter the fleet annually, which means that the ac- tive fleet, estimated in Chapter 2 to contain about 170,000 piston-engine aircraft, is not likely to have changed significantly over a decade. Second, and perhaps more importantly, many aircraft certified since the early 1970s specify only 100LL as the operating fuel in their TC because no other av- gas grade was available for purchase when the aircraft was produced. It seems likely, however, that many of those aircraft are equipped with older- design, lower-power engines that had been originally certified for an 80 or 91 MON grade. If so, those aircraft could presumably be safely operated using a 94 MON fuel, although their operating limitations would need to be amended to permit the fuel’s use. Consequently, it seems reasonable to assume that more than 57 percent of the existing piston-engine fleet could use a 94 MON avgas. Swift Fuels of West Lafayette, Indiana, is the only fuel manufacturer currently offering a UL94 grade based on a proprietary formulation.5 The company has estimated that some 26,500 aircraft identified in the 2011 FAA study as requiring 100LL could in fact operate on UL94, and conse- quently about 68 percent of the current fleet (rather than 57 percent) could use this grade if allowed by an amended TC, STC, or applicable SAIB.6 Indeed, Swift Fuels sells STCs to owners of aircraft that are operationally capable of using UL94 but have a TC that does not explicitly identify UL94 as a permissible fuel.7 In 2016, FAA issued SAIB-HQ-16-05R1, clarifying that aircraft that require 80 and 91 MON can safely operate on UL94 avgas, and the bul- letin also confirmed that aircraft types approved for UL94 under an earlier ASTM International standard (D7592) could use UL94 grades that conform 5  See the Swift Fuels website at https://www.swiftfuelsavgas.com. 6  Data supplied to the committee by Chris D’Acosta, Swift Fuels, March 16, 2020. 7  See https://www.aopa.org/news-and-media/all-news/2020/march/10/swift-fuels-cuts-price- of-unleaded-avgas-stc. 

94 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT to current ASTM D7547. Lycoming Engines, which is among the three larg- est producers of piston aircraft engines, has published a service bulletin (SI 1070) that specifies fuel compatibility for all of its engines.8 According to this bulletin, approximately one-third of Lycoming’s roughly 200 engine model variants can use unleaded avgas that meets ASTM D7547. Unfortunately, without knowing how many aircraft in the current fleet have these engine variants, it not possible to translate this information into aircraft fleet counts. The other current manufacturers of piston engines for aviation who have significant sales market shares are Continental Aerospace Technolo- gies and Rotax. All current Rotax engines are approved to operate using 91 AKI (anti-knock index) unleaded automotive gas, which is essentially 87 MON. So as not to confuse aircraft owners, Continental has not published engine-by-engine guidance on the use of unleaded fuels, deferring to the airframe manufacturers who hold greater authority over fuels used in the aircraft they produce.9 One potential technical challenge for users of UL94, or any other un- leaded gasoline, is finding a way to replace the often beneficial effect of lead on maintaining engine valve health. Lycoming explicitly requires a specific lubricity oil additive when using unleaded avgas, while Rotax recommends occasional modest 100LL use to provide lubrication. Continental’s position is that its engines do not depend on TEL for valve lubrication and wear resistance, although it anticipates that certain proposed unleaded fuel for- mulations may drive changes in aviation lubricants while remaining com- patible with the prevailing J1899/J1966 standards of SAE International.10 The uncertainty about the precise share of the piston-engine fleet that can operate on an unleaded 94 MON avgas is accompanied by additional uncertainty about the extent to which total 100LL consumption could be reduced by the use of this unleaded alternative by the eligible portion of the piston-engine fleet. The reason for uncertainty about the impacts on total consumption of leaded avgas is that 100 MON avgas is generally required by the higher performance aircraft in the fleet that are frequently described as the fleet’s most heavily used and high-fuel consuming “working” air- craft. The Aircraft Owners and Pilots Association (AOPA) has estimated that these high-performance aircraft consume up to 70 percent of the total amount of 100LL sold annually (AOPA, 2010). Hence, if this estimate is accurate and remains relevant to the current fleet, then even if UL94 were used by all eligible aircraft, the reduction in leaded fuel consumption would be substantially smaller (on the order of 30 percent) than simple counts of eligible aircraft (57 or 68 percent of the fleet) would suggest is possible.  8  See https://www.lycoming.com/service-instruction-no-1070-AB.  9  Personal communication, Christopher Pollitt, Continental, April 28, 2020. 10  Personal communication, Christopher Pollitt, Continental, April 28, 2020. 

EXISTING FUEL OPTIONS FOR PISTON-ENGINE AIRCRAFT 95 Nevertheless, even if aircraft that can use UL94 account for only about 30 percent of the 100LL consumed each year, there would be a proportional 30 percent reduction in lead emissions from their transition to UL94 and the co-benefit of savings to operators in engine maintenance due to less lead fouling.11 As noted earlier, however, the challenge facing a producer of UL94, or any other fuel alternative, is that the avgas market is already small, making it potentially uneconomic to produce and widely distribute a second low- volume fuel that would have accompanying requirements for investments in new fuel storage and dispensing systems at many small airports. MOGAS The Statement of Task for this study requires an examination of the ap- plicability to piston-engine aircraft of unleaded motor gasoline, presumed to be in reference to the MOGAS listed as an operating limitation in some aircraft STCs. While FAA has not defined MOGAS, it generally refers to the automotive gasoline that could be purchased by the aircraft owner at the time an STC was issued permitting the use of this octane fuel. MOGAS is identified as an operating fuel for thousands of aircraft in the current pis- ton-engine fleet because of STCs approved some 40 years ago. At that time, automotive gasoline based on ASTM International standard D439 was the fuel that would have been evaluated for the STC applications (FAA, 1980). It is important to keep in mind, however, that this standard is no longer valid and many changes have been made to automotive gasoline since the 1980s, raising questions about whether the MOGAS tested and approved for STCs many years ago is consistent with the automotive gasoline being produced and dispensed today. The STC application process, as noted earlier, is a method to demon- strate that aircraft and engines can meet performance and safety objectives when using fuels other than those identified in the primary TC. Motivated by a desire to use less expensive automotive gasoline, several innovators and entrepreneurs in the aviation community performed the testing needed to secure FAA approval of STCs with a MOGAS fuel operating limitation using FAA Advisory Circular 91-33A (FAA, 1984). Most of the MOGAS STCs were developed by the Experimental Aircraft Association (EAA) and Petersen Aviation in the early 1980s. Since that time, some 62,000 MOGAS STCs have been issued; 24,000 by EAA and 38,000 by Petersen Aviation.12 The STCs were largely for aircraft and engines whose TCs specified 80/87 MON avgas, which had been widely available decades ago but whose production had been phased out a decade earlier in favor of the universal 11  This presumes that leaded fuel is periodically used to ensure valve health. 12  See https://www.eaa.org/eaa/pilots/EAA-STC-Program/auto-fuel-stc. 

96 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT 100LL grade. The gasoline produced for automobile use at the time (early 1980s) satisfied the 80/87 MON requirements for the aircraft that obtained MOGAS STCs. Thus, when many of the STCs were issued, MOGAS would have been the finished conventional premium gasoline (either unleaded or leaded13) commonly dispensed at automobile filling stations.14 During the 1980s when the MOGAS STCs were approved, ethanol was sometimes blended into automotive gasoline due to tax incentives and as an oxygenate to increase the octane rating of unleaded grades. Because alcohol is a polar solvent that attracts water, its addition to fuel can greatly increase the chances of fuel system corrosion, shorten the storage life of fuel, and lead to phase separation in the aircraft fuel tank, causing potential vapor lock problems at altitude. Accordingly, ethanol-blended automotive gaso- line would not have been permitted, and indeed, the MOGAS STCs require the user to exercise caution in ensuring the use of a fuel that is alcohol- free. However, ethanol blending was not required, and finished leaded and unleaded gasoline that was free of alcohol would have been available at many filling stations throughout the decade. Moreover, because the ethanol was usually added to the gasoline at bulk terminals before distribution to retail outlets, these terminals could have been the source of ethanol-free, pre-finished gasoline supplies for use in aviation, as long as the fuel met the properties (such as Reid Vapor Pressure [RVP]) and quality control require- ments of the MOGAS STC. Given that more than 60,000 aircraft received MOGAS STCs dur- ing the 1980s, tens of thousands are likely to remain in the fleet today. However, much has changed in the automotive fuels market since these STCs were approved in the 1980s. A revised automotive gasoline stan- dard, ASTM D4814, was introduced in 1988 and replaced ASTM D439. This new specification accounted for the blending of oxygenates.15 FAA approved D4814 as a “non-applicable ASTM fuel specification,” which permits it to be identified as an operating limitation for fuel in a TC or STC. However, according to FAA A/C 20-24D, TC and STC holders must apply for an amendment each time the revision number changes for an ASTM International standard. Because D4814 did not exist at the time the MOGAS STCs were approved, fuels that conform to the standard do not necessarily meet the STC’s operating limitations. The applicability of MOGAS as an aviation fuel option has decreased even more over the past 30 years because of further developments in the 13  Conventionalleaded premium gasoline contained up to 1.12 grams of lead per liter. 14  Conventionalgasoline is defined as finished motor gasoline, not including oxygenated or reformulated gasolines. 15  ASTM International standard D4814-20a, Standard Specification for Automotive Spark Ignition Engine Fuel. 

EXISTING FUEL OPTIONS FOR PISTON-ENGINE AIRCRAFT 97 automotive fuels market. Of particular significance were new U.S. Envi- ronmental Protection Agency (EPA) regulatory requirements implementing the Clean Air Act Amendments of 1990 that would have marked effects on the composition and physical properties of automotive gasoline start- ing in the mid-1990s (Martel, 1995). Not only did the requirements cause changes in the composition of finished automotive gasoline, but they also led to changes in intermediate refinery stocks. The new intermediate prod- ucts, known as CBOB (conventional gasoline for oxygenate blending) and RBOB (reformulated gasoline for oxygenate blending),16 were formulated specifically to address the effects of ethanol in increasing RVP and octane. For example, a 10 percent ethanol blend will increase RVP by about 1 psi (from a 9 psi base gasoline) and increase octane by about 3 octane numbers of the AKI used for automotive gasoline (slightly less for MON) (Bailey and Russell, 1981). Accordingly, CBOB and RBOB refinery stocks were formulated to have a lower MON and RVP to account for the effects that ethanol would have on these properties when blended to produce the finished gasoline dispensed at filling stations. Accordingly, even refinery products could no longer be generally relied on as a source of MOGAS by the end of the 1990s. The composition of automotive gasoline was further impacted by the Renewable Fuel Standard (RFS) created under the Energy Policy Act of 2005 and amended by the Energy Independence and Security Act of 2007. The RFS program is a national policy that requires that a certain volume of renewable fuel be used per year to replace or reduce the quantity of petroleum-based transportation fuel, heating oil, and jet fuel. To meet the program’s requirements, which do not apply to avgas or gasoline used for non-highway applications, ethanol is blended at 10 percent in essentially all dispensed automotive gasoline.17 As a result, the supplies of unleaded auto- motive gasoline grades that are widely available at filling stations today are substantially different from the supplies that were widely available when thousands of MOGAS STCs were approved 40 years ago to take advantage of the lower-priced automotive gasoline. According to the website AiRNav, 87 fixed base operators (FBOs) dis- pensed a fuel said to be “MOGAS (auto)” (and thus presumably unleaded) during August and September 2020,18 or about 2.5 percent of the more than 3,572 FBOs reported to be dispensing 100LL. AiRNav also reported that the price of this unleaded fuel averaged about $1 per gallon less than the price of 100LL. However, neither the validity of this count nor the 16  See 40 CFR 80.2. 17  In 2006, EPA removed the regulatory requirement that reformulated gasoline have 2.0 percent oxygenate by weight. See 71 FR 26691, May 8, 2006. 18  See http://airnav.com/fuel/report.html as of September 16, 2020. 

98 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT properties of this fuel, including its full and consistent compliance with the operating limitations of aircraft that may be using it, could be assessed for this study. ANALYSIS OF EXISTING FUELS TO REDUCE LEAD Current lead emissions from avgas are estimated by EPA to be about 468 tons per year.19 Considering the three fuels that are discussed above as possible pathways to reducing these emissions, namely UL94, MOGAS, and 100VLL, there remain technical, regulatory, and market challenges that differ in each case. A significant challenge for the first two unleaded options (UL94 and MOGAS) is that they would still require significant use of 100LL or 100VLL to accommodate high-performance aircraft, and thus would require the production, distribution, storage, and dispensing of at least two aviation fuels, each in smaller quantities than 100LL today. As has been noted, by the early 1970s the avgas market had shrunk to the point that it was economically feasible to support only one avgas grade (100LL), even though GA fuel demand was about twice as large as it is today. Moreover, FAA now forecasts that demand for avgas will decline about 0.6 percent per year through 2040.20 This downward trend in avgas consump- tion, if it happens as forecast, would produce its own reductions in lead emissions of about 10 percent in 20 years. However, the same downward trend in fuel demand would also make a dual-fuel option even less economi- cally viable because it would need to be accompanied by investments by fuel suppliers and airports in additional fuel production, distribution, storage, and dispensing capacity. MOGAS as a Mitigation Option From a retail perspective, the most widely available lead-free gasoline is automotive gasoline. When considering the potential for this fuel alterna- tive to reduce lead emissions from aviation, however, this outcome seems questionable for several reasons. As discussed above, the formulations and grades of finished automotive gasoline as they existed 40 years ago (87 and 91 AKI or 83 and 87 MON) when STCs were approved to permit the use of MOGAS have essentially disappeared. The automotive gasoline that 19  2017 National Emissions Inventory. Note that this estimate of lead emissions may be high, as EPA assumes that all 100LL avgas contains the maximum amount of TEL permitted by ASTM International, or 0.56 grams per liter. CRC (2010) reported that the average lead content of 100LL is 0.47 grams per liter, which would reduce the EPA estimated annual total to 393 tons. However, the higher EPA estimate is used as a baseline here. 20  See Table 31 of FAA Aerospace Forecast FY 2020–2040 at https://www.faa.gov/data_ research/aviation/aerospace_forecasts. 

EXISTING FUEL OPTIONS FOR PISTON-ENGINE AIRCRAFT 99 is dispensed today almost invariably contains at least 10 percent ethanol, detergent additives, and significant variations in RVP. Some refiners and blenders may have access to or be able to create the alkylate-blend stocks needed to reformulate a portion of the CBOB and RBOB stocks used for premium automotive gasoline to make these supplies suitable for piston-engine aircraft (i.e., a product with AKI of 87 or 91 or 83 or 87 MON developed under ASTM D7547). However, that unleaded product specifically amended for aviation use would no longer be the same mass-produced, widely available, and relatively inexpensive fuel that prompted interest in using automotive gasoline when the MOGAS STCs were approved decades ago. In this regard, the pursuit of such a niche, aviation-tailored fuel would seem to offer no economic advantage over the unleaded avgas grade (e.g., UL94) now available. The investments that would be needed for the significant changes required at the refinery or blending facilities to make such a suitable MOGAS, and the added down- stream requirements for distribution and storage, suggest that interest in supplying another lower octane gasoline for aviation that is derived from automotive gasoline stocks is likely to be negligible. It merits noting that in 2015, EIA estimated that about 5.3 billion gal- lons per year of ethanol-free gasoline were provided to final consumers.21 Setting aside questions about the logistics and distribution of these stocks for aviation access, this volume greatly exceeds what would be needed to meet any MOGAS demand by GA. It is possible that this reported volume was an unused blending component for regular, mid-grade, or premium gasoline or some other refinery stream. However, the MON and RVP of this ethanol-free fuel are not known. Thus, even if similar volumes exist today and a diligent purchaser is able to find an ethanol-free gasoline that is commercially available locally, the product’s qualities such as RVP, oc- tane, and additive packages may not be readily known, and therefore its compliance with MOGAS STC requirements would not be assured. It is important to recognize the high level of quality control associated with all aviation products, including aviation fuel, which is sampled and checked for conformance to specifications at multiple points in the supply chain. Supplies of MOGAS do not have these quality controls. UL94 as a Mitigation Option The expanded availability of an unleaded 94 MON fuel holds more prom- ise than MOGAS as an approach for reducing lead use because this fuel is already approved for use by many aircraft and has the potential to be approved for use by more. Its use by the lower-performance aircraft in 21  See https://www.eia.gov/todayinenergy/detail.php?id=26092. 

100 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT the piston-engine fleet would not eliminate lead entirely but could reduce the amount of lead consumed substantially, on the order of 30 percent (as calculated using the assumptions above), if made widely available and pur- chased by operators of all eligible aircraft. A proprietary UL94 developed by at least one manufacturer (Swift Fuels) that is compliant with ASTM 7547 already exists in the marketplace, albeit in only a fraction of potential refueling locations (fewer than 100 airports). Because any transition to this fuel would require a change in operating limitations, many aircraft owners interested in using it would need to obtain an STC. The requirements to demonstrate that such a proposed fuel change would not adversely affect safety of flight are understandably onerous, and thus potentially expensive for owners of aircraft not already certified for UL94. As a result, Swift has developed STCs that can be purchased at nominal cost by owners of many aircraft types. It is notable that Section 565 of the FAA Reauthorization Act of 2018 authorizes FAA to permit the use of unleaded fuel in aircraft certified on leaded fuel in cases where the aircraft can operate safely on the unleaded alternative. Presumably, this authority could be used to streamline the regulatory process to allow widespread use of unleaded fuels such as UL94 without requiring individual STCs.22 If every aircraft that could use UL94 did use this unleaded grade, per- haps 57 to 68 percent of annual fleet lead emissions would be removed, or 267 to 318 tons from the annual baseline of 468 tons cited above. However, if previously cited AOPA estimates are correct (i.e., the residual 32 to 43 percent “working” fleet burns as much as 70 percent of the 100LL), reduc- tions in lead emissions would be about 140 tons. In either case, of course, the availability of UL94 would need to be almost universal if it were to be used by all eligible aircraft, and therefore nearly all airports that serve a wide variety of GA aircraft would need supplies of the fuel on site for dispensing. As a result, one would expect the lead reductions to be lower than calculated, under the assumption that many small airports would not have the capacity to dispense UL94, which might only be available at larger airports that have the financial capability and economic incentive to add the needed fueling capacity. It merits noting that these estimates of lead reductions from using UL94 do not consider the period over which the transition to this fuel would occur. Keeping in mind that overall consumption of avgas is expected to decline over the next two decades as a result of reduced GA activity gen- erally, the annual lead tonnage reductions attributable to the transition 22  In addition to an appropriate STC, suitable placarding and directions to maintain valve seat health may be necessary. For example, valve seat health could probably be maintained with the occasional use of 100LL as long as it remains available, or the use of oil lubricity additives, such as those specified by Lycoming. 

EXISTING FUEL OPTIONS FOR PISTON-ENGINE AIRCRAFT 101 to UL94 would be smaller in absolute terms the further out in time this transition begins. 100VLL as a Mitigation Option For the past decade, 100VLL has been approved for use by all aircraft that require 100LL or an avgas with a lower MON. Its availability for purchase could therefore bring about a meaningful reduction in lead emis- sions because the entire fleet could use it without any STCs, subject to the ability of the refiners to produce 100VLL within its more tightly specified lead additive range. In being universally usable, 100VLL would not require any new investments in fuel storage and dispensing capacity if it were to replace 100LL at all airports or even some airports. Misfueling or comin- gling of 100LL and 100VLL would not be a concern for operators of high- performance aircraft as they would for a leaded fuel being accompanied by a lower octane unleaded grade. As noted earlier, the average lead content of 100LL has been found to be 0.47 grams per liter. The average lead content of 100VLL cannot be determined because it is not being produced. However, if it is simply assumed that the average lead content of 100VLL would be 19.6 percent lower than 0.47 grams per liter (consistent with the 19.6 percent lower maximum allowable lead content in 100VLL), then the average lead con- tent for 100VLL would be about 0.36 grams per liter [0.47 – (0.196 × 0.47)]. If 100VLL were to replace 100LL entirely, then one would expect a corresponding 19.6 percent reduction in the amount of lead emissions from avgas consumed across the piston-engine fleet. A 19.6 percent reduc- tion in lead emissions, using the baseline of 468 tons per year referenced above, would yield a reduction of 92 tons per year. As noted earlier, these annual tonnage reductions would depend on the timing of the transition of 100VLL, because total lead emissions will also decline as a result of long- term reductions in GA activity. An advantage of transitioning to 100VLL is that it could yield appre- ciable reductions in lead use without dividing the already small avgas mar- ket or requiring new investments in fuel supply infrastructure. However, if a switch to 100VLL were accompanied by a switch to UL94 by all aircraft that can use it, much larger lead reductions could be achieved. Even if one assumes that 70 percent of all avgas is consumed by high-performance air- craft that require 100 MON, their use of 100VLL with 19.7 percent lower lead content than 100LL would still result in a 13.7 percent reduction in total lead use (0.7 × 19.6). When added to the 30 percent reduction in lead use that might be achieved by the remaining share of the fleet that fully transitions to UL94, the total reduction could exceed 40 percent, or about 205 tons of lead per year. 

102 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT FINDINGS AND RECOMMENDATIONS While the downward trend in GA activity should yield gradual reductions in lead emissions from avgas consumption, larger reductions will require lower-lead or unleaded fuel alternatives to 100LL. Because the activity of the piston-engine fleet has been declining by an average of 1.6 percent per year during the past four decades and is expected by FAA to continue to decline by 0.6 percent per year during the next two decades,23 total lead use by the GA sector has been on a modest downward trajectory and is projected to be 10 percent lower within 20 years (Finding 5.1). 100VLL is currently the only ASTM International–specified fuel other than 100LL that could be used by all piston-engine aircraft in the existing fleet. The upper lead limit of 100VLL is 19.6 percent lower than the up- per limit in 100LL. Fleetwide use of 100VLL, therefore, offers a potential means of reducing total lead emissions from avgas by an amount approach- ing 20 percent. However, 100VLL is not currently being produced, presum- ably because there are no regulatory requirements or apparent economic incentives for fuel producers to supply fuel that can meet the tighter lead ranges in its ASTM International standard (Finding 5.2). At least 57 percent, and perhaps as much as 68 percent, of the cur- rent piston-engine fleet could use UL94, which is the only existing grade of unleaded avgas. However, this outcome would require special FAA certifications for some aircraft. The eligible fleet consists mostly of smaller, lower-performance aircraft that are not used as frequently as the higher- performance fleet that requires leaded avgas. Therefore, the reduction in leaded avgas use from making UL94 widely available is not likely to be proportional to the large share of lower performance aircraft in the fleet. Nevertheless, if all of these aircraft were to use UL94, lead emissions would be reduced by an estimated 30 percent. In addition, if higher-performance aircraft were to use 100VLL, reductions in lead emissions would exceed 40 percent (Finding 5.3). An unleaded fuel, such as UL94, approved for only part of the piston- engine fleet would require creating a second supply chain and fuel distribu- tion system across the nation. Such a fragmentation of avgas supplies into two grades that are each produced in lower volumes could also lead to higher avgas prices due to the loss of scale economies. Furthermore, the cost for airports to add storage and distribution facilities for a second fuel could be significant and potentially prohibitive, especially for small airports. Con- sequently, widespread availability of UL94 might be overly optimistic, and 23  See Table 31 of FAA Aerospace Forecast FY 2020–2040 at https://www.faa.gov/data_ research/aviation/aerospace_forecasts. 

EXISTING FUEL OPTIONS FOR PISTON-ENGINE AIRCRAFT 103 more likely to be restricted to a portion of airports that have or can afford to add the required fueling facilities (Finding 5.4). Automotive gasoline formulations are no longer a viable option for reducing lead emissions from the piston-engine fleet. Thousands of piston- engine aircraft were approved during the 1980s to use automotive gasoline formulations—loosely called MOGAS—that were then deemed to be safe substitutes for low octane avgas grades (80/87 MON) permitted by the aircraft’s original TCs. However, the composition of automotive gasoline has changed considerably during the past 30 years, particularly to include ethanol blends that are not compatible with almost all aircraft engines (Finding 5.5). FAA should research public policy options, which could be imple- mented as quickly as possible at the federal and state levels, as well as by Congress, for motivating refiners to produce and airports to supply 100VLL. The objective would be to reduce lead emissions from the entire piston-engine fleet while unleaded alternatives are being pursued for fleetwide use (Recommendation 5.1). FAA should research public policy options that will enable and encour- age greater use of available unleaded avgas by the portion of the piston- engine fleet that can safely use it. Possible options include (a) issuing an SAIB that will permit such use and (b) providing airports with incen- tives and means to supply unleaded fuel, particularly airports that are eligible for FAA-administered federal aid as part of the National Plan for Integrated Airport Systems (Recommendation 5.2). A mechanism should be established for facilitating the increased avail- ability of existing grades of unleaded avgas across the fleet of piston- engine aircraft. Fulfilling that need would likely require congressional involvement, such as by providing incentives for pilots to use existing unleaded avgas and for more small airports to add requisite fuel storage and dispensing capacity (Recommendation 5.3). REFERENCES AOPA (Aircraft Owners and Pilots Association). 2010. Preparing for an Unleaded Future: Lay- ing the Groundwork to Find the Next Avgas. AOPA Pilot. September 5. https://www.aopa. org/news-and-media/all-news/2010/september/pilot/preparing-for-an-unleaded-future. Bailey, B., and J. Russell. 1981. Emergency Transportation Fuels: Properties and Performance. SAE Technical Paper 810444. February. CRC (Coordinating Research Council). 2010. Investigation of Reduced TEL Content in Commercial 100LL Avgas. CRC Report No. 657. October 14. https://crcao.org/?s= 67-2010&orderby=relevance&post_type=post%2Cpage%2Cguide. 

104 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT FAA (Federal Aviation Administration). 1980. Light Aircraft Engines, the Potential and Problems for Use of Automotive Fuels, Phase 1—Literature Search. Report CT 81-150. December. https://apps.dtic.mil/dtic/tr/fulltext/u2/a094154.pdf. FAA. 1984. Use of Alternate Grades of Aviation Gasoline for Grade 80/87, and Use of Au- tomotive Gasoline. Advisory Circular AC No. 91-33A. July 18. https://www.faa.gov/ documentLibrary/media/Advisory_Circular/AC91-33A.pdf. FAA. 2011. Aviation Fuels Research Reciprocating Engine Aircraft Fleet Fuel Distribution Report. DOT/FAA/AR-TN11/22. November. http://www.tc.faa.gov/its/worldpac/techrpt/ artn11-22.pdf. FAA. 2018. Approval of Propulsion Fuels, Additives, and Lubricating Oils. Advisory Circular 20-24D chg. 1. February 20. https://www.faa.gov/documentLibrary/media/Advisory_ Circular/AC_20-24D_Chg_1.pdf. Martel, J.S. 1995. The explosion of Clean Air Act regulation of fuels. The Environmental Law Reporter 25(10). ELR 10538. https://elr.info/news-analysis/25/10538/explosion-clean- air-act-regulation-fuels. NASEM (National Academies of Sciences, Engineering, and Medicine). 2019. Airport Man- agement Guide for Providing Aircraft Fueling Services. Washington, DC: The National Academies Press. https://doi.org/10.17226/25400.