Options for Reducing Lead Emissions from Piston-Engine Aircraft (2021) / Chapter Skim
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5 Existing Fuel Options for Piston-Engine Aircraft to Reduce Lead
5 Existing Fuel Options for Piston-Engine Aircraft to Reduce Lead
Pages 87-104
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From page 87... ...
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)
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From page 88... ...
. 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.
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From page 89... ...
, 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.
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From page 90... ...
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.
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From page 91... ...
This convergence permitted the use of a common avgas distribution 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)
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From page 92... ...
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 proprietary UL94 avgas and an appropriately formulated automotive gasoline, or MOGAS.
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From page 93... ...
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 consequently 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 bulletin also confirmed that aircraft types approved for UL94 under an earlier ASTM International standard (D7592)
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From page 94... ...
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 formulations may drive changes in aviation lubricants while remaining compatible 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.
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From page 95... ...
The STC application process, as noted earlier, is a method to demonstrate 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)
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From page 96... ...
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.
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From page 97... ...
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 automotive 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)
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From page 98... ...
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.
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From page 99... ...
. 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.
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From page 100... ...
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, perhaps 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.
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From page 101... ...
Misfueling or comingling of 100LL and 100VLL would not be a concern for operators of highperformance 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.
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From page 102... ...
However, 100VLL is not currently being produced, presumably 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)
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From page 103... ...
Thousands of pistonengine 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.
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From page 104... ...
2011. Aviation Fuels Research Reciprocating Engine Aircraft Fleet Fuel Distribution Report.
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