OIL AND GAS INDUSTRY GLOBAL: June 2017

Saturday, June 17, 2017

Avgas

Avgas (aviation gasoline, also known as aviation spirit in the UK) is an aviation fuel used in aircraft with spark-ignited internal combustion engines. Avgas is distinguished from conventional gasoline (petrol) used in motor vehicles, which is termed mogas (motor gasoline) in an aviation context. Unlike motor gasoline, which has been formulated without lead since the 1970s to allow the use of catalytic converters for pollution reduction, the most commonly used grades of avgas still contain tetraethyl lead, a toxic lead-containing additive used to aid in lubrication of the engine, increase octane rating, and prevent engine knocking (spark-knock). There are ongoing efforts to reduce or eliminate the use of lead in aviation gasoline.

Kerosene-based jet fuel is formulated to suit the requirements of turbine engines which have no octane requirement and operate over a much wider flight envelope than piston engines. Kerosene is also used by most diesel piston engines developed for aviation use, such as those by SMA Engines, Austro Engine, and Thielert.

Properties

The main petroleum component used in blending avgas is alkylate, which is a mixture of various isooctanes. Some refineries also use reformate. All grades of avgas that meet CAN 2–3[further explanation needed], 25-M82[further explanation needed] have a density of 6.01 pounds per US gallon (720 g/L) at 15 °C (59 °F). (6 lb/U.S. gal is commonly used in America for weight and balance computation.)[1] Density increases to 6.41 pounds per US gallon (768 g/L) at −40 °C (−40 °F), and decreases by about 0.1% per 1 °C (1.8 °F) increase in temperature.[2][3] Avgas has an emission coefficient (or factor) of 18.355 pounds per US gallon (2.1994 kg/L) of CO2[4][5] or about 3.07 units of weight CO2 produced per unit weight of fuel used. Avgas is less volatile, with a Reid vapor pressure range of 5.5 to 7 psi, than automotive gasoline, with a range of 8 to 14 psi. A minimum limit ensures adequate volatility for engine starting. The upper limits are related to atmospheric pressure at sea level, 14.7 psi, for motor vehicles and ambient pressure at 22,000 ft, 6.25 psi, for aircraft. The lower avgas volatility reduces the chance of vapor lock in fuel lines at altitudes up to 22,000 ft.[6]

The particular mixtures in use today are the same as when they were first developed in the 1940s, and were used in airline and military aero engines with high levels of supercharging; notably the Rolls-Royce Merlin engine used in the Spitfire and Hurricane fighters, Mosquito fighter-bomber and Lancaster heavy bomber (the Merlin II and later versions required 100-octane fuel), as well as the liquid-cooled Allison V-1710 engine, and air-cooled radial engines from Pratt & Whitney, Wright, and other manufacturers on both sides of the Atlantic. The high octane ratings were traditionally achieved by the addition of tetraethyllead, a highly toxic substance that was phased out of automotive use in most countries in the late 20th century.

Leaded avgas is currently available in several grades with differing maximum lead concentrations. (Unleaded avgas is also available.) Because tetraethyllead is a toxic additive, the minimum amount needed to bring the fuel to the required octane rating is used; actual concentrations are often lower than the permissible maximum.[citation needed] Historically, many post-WWII developed, low-powered 4- and 6-cylinder piston aircraft engines were designed to use leaded fuels; an unleaded replacement fuel is being developed and certified for these engines. Some reciprocating-engine aircraft still require leaded fuels, but some do not, and some can burn unleaded gasoline if a special oil additive is used.

Consumption

The annual US usage of avgas was 186 million US gallons (700,000 m3) in 2008, and was approximately 0.14% of the motor gasoline consumption. From 1983 through 2008, US usage of avgas declined consistently by approximately 7.5 million US gallons (28,000 m3) each year.[7] As of 2024, the annual US usage of avgas was 180 million US gallons (680,000 m3),[8] most of which contained lead,[9] and 170,000 aircraft in the US used leaded avgas.[9]

In Europe, avgas remains the most common piston-engine fuel. High prices have encouraged efforts to convert to diesel engines burning jet fuel, which is more readily available, less expensive, and has advantages for aviation use.[10]

Grades

Grades of avgas are identified by two numbers associated with its Motor Octane Number (MON).[11] The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the anti-knock index or "pump rating" given to automotive gasoline in the US. The second number indicates the octane rating of the fuel tested to the "aviation rich" standard, which tries to simulate a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure. For example, 100/130 avgas has an octane rating of 100 at the lean settings usually used for cruising and 130 at the rich settings used for take-off and other full-power conditions.[12]

Antiknock agents such as tetraethyl lead (TEL) help to control detonation and provide lubrication. One gram of TEL contains 640.6 milligrams of lead.

100LL (blue)

Taking a fuel sample from an under-wing drain using a GATS Jar fuel sampler. The blue dye indicates that this fuel is 100LL.

100LL (pronounced "one hundred low lead") may contain a maximum of one-half the tetraethyllead allowed in 100/130 (green) avgas.[16][39]

Some of the lower-powered (100–150 horsepower or 75–112 kilowatts) aviation engines that were developed in the late 1990s are designed to run on unleaded fuel and on 100LL, an example being the Rotax 912.[20]

Automotive gasoline

An EAA Cessna 150 used for American certification of auto fuel

Automotivegasoline –known as mogas or autogas among aviators –that does not contain ethanol may be used in certified aircraft that have a Supplemental Type Certificate (STC) for automotive gasoline, as well as in experimental aircraft and ultralight aircraft.[citation needed] Some oxygenates other than ethanol are approved, but these STC's prohibit ethanol-laced gasolines.[citation needed] Ethanol-treated gasoline is susceptible to phase-separation which is very possible due to the altitude/temperature changes light airplanes undergo in ordinary flight.[citation needed] This ethanol-treated fuel can flood the fuel system with water which can cause in-flight engine failure.[citation needed] Additionally, the phase-separated fuel can leave remaining portions that do not meet octane requirements due to the loss of the ethanol in the water-absorption process. Further, the ethanol can attack materials in aircraft construction which pre-date "gasahol" fuels.[citation needed] Most of these applicable aircraft have low-compression engines which were originally certified to run on 80/87 avgas and require only "regular" 87 anti-knock index automotive gasoline. Examples include the popular Cessna 172 Skyhawk or Piper Cherokee with the 150 hp (110 kW) variant of the Lycoming O-320.[citation needed]

Some aircraft engines were originally certified using a 91/96 avgas and have STC's available to run "premium" 91 anti-knock index (AKI) automotive gasoline. Examples include some Cherokees with the 160 hp (120 kW) Lycoming O-320 or 180 hp (130 kW) O-360, or the Cessna 152 with the O-235. The AKI rating of typical automotive fuel might not directly correspond to the 91/96 avgas used to certify engines, as motor vehicle pumps in the US use the so-called "(R + M)/2" averaged motor vehicle octane rating system as posted on gas station pumps. Sensitivity is roughly 8–10 points, meaning that a 91 AKI fuel might have a MON of as low as 86. The extensive testing process required to obtain an STC for the engine/airframe combination helps ensure that, for those eligible aircraft, 91 AKI fuel provides sufficient detonation margin under normal conditions.[citation needed]

Automotive gasoline is not a fully viable replacement for avgas in many aircraft, because many high-performance and/or turbocharged airplane engines require 100 octane fuel and modifications are necessary in order to use lower-octane fuel.[40][41]

Many general aviation aircraft engines were designed to run on 80/87 octane,[citation needed] roughly the standard (as unleaded fuel only, with the "{R+M}/2" 87 octane rating) for North American automobiles today. Direct conversions to run on automotive fuel are fairly common, by supplemental type certificate (STC). However, the alloys used in aviation engine construction are chosen for their durability and synergistic relationship with the protective features of lead, and engine wear in the valves is a potential problem on automotive gasoline conversions.[citation needed]

Fortunately, significant history of engines converted to mogas has shown that very few engine problems are caused by automotive gasoline[citation needed]. A larger problem stems from the higher and wider range of allowable vapor pressures found in automotive gasoline; this can pose some risk to aviation users if fuel system design considerations are not taken into account. Automotive gasoline can vaporize in fuel lines, causing a vapor lock (a bubble in the line) or fuel pump cavitation, thereby starving the engine of fuel. This does not constitute an insurmountable obstacle, but merely requires examination of the fuel system, ensuring adequate shielding from high temperatures and maintaining sufficient pressure in the fuel lines. This is the main reason why both the specific engine model as well as the aircraft in which it is installed must be supplementally certified for the conversion. A good example of this is the Piper Cherokee with high-compression 160 or 180 hp (120 or 130 kW) engines. Only later versions of the airframe with different engine cowling and exhaust arrangements are applicable for the automotive fuel STC, and even then require fuel-system modifications.[citation needed]

Vapor lock typically occurs in fuel systems where a mechanically-driven fuel pump mounted on the engine draws fuel from a tank mounted lower than the pump. The reduced pressure in the line can cause the more volatile components in automotive gasoline to flash into vapor, forming bubbles in the fuel line and interrupting fuel flow. If an electric boost pump is mounted in the fuel tank to push fuel toward the engine, as is common practice in fuel-injected automobiles, the fuel pressure in the lines is maintained above ambient pressure, preventing bubble formation. Likewise, if the fuel tank is mounted above the engine and fuel flows primarily due to gravity, as in a high-wing airplane, vapor lock cannot occur, using either aviation or automotive fuels. Fuel-injected engines in automobiles also usually have a "fuel return" line to send unused fuel back to the tank, which has the benefit of equalizing the fuel's temperature throughout the system, further reducing the chance of vapor lock developing.[citation needed]

In addition to vapor locking potential, automotive gasoline does not have the same quality tracking as aviation gasoline. To help solve this problem, the specification for an aviation fuel known as 82UL was developed as essentially automotive gasoline with additional quality tracking and restrictions on permissible additives. This fuel is not currently in production and no refiners have committed to producing it.[14]

Gasohol

Rotax allows up to 10% ethanol (similar to E10 fuel for cars) in the fuel for Rotax 912 engines. Light sport aircraft that are specified by the manufacturer to tolerate alcohol in the fuel system can use up to 10% ethanol.[20]

See also: Common ethanol fuel mixtures

Fuel dyes

Fuel dyes aid ground crew and pilots in identifying and distinguishing the fuel grades[13] and most are specified by ASTM D910 or other standards.[16] Dyes for the fuel are required in some countries.[42]

Phase-out of leaded aviation gasoline

The 100LL phase-out has been called "one of modern GA's most pressing problems",[43] because 70% of 100LL aviation fuel is used by the 30% of the aircraft in the general aviation fleet that cannot use any of the existing alternatives.[44][45][46]

There are three fundamental issues in using unleaded fuels without serious modification of the airframe/engine:

The fuel must have a high enough octane rating (and meet other specifications) to replace leaded fuels,

The engine must be certified to use the fuel, and

The airframe must also be certified to use the fuel.[47]

In February 2008, Teledyne Continental Motors (TCM) announced that the company is very concerned about future availability of 100LL, and as a result, they would develop a line of diesel engines.[48] In a February 2008 interview, TCM president Rhett Ross indicated belief that the aviation industry will be "forced out" of using 100LL in the near future, leaving automotive fuel and jet fuel as the only alternatives. In May 2010, TCM announced that they had licensed development of the SMA SR305 diesel engine.[49][50][51]

In November 2008, National Air Transportation Association president Jim Coyne indicated that the environmental impact of aviation is expected to be a big issue over the next few years and will result in the phasing out of 100LL because of its lead content.[52]

By May 2012, the US Federal Aviation Administration (FAA Unleaded Avgas Transition rulemaking committee) had put together a plan in conjunction with industry to replace leaded avgas with an unleaded alternative within 11 years. Given the progress already made on 100SF and G100UL, the replacement time might be shorter than that 2023 estimate. Each candidate fuel must meet a checklist of 12 fuel specification parameters and 4 distribution and storage parameters. The FAA has requested a maximum of US$60M to fund the administration of the changeover.[53][54] In July 2014, nine companies and consortiums submitted proposals to the Piston Aviation Fuels Initiative (PAFI) to assess fuels without tetraethyl lead. Phase one testing is performed at the William J. Hughes Technical Center for a FAA approved industry replacement by 2018.[55]

In July 2021, the first commercially-produced unleaded avgas, GAMI's G100UL, was approved by the Federal Aviation Administration through a Supplemental Type Certificate.[56]

Lycoming Engines provides a list of engines and fuels that are compatible with unleaded fuel. However, all of their engines require that an oil additive be used when unleaded fuel is used: "When using the unleaded fuels identified in Table 1, Lycoming oil additive P/N LW-16702, or an equivalent finished product such as Aeroshell 15W-50, must be used."[22] Lycoming also notes that the octane rating of the fuel used must also meet the requirements stated in the fuel specification, otherwise engine damage may occur due to detonation.

Prior to 2022, Teledyne Continental Motors (TCM) indicated that leaded avgas is required in their engines, and not unleaded auto fuels: "Current aircraft engines feature valve gear components which are designed for compatibility with the leaded ASTM D910 fuels. In such fuels, the lead acts as a lubricant, coating the contact areas between the valve, guide, and seat. The use of unleaded auto fuels with engines designed for leaded fuels can result in excessive exhaust valve seat wear due to the lack of lead with cylinder performance deteriorating to unacceptable levels in under 10 hours."[57]

In 2022, TCM changed its policy. They have announced a formal application to the FAA to approve the use of UL91 and UL94 in selected engines, stating that "Continental considers 91UL and 94UL fuel as a transitional step in a long-term strategy to reach a more sustainable aviation".[58]

New unleaded fuel grades

91UL (or UL91)

Hjelmco Oil first introduced unleaded Avgas grades in Europe in 2003, after its success with 80UL.[59] This grade of Avgas is manufactured to meet ASTM D7547.[60] Many common Lycoming engines are certified to run on this particular grade of Avgas,[22] and Cessna has approved the use of this fuel in a large number of their piston fleet.[61] This fuel is also usable in any aircraft in Europe[62] or the United Kingdom[63] where the engine is certified to use it, whether or not the airframe is certified to do so, too.

93UL (Ethanol-free 93AKI automotive gasoline)

The firm Airworthy AutoGas tested an ethanol-free 93 anti-knock index (AKI) premium auto gas on a Lycoming O-360-A4M in 2013. The fuel is certified under Lycoming Service Instruction 1070 and ASTM D4814.[64]

UL94 (formerly 94UL)

Unleaded 94 Motor octane fuel (UL94) is essentially 100LL without the lead. In March 2009, Teledyne Continental Motors (TCM) announced they had tested a 94UL fuel that might be the best replacement for 100LL. This 94UL meets the avgas specification including vapor pressure but has not been completely tested for detonation qualities in all Continental engines or under all conditions. Flight testing has been conducted in an IO-550-B powering a Beechcraft Bonanza and ground testing in Continental O-200, 240, O-470, and O-520 engines. In May 2010, TCM indicated that despite industry skepticism, they are proceeding with 94UL and that certification was expected in mid-2013.[65][66]

In June 2010, Lycoming Engines indicated their opposition to 94UL. Company general manager Michael Kraft stated that aircraft owners do not realize how much performance would be lost with 94UL and characterized the decision to pursue 94UL as a mistake that could cost the aviation industry billions in lost business. Lycoming believes the industry should be pursuing 100UL instead. The Lycoming position is supported by aircraft type clubs representing owners of aircraft that would be unable to run on lower octane fuel. In June 2010, clubs such as the American Bonanza Society, the Malibu Mirage Owners and Pilots Association, and the Cirrus Owners and Pilots Association collectively formed the Clean 100 Octane Coalition to represent them on this issue and push for unleaded 100 octane avgas.[67][68][69][70]

In November 2015, UL94 was added as a secondary grade of unleaded aviation gasoline to ASTM D7547, which is the specification that governs UL91 unleaded avgas. UL91 is currently being sold in Europe. UL94 meets all of the same specification property limits as 100LL with the exception of a lower motor octane number (94.0 minimum for UL94 vs. 99.6 minimum for 100LL) and a decreased maximum lead content. UL94 is an unleaded fuel, but as with all ASTM International unleaded gasoline specifications, a de minimis amount of unintentionally added lead is permitted.[60]

Since May 2016, UL94, now a product of Swift Fuels, is available for sale at dozens of airports in the United States. Swift Fuels has an agreement for distribution in Europe.[71][72][73]

UL94 is not intended to be a full replacement for 100LL, but rather is designed to be a drop-in replacement for aircraft with lower-octane-rated engines, such as those that are approved for operation on Grade 80 avgas (or lower), UL91, or mogas. It is estimated that up to 65% of the fleet of current general aviation piston-engine-powered aircraft can operate on UL94 with no modifications to either the engine or airframe. Some aircraft, however, do require a FAA-approved Supplemental Type Certificate (STC) to be purchased to allow for operation on UL94.[72][74][75]

UL94 has a minimum Motor octane number (MON, which is the octane rating employed for grading aviation gasoline) of 94.0. 100LL has a minimum MON of 99.6.[16][60]

AKI is the octane rating used to grade all U.S. automotive gasoline (typical values at the pump can include 87, 89, 91, and 93), and also the 93UL fuel from Airworthy AutoGas.

The minimum AKI of UL94, as sold by Swift Fuels, is 98.0.

Concurrent with the addition of UL94 to ASTM D7547, the FAA published Special Airworthiness Information Bulletin (SAIB) HQ-16-05, which states that "UL94 meets the operating limitations or aircraft and engines approved to operate with grade UL91 avgas," meaning that "Grade UL94 avgas that meets specification D7547 is acceptable to use on those aircraft and engines that are approved to operate with ... grade UL91 avgas that meets specification D7547."[76] In August 2016, the FAA revised SAIB HQ-16-05 to include similar wording regarding the acceptability of using UL94 in aircraft and engines that are approved to operate with avgas that has a minimum Motor octane rating of 80 or lower, including Grade 80/87.[77]

The publication of the SAIB, especially the August 2016 revision, eliminated the need for many of the UL94 STCs being sold by Swift Fuels, as the majority of the aircraft on the STC's Approved Model List are type-certified to use 80-octane or lower avgas.

On April 6, 2017, Lycoming Engines published Service Instruction 1070V, which adds UL94 as an approved grade of fuel for dozens of engine models, 60% of which are carbureted engines. Engines with displacements of 235, 320, 360, and 540 cubic inches make up almost 90% of the models approved for UL94.[22]

UL102 (formerly 100SF Swift Fuel)

Purdue University Cessna 150M Swift Fuel demonstrator

Swift Fuels, LLC, has attained approval to produce fuel for testing at its pilot plant in Indiana. Composed of approximately 85% mesitylene and 15% isopentane, the fuel is reportedly scheduled for extensive testing by the FAA to receive certification under the new ASTM D7719 guideline for unleaded 100LL replacement fuels. The company eventually intends to produce the fuel from renewable biomass feedstocks, and aims to produce something competitive in price with 100LL and currently available alternative fuels. Swift Fuels has suggested that the fuel, formerly referred to as 100SF, will be available for "high performance piston-powered aircraft" before 2020.[71]

John and Mary-Louise Rusek founded Swift Enterprises in 2001 to develop renewable fuels and hydrogen fuel cells. They began testing "Swift 142" in 2006[78] and patented several alternatives for non-alcohol based fuels which can be derived from biomass fermentation.[79] Over the next several years, the company sought to build a pilot plant to produce enough fuel for larger-scale testing[80][81] and submitted fuel to the FAA for testing.[82][83][84][85]

In 2008, an article by technology writer and aviation enthusiast Robert X. Cringely attracted popular attention to the fuel,[86] as also did a cross-country Swift-Fueled flight by the AOPA's Dave Hirschman.[87] Swift Enterprises' claims that the fuel could eventually be manufactured much more cheaply than 100LL have been debated in the aviation press.[82][88][89][90][91][92][93]

The FAA found Swift Fuel to have a motor octane number of 104.4, 96.3% of the energy per unit of mass, and 113% of the energy per unit of volume as 100LL, and to meet most of the ASTM D910 standard for leaded aviation fuel. Following tests in two Lycoming engines, the FAA concluded it performs better than 100LL in detonation testing and will provide a fuel saving of 8% per unit of volume, though it weighs 1 pound per US gallon (120 g/L) more than 100LL. GC–FID testing showed the fuel to be made primarily of two components — one about 85% by weight and the other about 14% by weight.[94][95] Soon afterward, AVweb reported that Continental had begun the process of certifying several of its engines to use the new fuel.[96]

From 2009 through 2011, 100SF was approved as a test fuel by ASTM International, allowing the company to pursue certification testing.[97][98] satisfactorily tested by the FAA,[99] tested by Purdue University,[100] and approved under ASTM specification D7719 for high-octane Grade UL102, allowing the company to test more economically in non-experimental aircraft.[101]

In 2012, Swift Fuels LLC was formed to bring in oil and gas industry experience, scale up production and bring the fuel to market. By November 2013, the company had built its pilot plant and received approval to produce fuel in it.[102] Its most recent patent, approved in 2013, describes methods by which the fuel can be produced from fermentable biomass.[103]

The FAA scheduled UL102 for 2 years of phase 2 testing in its PAFI initiative beginning in the summer of 2016.[104]

G100UL

In February 2010, General Aviation Modifications Inc. (GAMI) announced that it was in the process of developing a 100LL replacement to be called G100UL ("unleaded"). This fuel is made by blending existing refinery products and yields detonation margins comparable to 100LL. The new fuel is slightly more dense than 100LL, but has a 3.5% higher thermodynamic output. G100UL is compatible with 100LL and can be mixed with it in aircraft tanks for use.[90][31][105]

In demonstrations held in July 2010, G100UL performed better than 100LL that just meets the minimum specification and performed as well as average production 100LL.[106]

G100UL was approved by the Federal Aviation Administration by the issuance of a Supplemental Type Certificate at AirVenture in July 2021. The STC was initially only applicable to Lycoming-powered models of the Cessna 172. The company indicated that the retail cost was expected to be 0.60–0.85 US dollars per US gallon higher than 100LL.[56] This was later revised to 1.00 US dollar per US gallon.[31]

In 2022, Paul Bertorelli of AVweb reported that the FAA was dragging its feet on broadly certifying G100UL, delaying approval of the fuel for more engines and spending over $80 million on EAGLE to re-start a search for an unleaded fuel when G100UL had been under evaluation for over 10 years.[107]

In September 2022, in a surprise announcement, the FAA approved an STC for the use of the fuel for all piston-engined aircraft and engine combinations. In February 2023, GAMI began selling supplemental type certificates to allow aircraft owners to use the fuel when it becomes available.[108][109] In April 2024, GAMI announced that 1 million gallons of G100UL had been produced. Fuel availability in the US was forecast for airports in California, Washington and Oregon by the middle of 2024 and the rest of the country by 2026.[32][31][108][109]

In December 2024, shortly after G100UL was made available at selected airports in California, several concerns regarding material compatibility arose, as users reported fuel leaks, paint staining, and paint stripping.[110]

Shell Unleaded 100-Octane Fuel

In December 2013, Shell Oil announced that they had developed an unleaded 100 octane fuel and will submit it for FAA testing with certification expected within two to three years.[111] The fuel is alkylate-based with an additive package of aromatics. No information has yet been published with regard to its performance, producibility or price. Industry analysts have indicated that it will likely cost as much as or more than existing 100LL.[112]

UL100E

In 2018, LyondellBasell and VP Racing Fuels, an established motorsport fuel manufacturer, announced its intention to participate in developing an unleaded fuel for piston-powered aircraft. As of December 2024, it has reached Phase 4 of testing, according to the FAA.[113]

Environmental regulation

TEL found in leaded avgas and its combustion products are potent neurotoxins that have been shown in scientific research to interfere with brain development in children. Children in residences or childcare facilities in close proximity to airports with moderate to high piston engine aircraft traffic are at especially high risk of high blood lead levels.[114][115][116] The United States Environmental Protection Agency (EPA) has noted that exposure to even very low levels of lead contamination has been conclusively linked to loss of IQ in children's brain function tests, thus providing a high degree of motivation to eliminate lead and its compounds from the environment.[117][118]

While lead concentrations in the air have declined, scientific studies have demonstrated that children's neurological development is harmed by much lower levels of lead exposure than previously understood. Low level lead exposure has been clearly linked to loss of IQ in performance testing. Even an average IQ loss of 1–2 points in children has a meaningful impact for the nation as a whole, as it would result in an increase in children classified as mentally challenged, as well as a proportional decrease in the number of children considered "gifted".[118]

On November 16, 2007, the environmental group Friends of the Earth formally petitioned the EPA, asking them to regulate leaded avgas. The EPA responded with a notice of petition for rulemaking.[14]

The notice of petition stated:

Friends of the Earth has filed a petition with EPA, requesting that EPA find pursuant to section 231 of the Clean Air Act that lead emissions from general aviation aircraft cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare and that EPA propose emissions standards for lead from general aviation aircraft. Alternatively, Friends of the Earth requests that EPA commence a study and investigation of the health and environmental impacts of lead emissions from general aviation aircraft, if EPA believes that insufficient information exists to make such a finding. The petition submitted by Friends of the Earth explains their view that lead emissions from general aviation aircraft endanger the public health and welfare, creating a duty for the EPA to propose emission standards.[119]

The public comment period on this petition closed on March 17, 2008.[119]

Under a federal court order to set a new standard by October 15, 2008, the EPA cut the acceptable limits for atmospheric lead from the previous standard of 1.5 μg/m3 to 0.15 μg/m3. This was the first change to the standard since 1978 and represents an order of magnitude reduction over previous levels. The new standard requires the 16,000 remaining USA sources of lead, which include lead smelting, airplane fuels, military installations, mining and metal smelting, iron and steel manufacturing, industrial boilers and process heaters, hazardous waste incineration, and production of batteries, to reduce their emissions by October 2011.[117][118][120]

The EPA's own studies have shown that to prevent a measurable decrease in IQ for children deemed most vulnerable, the standard needs to be set much lower, to 0.02 μg/m3. The EPA identified avgas as one of the most "significant sources of lead".[121][122]

At an EPA public consultation held in June 2008 on the new standards, Andy Cebula, the Aircraft Owners and Pilots Association's executive vice president of government affairs, stated that general aviation plays a valuable role in the USA economy and any changes in lead standards that would change the current composition of avgas would have a "direct impact on the safety of flight and the very future of light aircraft in this country".[123]

In December 2008, AOPA filed formal comments to the new EPA regulations. AOPA has asked the EPA to account for the cost and the safety issues involved with removing lead from avgas. They cited that the aviation sector employs more than 1.3 million people in the US and has an economic direct and indirect effect that "exceeds $150 billion annually". AOPA interprets the new regulations as not affecting general aviation as they are currently written.[124]

Publication in the USA Federal Register of an Advance Notice of Proposed Rulemaking by the USA EPA occurred in April 2010. The EPA indicated: "This action will describe the lead inventory related to use of leaded avgas, air quality and exposure information, additional information the Agency is collecting related to the impact of lead emissions from piston-engine aircraft on air quality and will request comments on this information."[125][126]

Despite assertions in the media that leaded avgas will be eliminated in the US by 2017 at the latest date, the EPA confirmed in July 2010 that there is no phase-out date and that setting one would be an FAA responsibility as the EPA has no authority over avgas. The FAA administrator stated that regulating lead in avgas is an EPA responsibility, resulting in widespread criticism of both organizations for causing confusion and delaying solutions.[127][128][129][130][131]

In April 2011 at Sun 'n Fun, Pete Bunce, head of the General Aviation Manufacturers Association (GAMA), and Craig Fuller, president and CEO of the Aircraft Owners and Pilots Association, indicated that they both are confident that leaded avgas will not be eliminated until a suitable replacement is in place. "There is no reason to believe 100 low-lead will become unavailable in the foreseeable future," Fuller stated.[132]

Final results from EPA's lead modeling study at the Santa Monica Airport shows off-airport levels below current 150 ng/m3 and possible future 20 ng/m3 levels.[133] Fifteen of 17 airports monitored during a year-long study in the US by the EPA have lead emissions well below the current National Ambient Air Quality Standard (NAAQS) for lead.[134]

Other uses

Avgas is occasionally used in amateur auto racing cars as its octane rating is higher than automotive gasoline thus allowing the engines to run at higher compression ratios.[citation needed]

Rocket propellant

Rocket propellant is used as reaction mass ejected from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines.

Overview

Rockets create thrust by expelling mass rear-ward, at high velocity. The thrust produced can be calculated by multiplying the mass flow rate of the propellants by their exhaust velocity relative to the rocket (specific impulse). A rocket can be thought of as being accelerated by the pressure of the combusting gases against the combustion chamber and nozzle, not by "pushing" against the air behind or below it. Rocket engines perform best in outer space because of the lack of air pressure on the outside of the engine. In space it is also possible to fit a longer nozzle without suffering from flow separation.

Most chemical propellants release energy through redox chemistry, more specifically combustion. As such, both an oxidizing agent and a reducing agent (fuel) must be present in the mixture. Decomposition, such as that of highly unstable peroxide bonds in monopropellant rockets, can also be the source of energy.

In the case of bipropellant liquid rockets, a mixture of reducing fuel and oxidizing oxidizer is introduced into a combustion chamber, typically using a turbopump to overcome the pressure. As combustion takes place, the liquid propellant mass is converted into a huge volume of gas at high temperature and pressure. This exhaust stream is ejected from the engine nozzle at high velocity, creating an opposing force that propels the rocket forward in accordance with Newton's laws of motion.

Chemical rockets can be grouped by phase. Solid rockets use propellant in the solid phase, liquid fuel rockets use propellant in the liquid phase, gas fuel rockets use propellant in the gas phase, and hybrid rockets use a combination of solid and liquid or gaseous propellants.

In the case of solid rocket motors, the fuel and oxidizer are combined when the motor is cast. Propellant combustion occurs inside the motor casing, which must contain the pressures developed. Solid rockets typically have higher thrust, less specific impulse, shorter burn times, and a higher mass than liquid rockets, and additionally cannot be stopped once lit.

Rocket stages

In space, the maximum change in velocity that a rocket stage can impart on its payload is primarily a function of its mass ratio and its exhaust velocity. This relationship is described by the rocket equation. Exhaust velocity is dependent on the propellant and engine used and closely related to specific impulse, the total energy delivered to the rocket vehicle per unit of propellant mass consumed. The mass ratio can also be affected by the choice of a given propellant.

Rocket stages that fly through the atmosphere usually use lower-performing, high-molecular-mass, high-density propellants due to the smaller and lighter tankage required. Upper stages, which mostly or only operate in the vacuum of space, tend to use the high-energy, high-performance, low-density liquid hydrogen fuel. For future planetary missions the use of local resources and solar energy for in situ propellant production is considered.[1]

Solid chemical propellants

Solid propellants come in two main types. "Composites" are mostly a mixture of granules of solid oxidizer, such as ammonium nitrate, ammonium dinitramide, ammonium perchlorate, or potassium nitrate in a polymer binding agent, with flakes or powders of energetic fuel compounds such as RDX, HMX, aluminium, beryllium. Plasticizers, stabilizers, and burn rate modifiers (iron oxide, copper oxide) can be added.

Single-, double-, or triple-bases are homogeneous mixtures of one to three primary ingredients, which must include fuel and oxidizer, and often include binders and plasticizers. All components are macroscopically indistinguishable and often blended as liquids and cured in a single batch. Ingredients can have multiple roles: RDX is both fuel and oxidizer, while nitrocellulose is fuel, oxidizer, and structural polymer.

Many propellants contain elements of double-base and composite propellants, and often contain energetic additives homogeneously mixed into the binder. In the case of gunpowder (a pressed composite without a polymeric binder), the fuel is charcoal, the oxidizer is potassium nitrate, and sulphur serves as a reaction catalyst while also being consumed to form reaction products such as potassium sulfide.

The newest nitramine solid propellants based on CL-20 (HNIW) can match the performance of NTO/UDMH storable liquid propellants, but cannot be throttled or restarted.

Extraterrestrial on-site production is being explored by combining aluminum and ice (ALICE).

Advantages

Solid-propellant rockets are much easier to store and handle than liquid-propellant rockets. High propellant density makes for compact size as well. These features plus simplicity and low cost make solid-propellant rockets ideal for military applications.

Their simplicity also makes solid rockets a good choice whenever large amounts of thrust are needed and cost is an issue. The Space Shuttle and many other orbital launch vehicles use solid-fueled rockets in their boost stages (solid rocket boosters) for this reason.

Disadvantages

Solid-fuel rockets have lower specific impulse, a measure of propellant efficiency, than liquid-fuel rockets. As a result, the overall performance of solid upper stages is less than liquid stages even though the solid mass ratios are usually in the .91 to .93 range, as good as or better than most liquid-propellant upper stages. The high mass ratios possible with these unsegmented solid upper stages is a result of high propellant density and very high strength-to-weight ratio filament-wound motor casings.[citation needed]

A drawback to solid rockets is that they cannot be throttled in real time, although a programmed thrust schedule can be created by adjusting the interior propellant geometry. Solid rockets can be vented to extinguish combustion or reverse thrust as a means of controlling range or accommodating stage separation. Casting large amounts of propellant requires consistency and repeatability to avoid cracks and voids in the completed motor. The blending and casting take place under computer control in a vacuum, and the propellant blend is spread thin and scanned to ensure that no large gas bubbles are introduced into the motor.

Solid-fuel rockets are intolerant to cracks and voids and require post-processing such as X-ray scans to identify faults. The combustion process is dependent on the surface area of the fuel. Voids and cracks represent local increases in burning surface area, increasing the local temperature, which increases the local rate of combustion. This positive feedback loop can easily lead to catastrophic failure of the case or nozzle.

History

Solid rocket propellant was first developed during the 7th century under the Chinese Song dynasty. The Song Chinese first used gunpowder in 732 during the military siege of Kaifeng.[2][3][4][5][6]

During the 1950s and 60s, researchers in the United States developed ammonium perchlorate composite propellant (APCP). This mixture is typically 69-70% finely ground ammonium perchlorate (an oxidizer), combined with 16-20% fine aluminium powder (a fuel), held together in a base of 11-14% polybutadiene acrylonitrile (PBAN) or hydroxyl-terminated polybutadiene (polybutadiene rubber fuel). The mixture is formed as a thickened liquid and then cast into the correct shape and cured into a firm but flexible load-bearing solid. Historically, the tally of APCP solid propellants is relatively small. The military, however, uses a wide variety of different types of solid propellants, some of which exceed the performance of APCP. A comparison of the highest specific impulses achieved with the various solid and liquid propellant combinations used in current launch vehicles is given in the article on solid-fuel rockets.[7]

In the 1970s and 1980s, the U.S. switched entirely to solid-fueled ICBMs: the LGM-30 Minuteman and LG-118A Peacekeeper (MX). In the 1980s and 1990s, the USSR/Russia also deployed solid-fueled ICBMs (RT-23, RT-2PM, and RT-2UTTH), but retains two liquid-fueled ICBMs (R-36 and UR-100N). All solid-fueled ICBMs on both sides had three initial solid stages, and those with multiple independently targeted warheads had a precision maneuverable bus used to fine-tune the trajectory of the re-entry vehicles.

Liquid chemical propellants

Main article: Liquid-propellant rocket

Advantages

Liquid-fueled rockets have higher specific impulse than solid rockets and are capable of being throttled, shut down, and restarted. Only the combustion chamber of a liquid-fueled rocket needs to withstand high combustion pressures and temperatures. Cooling can be done regeneratively with the liquid propellant. On vehicles employing turbopumps, the propellant tanks are at a lower pressure than the combustion chamber, decreasing tank mass. For these reasons, most orbital launch vehicles use liquid propellants.

The primary specific impulse advantage of liquid propellants is due to the availability of high-performance oxidizers. Several practical liquid oxidizers (liquid oxygen, dinitrogen tetroxide, and hydrogen peroxide) are available which have better specific impulse than the ammonium perchlorate used in most solid rockets when paired with suitable fuels.

Some gases, notably oxygen and nitrogen, may be able to be collected from the upper atmosphere, and transferred up to low Earth orbit for use in propellant depots at substantially reduced cost.[8]

Disadvantages

The main difficulties with liquid propellants are also with the oxidizers. Storable oxidizers, such as nitric acid and nitrogen tetroxide, tend to be extremely toxic and highly reactive, while cryogenic propellants by definition must be stored at low temperature and can also have reactivity/toxicity issues. Liquid oxygen (LOX) is the only flown cryogenic oxidizer. Others such as FLOX, a fluorine/LOX mix, have never been flown due to instability, toxicity, and explosivity.[9] Several other unstable, energetic, and toxic oxidizers have been proposed: liquid ozone (O3), ClF3, and ClF5.

Liquid-fueled rockets require potentially troublesome valves, seals, and turbopumps, which increase the cost of the launch vehicle. Turbopumps are particularly troublesome due to high performance requirements.

Current cryogenic types

Liquid oxygen (LOX) and highly refined kerosene (RP-1). Used for the first stages of the Atlas V, Falcon 9, Falcon Heavy, Soyuz, Zenit, Angara, and Long March 6, among others. This combination is widely regarded as the most practical for boosters that lift off at ground level and therefore must operate at full atmospheric pressure.

LOX and liquid hydrogen. Used on the Centaur upper stage, the Delta IV rocket, the H-IIA rocket, most stages of the European Ariane 5, and the Space Launch System core and upper stages.

LOX and liquid methane (from liquefied natural gas). Used on Zhuque-2, Vulcan, and also planned for use on several rockets in development, including New Glenn, Soyuz-7, SpaceX Starship, and Rocket Lab Neutron.

Current storable types

Dinitrogen tetroxide (N2O4) and hydrazine (N2H4), MMH, or UDMH. Used in military, orbital, and deep-space rockets because both liquids are storable for long periods at reasonable temperatures and pressures. N2O4/UDMH is the main fuel for the Proton rocket, older Long March rockets (LM 1-4), PSLV, Fregat, and Briz-M upper stages. This combination is hypergolic, making for attractively simple ignition sequences. The major inconvenience is that these propellants are highly toxic and require careful handling.

Monopropellants such as hydrogen peroxide, hydrazine, and nitrous oxide are primarily used for attitude control and spacecraft station-keeping where their long-term storability, simplicity of use, and ability to provide the tiny impulses needed outweighs their lower specific impulse as compared to bipropellants. Hydrogen peroxide is also used to drive the turbopumps on the first stage of the Soyuz launch vehicle.[citation needed]

Mixture ratio

The theoretical exhaust velocity of a given propellant chemistry is proportional to the energy released per unit of propellant mass (specific energy). In chemical rockets, unburned fuel or oxidizer represents the loss of chemical potential energy, which reduces the specific energy. However, most rockets run fuel-rich mixtures, which result in lower theoretical exhaust velocities.[10]

However, fuel-rich mixtures also have lower molecular weight exhaust species. The nozzle of the rocket converts the thermal energy of the propellants into directed kinetic energy. This conversion happens in the time it takes for the propellants to flow from the combustion chamber through the engine throat and out the nozzle, usually on the order of one millisecond. Molecules store thermal energy in rotation, vibration, and translation, of which only the latter can easily be used to add energy to the rocket stage. Molecules with fewer atoms (like CO and H2) have fewer available vibrational and rotational modes than molecules with more atoms (like CO2 and H2O). Consequently, smaller molecules store less vibrational and rotational energy for a given amount of heat input, resulting in more translation energy being available to be converted to kinetic energy. The resulting improvement in nozzle efficiency is large enough that real rocket engines improve their actual exhaust velocity by running rich mixtures with somewhat lower theoretical exhaust velocities.[10]

The effect of exhaust molecular weight on nozzle efficiency is most important for nozzles operating near sea level. High-expansion rockets operating in a vacuum see a much smaller effect, and so are run less rich.

LOX/hydrocarbon rockets are run slightly rich (O/F mass ratio of 3 rather than stoichiometric of 3.4 to 4) because the energy release per unit mass drops off quickly as the mixture ratio deviates from stoichiometric. LOX/LH2 rockets are run very rich (O/F mass ratio of 4 rather than stoichiometric 8) because hydrogen is so light that the energy release per unit mass of propellant drops very slowly with extra hydrogen. In fact, LOX/LH2 rockets are generally limited in how rich they run by the performance penalty of the mass of the extra hydrogen tankage instead of the underlying chemistry.[10]

Another reason for running rich is that off-stoichiometric mixtures burn cooler than stoichiometric mixtures, which makes engine cooling easier. Because fuel-rich combustion products are less chemically reactive (corrosive) than oxidizer-rich combustion products, a vast majority of rocket engines are designed to run fuel-rich. At least one exception exists: the Russian RD-180 preburner, which burns LOX and RP-1 at a ratio of 2.72.

Additionally, mixture ratios can be dynamic during launch. This can be exploited with designs that adjust the oxidizer-to-fuel ratio (along with overall thrust) throughout a flight to maximize overall system performance. For instance, during lift-off, thrust is more valuable than specific impulse, and careful adjustment of the O/F ratio may allow higher thrust levels. Once the rocket is away from the launchpad, the engine O/F ratio can be tuned for higher efficiency.

Propellant density

Although liquid hydrogen gives a high Isp, its low density is a disadvantage: hydrogen occupies about 7 times more volume per kilogram than dense fuels such as kerosene. The fuel tankage, plumbing, and pump must be correspondingly larger. This increases the vehicle's dry mass, reducing performance. Liquid hydrogen is also relatively expensive to produce and store, and causes difficulties with design, manufacture, and operation of the vehicle. However, liquid hydrogen is extremely well-suited to upper stage use where Isp is at a premium and thrust-to-weight ratios are less relevant.

Dense-propellant launch vehicles have a higher takeoff mass due to lower Isp, but can more easily develop high takeoff thrusts due to the reduced volume of engine components. This means that vehicles with dense-fueled booster stages reach orbit earlier, minimizing losses due to gravity drag and reducing the effective delta-v requirement.

The proposed tripropellant rocket uses mainly dense fuel while at low altitude and switches across to hydrogen at higher altitude. Studies in the 1960s proposed single-stage-to-orbit vehicles using this technique.[11] The Space Shuttle approximated this by using dense solid rocket boosters for the majority of the thrust during the first 120 seconds. The main engines burned a fuel-rich hydrogen and oxygen mixture, operating continuously throughout the launch but providing the majority of thrust at higher altitudes after SRB burnout.

Other chemical propellants

Hybrid propellants

Main article: Hybrid rocket

Hybrid propellants consist of a storable liquid oxidizer used with a solid fuel, which retains most virtues of both liquids (high ISP) and solids (simplicity).

A hybrid-propellant rocket usually has a solid fuel and a liquid or NEMA oxidizer.[clarification needed] The fluid oxidizer can make it possible to throttle and restart the motor just like a liquid-fueled rocket. Hybrid rockets can also be environmentally safer than solid rockets since some high-performance solid-phase oxidizers contain chlorine (specifically composites with ammonium perchlorate), versus the more benign liquid oxygen or nitrous oxide often used in hybrids. This is only true for specific hybrid systems. There have been hybrids which have used chlorine or fluorine compounds as oxidizers and hazardous materials such as beryllium compounds mixed into the solid fuel grain. Because just one constituent is a fluid, hybrids can be simpler than liquid rockets that depend on the rocket's acceleration to transport the fluid into the combustion chamber. Fewer fluids typically mean fewer and smaller piping systems, valves, and pumps.

Hybrid motors suffer two major drawbacks. The first, shared with solid rocket motors, is that the casing around the fuel grain must be built to withstand the full combustion pressure and often extreme temperatures as well. However, modern composite structures handle this problem well, and when used with nitrous oxide and a solid rubber propellant (HTPB), a relatively small percentage of fuel is needed anyway, so the combustion chamber is not especially large.[citation needed]

The primary remaining difficulty with hybrids is with mixing the propellants during the combustion process. In solid propellants, the oxidizer and fuel are mixed in a factory in carefully controlled conditions. Liquid propellants are generally mixed by the injector at the top of the combustion chamber, which directs many small swift-moving streams of fuel and oxidizer into one another. Liquid-fueled rocket injector design has been studied at great length and still resists reliable performance prediction. In a hybrid motor, the mixing happens at the melting or evaporating surface of the fuel. The mixing is not a well-controlled process and generally, quite a lot of propellant is left unburned,[12] which limits the efficiency of the motor. The combustion rate of the fuel is largely determined by the oxidizer flux and exposed fuel surface area. This combustion rate is not usually sufficient for high-power operations such as boost stages unless the surface area or oxidizer flux is high. Too-high oxidizer flux can lead to flooding and loss of flame-holding, which locally extinguishes the combustion. Surface area can be increased, typically by longer grains or multiple ports, but this can increase combustion chamber size, reduce grain strength, and reduce volumetric loading. Additionally, as the burn continues, the hole down the center of the grain (the "port") widens, and the mixture ratio tends to become more oxidizer-rich.

There has been much less development of hybrid motors than of solid and liquid motors. For military use, ease of handling and maintenance have driven the use of solid rockets. For orbital work, liquid fuels are more efficient than hybrids, and most development has concentrated there. There has recently[when?] been an increase in hybrid motor development for nonmilitary suborbital work:

Several universities have recently[when?] experimented with hybrid rockets. Brigham Young University, the University of Utah, and Utah State University launched a student-designed rocket called Unity IV in 1995 which burned the solid fuel hydroxy-terminated polybutadiene (HTPB) with an oxidizer of gaseous oxygen, and in 2003 launched a larger version which burned HTPB with nitrous oxide. Stanford University researches nitrous-oxide/paraffin wax hybrid motors. UCLA has launched hybrid rockets through an undergraduate student group since 2009 using HTPB.[13]

The Rochester Institute of Technology was building an HTPB hybrid rocket to launch small payloads into space and to several near-Earth objects. Its first launch was in the Summer of 2007.

Scaled Composites SpaceShipOne, the first private crewed spacecraft, was powered by a hybrid rocket burning HTPB with nitrous oxide: RocketMotorOne. The hybrid rocket engine was manufactured by SpaceDev. SpaceDev partially based its motors on experimental data collected from the testing of AMROC's (American Rocket Company) motors at NASA's Stennis Space Center's E1 test stand.

Gaseous propellants

GOX (gaseous oxygen) was used as the oxidizer for the Buran program's orbital maneuvering system.

Inert propellants

Some rocket designs impart energy to their propellants with external energy sources. For example, water rockets use a compressed gas, typically air, to force the water reaction mass out of the rocket.

Ion thruster

Environmental impact of aviation

From Wikipedia, the free encyclopedia

Between 1940 and 2018, aviation CO2 emissions grew from 0.7% to 2.65% of all CO2 emissions[1]

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Aircraft engines produce gases, noise, and particulates from fossil fuel combustion, raising environmental concerns over their global effects and their effects on local air quality.[2] Jet airliners contribute to climate change by emitting carbon dioxide (CO2), the best understood greenhouse gas, and, with less scientific understanding, nitrogen oxides, contrails and particulates. Their radiative forcing is estimated at 1.3–1.4 that of CO2 alone, excluding induced cirrus cloud with a very low level of scientific understanding. In 2018, global commercial operations generated 2.4% of all CO2 emissions.[3]

Jet airliners have become 70% more fuel efficient between 1967 and 2007, and CO2 emissions per revenue ton-kilometer (RTK) in 2018 were 47% of those in 1990. In 2018, CO2 emissions averaged 88 grams of CO2 per revenue passenger per km. While the aviation industry is more fuel efficient, overall emissions have risen as the volume of air travel has increased. By 2020, aviation emissions were 70% higher than in 2005 and they could grow by 300% by 2050.[4]

Aircraft noise pollution disrupts sleep, children's education and could increase cardiovascular risk. Airports can generate water pollution due to their extensive handling of jet fuel and deicing chemicals if not contained, contaminating nearby water bodies. Aviation activities emit ozone and ultrafine particles, both of which are health hazards. Piston engines used in general aviation burn Avgas, releasing toxic lead.

Aviation's environmental footprint can be reduced by better fuel economy in aircraft, or air traffic control and flight routes can be optimized to lower non-CO2 effects on climate from NO

x, particulates or contrails. Aviation biofuel, emissions trading and carbon offsetting, part of the ICAO's CORSIA, can lower CO2 emissions. Aviation usage can be lowered by short-haul flight bans, train connections, personal choices and aviation taxation and subsidies. Fuel-powered aircraft may be replaced by hybrid electric aircraft and electric aircraft or by hydrogen-powered aircraft. Since 2021, the IATA members plan net-zero carbon emissions by 2050, followed by the ICAO in 2022.

Climate change

Factors

Radiative forcings from aviation emissions, estimated in 2020[1]

See also: Aircraft deicing fluid

Air pollution

See also: Air pollution and Avgas § environmental regulation

Aviation is the main human source of ozone, a respiratory health hazard, causing an estimated 6,800 premature deaths per year.[59]

Aircraft engines emit ultrafine particles (UFPs) in and near airports, as does ground support equipment. During takeoff, 3 to 50 × 1015 particles were measured per kg of fuel burned,[60] while significant differences are observed depending on the engine.[61] Other estimates include 4 to 200 × 1015 particles for 0.1–0.7 gram,[62] or 14 to 710 × 1015 particles,[63] or 0.1–10 × 1015 black carbon particles for 0.046–0.941 g.[64]

In the United States, 167,000 piston aircraft engines, representing three-quarters of private airplanes, burn Avgas, releasing lead into the air.[65] The Environmental Protection Agency estimated this released 34,000 tons of lead into the atmosphere between 1970 and 2007.[66] The Federal Aviation Administration recognizes inhaled or ingested lead leads to adverse effects on the nervous system, red blood cells, and cardiovascular and immune systems. Lead exposure in infants and young children may contribute to behavioral and learning problems and lower IQ.[67]

Private jet travel

A 2024 study published in Communications Earth & Environment revealed that carbon dioxide emissions from private jet travel surged to 15.6 million tonnes in 2023, a 46% increase compared to 2019. Despite serving only 256,000 individuals—approximately 0.003% of the global population—the industry contributes significantly to greenhouse gas emissions.[68]

The research further highlights that nearly half of these flights covered distances shorter than 500 kilometers. Moreover, many flights involved empty legs, where aircraft traveled without passengers, often for repositioning or ferry flights.[68]

The private jet industry is poised for further growth, with projections indicating a 33% increase in the global fleet to 26,000 aircraft by 2033.[68]

Mitigation

See also: Air travel demand reduction, Climate change mitigation, Electric aircraft, Hydrogen-powered aircraft, Ram air turbine, and Solar-powered aircraft

Aviation's environmental footprint can be mitigated by reducing air travel, optimizing flight routes, capping emissions, restricting short-distance flights, increasing taxation and decreasing subsidies to the aviation industry. Technological innovation could also mitigate damage to the environment and climate, for example, through the development of electric aircraft, biofuels, and increased fuel efficiency.

In 2016, the International Civil Aviation Organization (ICAO) committed to improve aviation fuel efficiency by 2% per year and to keeping the carbon emissions from 2020 onwards at the same level as those from 2010.[69] To achieve these goals, multiple measures were identified: more fuel-efficient aircraft technology; development and deployment of sustainable aviation fuels (SAFs); improved air traffic management (ATM); market-based measures like emission trading, levies, and carbon offsetting,[69] the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA).[70]

In December 2020, the UK Climate Change Committee said that: "Mitigation options considered include demand management, improvements in aircraft efficiency (including use of hybrid electric aircraft), and use of sustainable aviation fuels (biofuels, biowaste to jet and synthetic jet fuels) to displace fossil jet fuel."[71]

In February 2021, Europe's aviation sector unveiled its Destination 2050 sustainability initiative towards zero CO2 emissions by 2050:

aircraft technology improvements for 37% emission reductions;

SAFs for 34%;

economic measures for 8%;

ATM and operations improvements for 6%;

while air traffic should grow by 1.4% per year between 2018 and 2050.[72] The initiative is led by ACI Europe, ASD Europe, A4E, CANSO and ERA.[72] This would apply to flights within and departing the European single market and the UK.[72]

In October 2021, the IATA committed to net-zero carbon emissions by 2050.[73] In 2022, the ICAO agreed to support a net-zero carbon emission target for 2050.[74]

The aviation sector could be decarbonized by 2050 with moderate demand growth, continuous efficiency improvements, new short-haul engines, higher SAF production and CO2 removal to compensate for non-CO2 forcing.[75] With constant air transport demand and aircraft efficiency, decarbonizing aviation would require nearly five times the 2019 worldwide biofuel production, competing with other hard-to-decarbonize sectors, and 0.2 to 3.4 Gt of CO2 removal to compensate for non-CO2 forcing.[75] Carbon offsets would be preferred if carbon credits are less expensive than SAFs, but they may be unreliable, while specific routing could avoid contrails.[75] As of 2023, fuel represents 20–30% of the airlines' operating costs, while SAF is 2–4 times more expensive than fossil jet fuel.[75] Projected cost decreases of green hydrogen and carbon capture could make synthetic fuels more affordable, and lower feedstock costs and higher conversion efficiencies would help FT and HEFA biofuels.[75] Policy incentives like cleaner aviation fuel tax credits and low-carbon fuel standards could induce improvements, and carbon pricing could render SAFs more competitive, accelerating their deployment and reducing their costs through learning and economies of scale.[75]

According to a 2023 Royal Society study, reaching net zero would need replacing fossil aviation fuel with a low or zero carbon energy source, as battery technologies are unlikely to give enough specific energy.[76] Biofuels can be introduced quickly and with little aircraft modification, but are restricted by scale and feedstock availability, and few are low-carbon.[76] Producing enough renewable electricity to produce green hydrogen would be a costly challenge and would need substantial aircraft and infrastructure modification.[76] Synthetic fuels would need little aircraft modification, but necessitates green hydrogen feedstock and large scale direct CO2 air capture at high costs.[76] Low-carbon Ammonia would also need costly green hydrogen at scale, and would need substantial aircraft and infrastructure modifications.[76]

In its Sixth Assessment Report, the IPCC notes that sustainable biofuels, low-emissions hydrogen, and derivatives (including ammonia and synthetic fuels) can support mitigation of CO2 emissions but some hard-to-abate residual GHG emissions remain and would need to be counterbalanced by deployment of carbon dioxide removal methods.[77] On 29 March 2003, during a Senate hearing, hydrogen propulsion proponents like ZeroAvia or Universal Hydrogen bemoaned that the incumbents like GE Aerospace or Boeing were supporting sustainable aviation fuel (SAF) because it does not require major changes to existing infrastructure.[78]

An April 2023 report of the Sustainable Aero Lab estimate current in-production aircraft will be the vast majority of the 2050 fleet as electric aircraft will not have enough range and hydrogen aircraft will not be available soon enough : the main decarbonisation drivers will be SAF; replacing regional jets with turboprop aircraft; and incentives to replace older jets with new generation ones.[79]

The airline industry faces a significant climate challenge due to the scarcity of clean fuel options, exemplified by the recent establishment of LanzaJet Inc.'s $200 million facility in Georgia, the first to convert ethanol into jet engine-compatible fuel, with an annual production target of 9 million gallons of sustainable aviation fuel (SAF). This volume, however, is minuscule compared to the global demand, as evidenced by the world's airlines consuming 90 billion gallons of jet fuel last year, and even major airlines like IAG SA (parent company of British Airways) using only 0.66% of their total fuel consumption as SAF, with a goal to increase this to 10% by 2030. Incentives such as the $1.75 per gallon SAF credit offered by the US Inflation Reduction Act, set to expire in 2027, aim to boost SAF usage, while L.E.K. Consulting forecasts that alcohol-to-jet technology will become the dominant source of SAF by the mid-next decade. Meanwhile, emerging technologies like e-kerosene, though potentially reducing climate impacts significantly, face economic challenges as they cost nearly seven times more than traditional jet fuel, and the future of 45 proposed power-to-liquids plants in Europe remains uncertain, according to Transport & Environment.[80]

Technology improvements

Electric aircraft

The Velis Electro was the first type certificated electric aircraft on 10 June 2020.

Main articles: Electric aircraft and Hybrid electric aircraft

Electric aircraft operations do not produce any emissions and electricity can be generated by renewable energy. Lithium-ion batteries including packaging and accessories gives a 160 Wh/kg energy density while aviation fuel gives 12,500 Wh/kg.[81] As electric machines and converters are more efficient, their shaft power available is closer to 145 Wh/kg of battery while a gas turbine gives 6,555 Wh/kg of fuel: a 45:1 ratio.[82] For Collins Aerospace, this 1:50 ratio forbids electric propulsion for long-range aircraft.[83] By November 2019, the German Aerospace Center estimated large electric planes could be available by 2040. Large, long-haul aircraft are unlikely to become electric before 2070 or within the 21st century, whilst smaller aircraft can be electrified.[84] As of May 2020, the largest electric airplane was a modified Cessna 208B Caravan.

For the UK's Committee on Climate Change (CCC), huge technology shifts are uncertain, but consultancy Roland Berger points to 80 new electric aircraft programmes in 2016–2018, all-electric for the smaller two-thirds and hybrid for larger aircraft, with forecast commercial service dates in the early 2030s on short-haul routes like London to Paris, with all-electric aircraft not expected before 2045.[85] Berger predicts a 24% CO2 share for aviation by 2050 if fuel efficiency improves by 1% per year and if there are no electric or hybrid aircraft, dropping to 3–6% if 10-year-old aircraft are replaced by electric or hybrid aircraft due to regulatory constraints, starting in 2030, to reach 70% of the 2050 fleet.[85] This would greatly reduce the value of the existing fleet of aircraft, however.[85] Limits to the supply of battery cells could hamper their aviation adoption, as they compete with other industries like electric vehicles. Lithium-ion batteries have proven fragile and fire-prone and their capacity deteriorates with age. However, alternatives are being pursued, such as sodium-ion batteries.[85]

Hydrogen-powered aircraft

Main article: Hydrogen-powered aircraft

In 2020, Airbus unveiled liquid-hydrogen-powered aircraft concepts as zero-emissions airliners, poised for 2035.[86] Aviation, like industrial processes that cannot be electrified, could use primarily Hydrogen-based fuel.[87]

A 2020 study by the EU Clean Sky 2 and Fuel Cells and Hydrogen 2 Joint Undertakings found that hydrogen could power aircraft by 2035 for short-range aircraft.[88] A short-range aircraft (< 2,000 km, 1,100 nmi) with hybrid Fuel cell/Turbines could reduce climate impact by 70–80% for a 20–30% additional cost, a medium-range airliner with H2 turbines could have a 50–60% reduced climate impact for a 30–40% overcost, and a long-range aircraft (> 7,000 km, 3,800 nmi) also with H2 turbines could reduce climate impact by 40–50% for a 40–50% additional cost.[88] Research and development would be required, in aircraft technology and into hydrogen infrastructure, regulations and certification standards.[88]

Sustainable aviation fuels (SAF)

Main article: Aviation biofuel

Refueling an Airbus A320 with biofuel in 2011

An aviation biofuel (also known as bio-jet fuel,[89] sustainable aviation fuel (SAF), or bio-aviation fuel (BAF)[90]) is a biofuel used to power aircraft. The International Air Transport Association (IATA) considers it a key element in reducing the environmental impact of aviation.[91] Aviation biofuel is used to decarbonize medium and long-haul air travel. These types of travel generate the most emissions and could extend the life of older aircraft types by lowering their carbon footprint. Synthetic paraffinic kerosene (SPK) refers to any non-petroleum-based fuel designed to replace kerosene jet fuel, which is often, but not always, made from biomass.

Biofuels are biomass-derived fuels from plants, animals, or waste; depending on which type of biomass is used, they could lower CO2 emissions by 20–98% compared to conventional jet fuel.[92] The first test flight using blended biofuel was in 2008, and in 2011, blended fuels with 50% biofuels were allowed on commercial flights. In 2023 SAF production was 600 million liters, representing 0.2% of global jet fuel use.[93] By 2024, SAF production was to increase to 1.3 billion liters (1 million tonnes), representing 0.3% of global jet fuel consumption and 11% of global renewable fuel production.[94] This increase came as major US production facilities delayed their ramp-up until 2025, having initially been expected to reach 1.9 billion liters.

Aviation biofuel can be produced from plant or animal sources such as Jatropha, algae, tallows, waste oils, palm oil, Babassu, and Camelina (bio-SPK); from solid biomass using pyrolysis processed with a Fischer–Tropsch process (FT-SPK); with an alcohol-to-jet (ATJ) process from waste fermentation; or from synthetic biology through a solar reactor. Small piston engines can be modified to burn ethanol.

Sustainable biofuels are an alternative to electrofuels.[95] Sustainable aviation fuel is certified as being sustainable by a third-party organisation[citation needed].

Electrofuels (e-fuels)

The Potsdam Institute for Climate Impact Research reported a €800–1,200 mitigation cost per ton of CO2 for hydrogen-based e-fuels.[96] Those could be reduced to €20–270 per ton of CO2 in 2050, but maybe not early enough to replace fossil fuels.[96] Climate policies could bear the risk of e-fuel uncertain availability, and Hydrogen and e-fuels may be prioritised when direct electrification is inaccessible.[96]

Aircraft with lower design speed and altitude

According to a research project focusing on short to medium range passenger aircraft, design for subsonic instead of transonic speed (about 15% less speed) would save 21% of fuel compared to an aircraft of conventional design speed and similar characteristics in terms of size, range and expected general technology improvements. The lower mach number and turboprop instead of turbofan propulsion leads to lower flight altitude with a disproportionately high reduction in Non-CO2 emissions. Thus, over 60% climate impact reduction can be potentially achieved by such advanced turboprop aircraft compared to current short to medium range passenger aircraft, yet before switching to synthetic fuels. [97][98]

Reducing air travel

Main article: Air travel demand reduction

UK air travel by income quintile through time[99]

Global distribution of aviation fuel use[100]

Aviation is one of three sectors identified in a study where "demand-side options" can have a large effect in "reaching SDS levels".[101] According to a study, the attainment of the 1.5–2 °C global temperature goal necessitates substantial demand reductions in the critical sectors of aviation, shipping, road freight, and industry, should large-scale negative emissions not be realized.[102] According to the IMAGE model used to project scenarios aimed at limiting global temperature increases to 1.5 °C and 2 °C, it is suggested that achieving deep decarbonization within the aviation sector within the specified timeframe is contingent upon a reduction in air travel in certain markets.[102] The decreases in carbon intensity of aviation energy in net-zero scenarios "are heavily dependent on projected changes in aviation demand and energy intensity".[103] The significant challenges of sustainable aviation fuel expansion, including food security, local community impacts, and land use issues, underscore the importance of simultaneous demand reduction efforts.[103] For instance, according to a report by the Royal Society, to produce enough biofuel to supply the UK's aviation industry would require using half of Britain's farming land which would put major pressures on food supplies.[104][105]

Tourism is projected to generate up to 40% of total global CO2 emissions by 2050.[106] Of climate change mitigation consumption options investigated by a review, the consumption options with "the highest mitigation potential advocate reduction in car and air travel".[107] A study projected a potential reduction of "transport direct CO2 emissions by around 50% in the end of the century compared to the baseline" via combined behavioral factors.[108]

Measures

The Taiwan High Speed Rail in 2007

According to the IPCC Sixth Assessment Report, "the greatest Avoid potential" in demand-side mitigation, which consists of Avoid-Shift-Improve (ASI) options, "comes from reducing long-haul aviation and providing short-distance low-carbon urban infrastructure".[109] It lists the following related mobility measures:[109]

Avoid: integrate transport & land use planning, tele-working, fewer long-haul flights, local holidays[109]

Shift: from air travel to high-speed rail[109]

It found that socio-cultural factors promoting a preference for train travel over long-haul flights have the potential to reduce aviation greenhouse gas emissions by 10% to 40% by 2050.[109]

The ICCT estimates that 3% of the global population take regular flights.[24] Stefan Gössling of the Western Norway Research Institute estimates 1% of the world population emits half of commercial aviation's CO2, while close to 90% does not fly in a given year.[110]

Per capita emissions from domestic and international flights

In early 2022, the European Investment Bank published the results of its 2021–2022 Climate Survey, showing that 52% of Europeans under 30, 37% of people between 30 and 64 and 25% for people aged 65 and above plan to travel by air for their summer holidays in 2022; and 27% of those under 30, 17% for people aged 30–64 and 12% for people aged 65 and above plan to travel by air to a faraway destination.[111]

Short-haul flight ban

Main article: Short-haul flight ban

A short-haul flight ban is a prohibition imposed by governments on airlines to establish and maintain a flight connection over a certain distance, or by organisations or companies on their employees for business travel using existing flight connections over a certain distance, in order to mitigate the environmental impact of aviation (most notably to reduce anthropogenic greenhouse gas emissions which is the leading cause of climate change). In the 21st century, several governments, organisations and companies have imposed restrictions and even prohibitions on short-haul flights, stimulating or pressuring travellers to opt for more environmentally friendly means of transportation, especially trains.[112]

Flight shame

In Sweden the concept of "flight shame" or "flygskam" has been cited as a cause of falling air travel.[113] Swedish rail company SJ AB reports that twice as many Swedish people chose to travel by train instead of by air in summer 2019 compared with the previous year.[114] Swedish airports operator Swedavia reported 4% fewer passengers across its 10 airports in 2019 compared to the previous year: a 9% drop for domestic passengers and 2% for international passengers.[115]

Personal allowances

Climate change mitigation can be backed by Personal carbon allowances (PCAs) where all adults receive "an equal, tradable carbon allowance that reduces over time in line with national targets."[116][117][118][excessive citations] Everyone would have a share of allowed carbon emissions and would need to trade further emissions allowances.[119][importance?] An alternative would be rationing everyone's flights: an "individual cap on air travel, that people can trade with each other".[120]

Economic measures

Emissions trading

CO2 price in the European Union Emission Trading Scheme

ICAO has endorsed emissions trading to reduce aviation CO2 emission, guidelines were to be presented to the 2007 ICAO Assembly.[121] Within the European Union, the European Commission has included aviation in the European Union Emissions Trading Scheme operated since 2012, capping airline emissions, providing incentives to lower emissions through more efficient technology or to buy carbon credits from other companies.[122][123] The Centre for Aviation, Transport and Environment at Manchester Metropolitan University estimates the only way to lower emissions is to put a price on carbon and to use market-based measures like the EU ETS.[124]

Taxation and subsidies

Main article: Aviation taxation and subsidies

Financial measures can discourage airline passengers and promote other transportation modes and motivates airlines to improve fuel efficiency. Aviation taxation include:

air passenger taxes, paid by passengers for environmental reasons, may be variable by distance and include domestic flights;

departure taxes, paid by passengers leaving the country, sometimes also applies outside aviation;

jet fuel taxes, paid by airlines for the consumed jet fuel. Jet fuel taxation is applied in the United States, but banned in the European Union.

Consumer behavior can be influenced by cutting subsidies for unsustainable aviation and subsidising the development of sustainable alternatives. By September–October 2019, a carbon tax on flights would be supported by 72% of the EU citizens, in a poll conducted for the European Investment Bank.[125]

Aviation taxation could reflect all its external costs and could be included in an emissions trading scheme.[126] International aviation emissions escaped international regulation until the ICAO triennial conference in 2016 agreed on the CORSIA offset scheme.[127] Due to low or nonexistent taxes on aviation fuel, air travel has a competitive advantage over other transportation modes.[128][129]

Carbon offsetting

Money generated by carbon offsets from airlines often go to fund green-energy projects such as wind farms.

A carbon offset is a means of compensating aviation emissions by saving enough carbon or absorbing carbon back into plants through photosynthesis (for example, by planting trees through reforestation or afforestation) to balance the carbon emitted by a particular action.

However, carbon credits permanence and additionality can be questionable.[75] More than 90% of rainforest offset credits certified by Verra's Verified Carbon Standard may not represent genuine carbon reductions.[130]

Consumer option

Some airlines offer carbon offsets to passengers to cover the emissions created by their flight, invested in green technology such as renewable energy and research into future technology. Airlines offering carbon offsets include British Airways,[131] Continental Airlines,[132][133] easyJet,;[134] and also Air Canada, Air New Zealand, Delta Air Lines, Emirates Airlines, Gulf Air, Jetstar, Lufthansa, Qantas, United Airlines and Virgin Australia.[135] Consumers can also purchase offsets on the individual market. There are certification standards for these,[136] including the Gold Standard[137] and the Green-e.[138]

National carbon budgets

In UK, transportation replaced power generation as the largest emissions source. This includes aviation's 4% contribution. This is expected to expand until 2050 and passenger demand may need to be reduced.[85] For the UK Committee on Climate Change (CCC), the UK target of an 80% reduction from 1990 to 2050 was still achievable from 2019, but the committee suggests that the Paris Agreement should tighten its emission targets.[85] Their position is that emissions in problematic sectors, like aviation, should be offset by greenhouse gas removal, carbon capture and storage and reforestation.[85] The UK will include international aviation and shipping in their carbon budgets and hopes other countries will too.[139]