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PREPUBLICATION COPY â Uncorrected Proofs 23 2 Background on the Piston-Engine Aircraft Fleet and Airports The U.S. active piston-engine fleet consists of approximately 144,000 aircraft in civilian use (FAA, 2020a). Adding in the roughly 27,000 experimental aircraft without Federal Aviation Administration (FAA) type certifications, the piston-engine fleet has about 170,000 active aircraft in total. Nearly all of these aircraft, which consist of airplanes and helicopters, are the basis of most general aviation (GA) operations, which encompasses a diverse range of functions including transportation, recreation, pilot training, emergency services, and other commercial, sport, and government purposes. This chapter begins with an overview of the basic types of aircraft in the piston-engine GA fleet and their varied GA uses. Background is then provided on some of the characteristics of the approximately 13,100 airports from which most piston-engine aircraft are based and operate. These airports vary widely in size, activity levels, features, and function. On one end of the spectrum are heavily used, state-, county- or municipally-owned airports that accommodate both turbine and piston-engine aircraft, often with intensive operations such as pilot training and business aviation. On the other end are airfields that may be privately owned and consist of grass, water, and sand landing strips that may have highly specialized and seasonal applications such as crop spraying, fire protection, and sightseeing and sport flying. In some rural and remote parts of the country, and particularly in Alaska, the small airfields and aircraft that operate from them are the principal means of transportation for residents and visitors and for access to critical supplies and services. Consideration is also given to aircraft operations at airports. As defined by FAA, an aircraft âoperationâ is either a takeoff or landing. Each operation involves pilots taxiing to and from the runway and performing checks, including engine ârun-upsâ while stopped before takeoff to confirm that the engine can safely attain full power with normal indications. Run-ups may also be performed during aircraft maintenance and repair. So as not to interfere with the operations of other aircraft and to orient propeller wash away from people and structures, airports will sometimes designate special areas for these pre-takeoff checks, often just before the aircraft turns onto the runway for takeoff. All but the smallest and most specialized airports will also have areas and facilities designated for aircraft repair, maintenance, and refueling. Background is provided on some of these features because their wide variability across the many airports that serve piston-engine aircraft can have important implications on the opportunities available to reduce emissions and concentrations from leaded aviation gasoline (avgas) through measures such as adding an unleaded fuel choice and relocating run-up areas. U.S. PISTON-ENGINE FLEET As noted above, the active GA fleet includes about 144,000 piston-engine aircraft with type certifications, and a further 27,000 experimental aircraft, a large portion of which are amateur built but also include the countryâs more than 2,000 show and vintage airplanes (e.g., warbirds) (FAA, 2020a). While data on the engine types of experimental aircraft are not readily availableâand therefore not included in the following description of the piston-engine fleetâit
PREPUBLICATION COPY â Uncorrected Proofs 24 is reasonable to assume the vast majority have piston-engines, for a total of about 170,000 aircraft that burn gasoline. While the countryâs 144,000 type-certified piston-engine aircraft can be grouped in many ways, the most common groupings are by type of wing (fixed- and rotary-wing, or airplane and helicopter) and number of engines (single- and multi-engine). Most of these airplanesânearly 90 percentâhave a single, gasoline-powered reciprocating engine, and are small (e.g., they have six or fewer seats, weigh less than 5,000 pounds when fully loaded, and require only 750- to 2,500- foot runways). They are seldom flown higher than 10,000 to 15,000 feet (because few are pressurized or designed for efficient operations at higher altitudes), further than 1,200 miles, or faster than 175 mph; however, some high-performance single-engine piston aircraft can operate at higher speeds and altitudes. In the piston-engine fleet, the large number of single-engine airplanes is accompanied by about 12,500 multi-engine airplanes and about 3,000 rotary-wing aircraft. These latter aircraft are flown more hours on average than the single-engine planes, as they are more likely to be used for commercial purposes (see Table 2-1). TABLE 2-1 U.S. Active Type-Certified Piston-Engine Aircraft Fleet Size and Hours Flown, 2019 Aircraft Type Number of Aircraft Percent of Total Fleet Hours Flown During a Year Percent of Total Fleet Hours Flown Average Hours Flown per Year Single-engine, Fixed-wing 128,470 89 12,700,000 84 93 Multi-engine Fixed-wing, 12,470 9 1,731,000 12 139 Rotary-wing 3,082 2 628,000 4 204 Total 144,485 15,059,000 104a a Based on total values shown in the table. NOTE: The table does not include the approximately 27,000 experimental aircraft, most of which are likely to have gasoline engines. SOURCE: FAA, 2020a. Over the past several decades, the demand for new piston-engine airplanes has been trending down, especially among the most basic small aircraft. Domestic deliveries of new airplanes declined from more than 10,000 units in 1980 to about 900 in 2019 (GAMA, 2020, 9- 10, 16). There has been much speculation about the causes of this decline, from product liability costs that have increased new aircraft prices to a shrinking population of private pilots. The average price of a new piston-engine aircraft in 2019 was more than $550,000 (GAMA, 2020, 9). While these new aircraft tend to have increasingly more advanced avionics and other sophisticated features, the needs of many private pilots can be met by the large selection of well- maintained and reconditioned used airplanes already in the fleet. Flown an average of about 100 hours per year, piston-engine airplanes have long service lives, as the average age of the large number of single-engine airplanes in the fleet is nearly 50 years (FAA, 2020a). The combination of fewer pilots and highly durable aircraft is reflected in low annual fleet turnover rates and modest aircraft resale prices. FAA has estimated that the average market value of a single-engine GA aircraft (based on the 2017 fleet in 2018 dollars) was approximately $60,000, while the average value an aircraft manufactured before 1984 was less than $45,000 (FAA, 2020b).
PREPUBLICATION COPY â Uncorrected Proofs 25 USES OF PISTON-ENGINE AIRCRAFT To better understand the characteristics of the GA sector, and resulting demands on airport and air traffic control systems, FAA conducts annual surveys of GA pilots and aircraft owners. Respondents are asked to report on the number of hours flown, basic type of aircraft used (e.g., number of engines), and reasons for flying. The 2019 survey data indicates the piston-engine fleet logged more than 15 million hours of flying that year, with single- and multi-engine airplanes accounting for about 96 percent of the hours and rotary-wing aircraft accounting for the remainder (see Table 2-2). While some of this flying was for transporting people and goods from point to point, most of it was for other purposes. For reasons explained below, because the FAA survey data are aggregated nationally, they can mask considerable geographic variation in how often and for what purposes piston-engine aircraft are flown, particularly in remote and rural communities lacking good roads, commercial airline service, and other means of access. TABLE 2-2 Uses of the U.S. Piston-Engine Aircraft Fleet, 2019 Percent of Total Hours Flown by Purpose Number of Aircraft Total Hours Flown For-Hire Transport Business Transport Personal and Recreation Pilot Training Aerial Observation and Agriculture Other Single- engine, fixed- wing 128,470 12,700,000 1.6 7.4 42.7 39.8 3.4 5.1 Multi-engine, fixed-wing 12,470 1,731,000 10.1 15.4 24.1 42.4 2.8 5.3 Rotary-wing 3,082 628,000 2.2 2.5 9.2 50.0 18.3 18.2 Total 144,485 15,059,000 2.6 8.1 39.2 40.5 4.0 5.6 NOTE: The table does not include uses of the approximately 27,000 experimental aircraft. SOURCE: FAA, 2020a. For-Hire Transportation (Air Taxi) In regulating air transport operations and flight standards, FAA has long distinguished between for-hire and private transportation. The rationale for making this distinction is that customers of for-hire carriers do not have direct control over their own flying safety; and therefore the government must assume a more prominent role in ensuring airworthiness and safe operation. Piston-engine aircraft are more common among providers of for-hire air taxi services, and are rarely used by commercial airlines. Operating much like short-distance air charter services, these carriers typically fly aircraft with fewer than 10 seats, but sometimes more. Because they are often used for purposes in addition to for-hire transportation, the aircraft are usually counted as part of the GA fleet. According to the FAA survey, about 1,400 piston-engine aircraft provided air taxi service in 2019 (FAA, 2020a). They accounted for about 2 percent of hours flown by the fleet that year, with the multi-engine, high-performance airplanes accounting for a disproportionately high share of their hours (see Table 2-2).
PREPUBLICATION COPY â Uncorrected Proofs 26 Business Transportation From a regulatory standpoint, private aircraft that are used for business transportation are treated like other kinds of private GA aircraft because they are used for in-house transportation incidental to the ownerâs main line of business. Nevertheless, many of these GA aircraft are flown by professional pilots. While most corporations that operate business aircraft use turbine airplanes, about 12,000 piston-engine airplanes are also used for GA business transportation (FAA, 2020a). They are usually (about 90 percent of the time) self-piloted by the person conducting the business rather than by a hired crew. In total, business aviation accounts for about 8 percent of the 15 million hours flown by piston-engine aircraft annually (see Table 2-2). Personal and Recreational Use About three-quarters of the aircraft in the piston-engine fleet, consisting of about 108,000 airplanes and helicopters, are used exclusively for personal and recreational flying, often by private pilots who are not instrument-rated (FAA, 2020a). Often weather- and daylight- dependent for operations, much of this fleet sits idle for long periods. Thus, despite accounting for a large majority of piston-engine aircraft, the personal and recreational fleet accounts for less than 45 percent of total fleet hours flown (see Table 2-2). Most of the smallest, low-performance airplanes in the piston-engine fleet are used for these private purposes, and much of the flying is local in nature operating at relatively low altitude and low speeds. Pilot Training The piston-engine fleet used for pilot training totals to more than 15,000 aircraft, or about 10 percent of the fleet (FAA, 2020a). Because these aircraft are used intensely, they account for more than 40 percent of fleet hours flown (see Table 2-2). Whatâs more, these flying hours are often logged by multiple training sessions in a single day, and as such, the flying hours of these aircraft can be accompanied by a significant amount of additional time spent with engines running while on the ground practicing procedures, taxiing, and performing pre-takeoff checks. Aerial Observation and Agricultural Services Aerial observation and agricultural services are important local uses of piston-engine aircraftâ for instance, for ensuring that utility rights-of-way are clear, photographing land uses, and treating crops. While only less than 2 percent of the piston-engine fleet is used mainly for these purposes, these activities account for about 6 percent of fleet hours flown (see Table 2-2 and FAA, 2020a). Other Uses Fixed- and rotary-wing piston-engine aircraft are used for a wide range of public and commercial purposes. Public purposes include search and rescue, aerial firefighting, police aviation, traffic reporting, and emergency medical airlifts (medevac) (FAA, 2020a). Examples of commercial uses are entertainment and sport applications such as air tours and sightseeing, airshows, air racing, and parachute jumping. About 4 percent of the piston-engine fleet is used primarily for
PREPUBLICATION COPY â Uncorrected Proofs 27 these purposes, and these aircraft account for about 6 percent of hours flown (see Table 2-2 and FAA, 2020a). AIRPORTS WHERE PISTON AIRCRAFT OPERATE Excluding about 6,000 heliports and seaplane bases, FAA has identified 13,117 airports in the United States, including 4,815 public-use and 8,302 private-use airports (see Table 2-3). Among these 4,815 public-use airports (which are mostly owned by cities, counties, and states), FAA has designated 3,249 to be part of the national airport system, known as the National Plan of Integrated Airport Systems (NPIAS). Another 72 private-use airports are included in the NPIAS, bringing the total to 3,321 public- and private-use airports in the national system. These airports are eligible for federal funding assistance for infrastructure improvements. TABLE 2-3 Percentage of the U.S. GA Fleet Based at Airports by Category Airports Number of Airports Percent of GA Aircraft Based (includes mostly piston- engine aircraft but also some turbine-engine aircraft) NPIAS Primary 380 16.7 NPIAS Non-primary 2,941 58.5 National 88 10.5 Regional 492 22.4 Local 1,278 21.3 Basic/Unclassed 1,083 4.3 Public and Private Airports Not in NPIAS 9,790 24.8 TOTAL 13,117 100 NOTES: NPIAS = National Plan of Integrated Airport Systems eligible for federal grants. The GA fleet consists of mostly piston-engine aircraft but some turbine-engine aircraft as well. SOURCE: FAA, 2019b. The 3,321 airports in the NPIAS consist of 380 âprimaryâ airports and 2,941 ânon- primaryâ airports. The primary airports account for nearly all scheduled airline enplanements and freight loadings and serve as bases for nearly all of the commercial passenger and cargo airline fleet. The 30 busiest primary airports, referred to as large hubs, house few piston-engine or other GA aircraft, but there are some exceptions such as Honolulu (HON), Las Vegas (LAS), and Salt Lake City (SLC). By comparison, most of the countryâs approximately 30 medium- and 70 small-hub airports have sufficient capacity to accommodate both airline operations and GA users. Together with the other 260 primary airports that are classified as ânonhubsâ because they have little or no commercial service, the 380 primary airports serve as the bases for about 17 percent of the total GA fleet. The vast majority of GA aircraftânearly 60 percent of the fleetâare based at the countryâs 2,941 non-primary airports that are used only for GA. This large grouping of airports is
PREPUBLICATION COPY â Uncorrected Proofs 28 further subdivided into ânational,â âregional,â âlocal,â and âbasic/unclassedâ facilities. The 88 national airports are usually located in metropolitan areas, often near major business centers, and therefore they accommodate large amounts of business aviation, often with significant operations by jets and multi-engine airplanes in GA service. Comprised of the countryâs busiest GA airports, they serve as bases for about 11 percent of the GA fleet. By far the largest segment of the GA fleet, about 43 percent, is based at the countryâs nearly 1,800 regional and local non-primary airports. Typically, these airports are located near population centers but not necessarily in major metropolitan areas. While they support some longer-distance flying (especially the regional airports), they are mostly used for local flying, flight training, and emergency services and the GA fleet they serve consists almost entirely of piston-engine aircraft. By comparison, the 1,083 âbasic/unclassifiedâ airports in the NPIAS are mostly located in rural areas. About 3 percent of the GA fleet is based at these airports, which receive some federal funding because of their roles in keeping remote communities connected to the countryâs aviation system. The remaining 25 percent of the GA fleet, consisting almost entirely of piston-engine aircraft, is based at the nearly 10,000 other small airports that are not part of the NPIAS. Geographic Variability in GA Uses and Airports The national-level data presented above do not convey the geographic variability that exists in GA aircraft uses and airports. The national data can be particularly misleading when considering GAâs role in vast, sparsely populated states such as Montana, Nevada, and other western states, but especially Alaska, whose communities are scattered across more than 580,000 square miles of land and on islands who inhabitants that have no or limited access to roads, airline service, or other long-distance transport modes. For most of Alaskaâs communitiesâmore than 80 percent of which are inaccessible to a state or long-distance highwayâGA flights are the only option for year-round passenger and cargo transportation (ADTPF, 2009). Likewise, GA is essential for medical airlifts, search and rescue, and other emergency services. Alaska has about 400 public use airports and seaplane bases, or nearly 10 percent of countryâs total. Moreover, the state has hundreds of other private airfields, and pilots routinely operate from many of the stateâs thousands of lakes and gravel bars where there are no constructed facilities (FAA, 2016). The state of Alaska estimates that about 40 percent of the stateâs economic output and 25 percent of its jobs depend on access to aviation, most of it provided at rural airports and airfields by GA aircraft (ADTPF, 2009). GAâs critical importance to Alaska, as well as many other rural states and remote locations across in the country, means that measures aimed at reducing aviation lead use need to be carefully considered so as not to create undesirable side effects, the distribution and magnitude of which could differ significantly by region. Airport Facilities and On-Airport Operations General Characteristics An airport has both airside and landside features. Airside features consist of runways, taxiways, apron areas, aircraft parking positions and maintenance buildings, hangars, refueling stations, air traffic control facilities, and navigational aids. Landside features include terminal and cargo
PREPUBLICATION COPY â Uncorrected Proofs 29 buildings, access roads, automobile parking lots, and other facilities for airport employees and users. The airside and landside features of airports can vary greatly. Some airports have control towers, instrumented landing systems, aircraft maintenance services, and multiple runways of varying length and orientation. They may also have terminal and cargo facilities. Other airports can have little more than a short landing strip or sea lane, sometimes with aircraft parking, maintenance, and refueling areas, but seldom many landside facilities. A review of certain airside features, including the presence of paved and lighted runways, reveals the wide variability in the countryâs airports that serve primarily GA traffic. Table 2-4 shows that most private airports do not have a paved or lighted runway, and that the situation is similar for about 25 percent of public-use airports. More than two-thirds of airports do not have a runway longer than 4,000 feet, including nearly half of all public-use airports. It is reasonable to assume that the other airside facilities at these small airports are also limited, including traffic control and aircraft maintenance and fueling services. TABLE 2-4 Runway Characteristics of U.S. Civilian Airportsa Airport Type Characteristic Public Use Private Use Total Total number of airports 4,776 8,266 13,042 Airports without a paved runway 24% 86% 63% Airports without a lighted runway 28% 90% 67% Airports without paved or lighted runway 48% 90% 64% Airports with longest runway length less than 4,000 feet 48% 84% 71% a Only operational airports in the United States are included. NOTE: Airport totals do not align with those in Table 2-3 because of differences in survey periods. SOURCE: Data available from the Airport Planning and Programming Office, Federal Aviation Administration. Personal communication, Boyd Rodeman, FAA, October 28, 2020. Fueling Facilities and Operations The variability among airports in fueling services is an important consideration for this study because one option for reducing lead from avgas is to ensure that both unleaded and leaded avgas grades are available for pilots depending on the requirements of their aircraft. There is currently considerable variability in the fueling services available at airports in accordance with variability in airport size, types of aircraft served, and levels of traffic activity (NASEM, 2019). Because all piston-engine aircraft can use 100LL, most airports that serve only gasoline-powered aircraft have a common fueling system that dispenses this grade only. By selling only one universally usable grade of avgas, the airport can reduce its investment in fueling infrastructure and avoid concerns about having to ensure that different types of fuels are physically separated during storage and dispensing to avoid co-mingling or misfueling by pilots. According to data from the National Air Transportation Association (NATA), the average installation cost of a 5,000 gallon fuel storage tank is $110,000 while the
PREPUBLICATION COPY â Uncorrected Proofs 30 cost of a 10,000 gallon tank is $150,000.1 Thus, the added cost of having to invest in two smaller tanks (one for leaded avgas and another for unleaded avgas) to hold the same total volume of fuel as one larger tank containing a single grade of avgas can be significant for an airport with limited revenues and financial capability. It also merits noting that over the past 20 years, the consumption of avgas has declined by about one third, reflecting the downward trend in GA flying (GAMA, 2020, 24). Hence, as avgas demand has declined, most small airports have had little incentive to expand their fueling infrastructure. While data could not be found on the total number of airports that only have multiple tanks and dispensing systems for avgas, it is reasonable to assume that a large majority of the smallest facilities have no more than one. As reported in Table 2-3, the smallest 70 percent of the 3,321 NPIAS airports (1,278 local and 1,083 basic/unclassed) serve as the bases for 25 percent of the GA fleet, thus averaging about 15 to 20 aircraft each. One would not expect these airports to have multiple avgas storage and dispensing systems. Additionally, the nearly 9,800 non-NPIAS airports serve as the bases for an average of about 5 aircraft each, suggesting that the vast majority of these airports have no more than one system, if any at all. While data are not available on the financial capacity of these small airports to add refueling facilities or to initiate other lead mitigation assessments and measures, it is reasonable to assume this capacity is limited given the small number of aircraft these airports house. FINDINGS The U.S. piston-engine fleet, which consumes most of the leaded gasoline used in aviation, numbers about 170,000 aircraft, including about 27,000 experimental aircraft (Finding 2.1). Piston-engine aircraft serve many different purposes, some with particular significance to specific regions. Typically, the smallest, most basic aircraft are used for personal and recreational flying, while another important purpose is pilot flight training. Aerial observation, medical airlift, and business transport are examples of important GA functions across the country, while some functions, such as crop dusting, aerial firefighting, search and rescue, and air taxi service, have particular significance to communities in rural and remote locations (Finding 2.2). The different GA functions affect flight hours and fuel consumption by segments of the piston-engine fleet. Personal and recreational flying accounts for about half of all hours flown, and involves about 75 percent of the piston-engine fleet. The aircraft in the remaining one- quarter of the fleet, flown for business, government, and commercial purposes, are used most intensely and account for about half of all hours flown. Because this âworkingâ segment of the piston-engine fleet consists disproportionately of multi-engine and high-performance airplanes and helicopters that burn fuel at higher rates, it is likely to consume more than half of all the avgas used by the fleet (Finding 2.3). The size of the piston-engine fleet has been fairly stable for decades, consisting of many older well-maintained and reconditioned aircraft that are augmented by about 900 new aircraft per year. Annual turnover of the piston-engine fleet is therefore very low, resulting in average aircraft age approaching 50 years. Aircraft piston engines are carefully monitored for maintenance issues and regularly overhauled. Retrofits of aircraft systems, including installing new engines on current airframes, can require extensive and expensive testing and FAA certification (Finding 2.4). 1 Presentation to the committee, M. Eisenstein, NATA, February 18, 2020.
PREPUBLICATION COPY â Uncorrected Proofs 31 The piston-engine fleet operates from about 13,100 airports, consisting of a combination of publicly (municipal, county, and state) and privately owned facilities having wide range of traffic activity and financial capability. About 3,300 airportsâmostly publicly ownedâare eligible to receive federal assistance for certain types of infrastructure improvements because they are part of the national airport system (NPIAS). While about three quarters of the piston-engine fleet is based at these NPIAS airports, the remaining 25 percent of the fleet is spread across the remaining 9,800 other airports, many of which are very small with limited financial or technical capability to add more fueling infrastructure or to perform assessments of lead impacts for mitigations such as changing airport layouts (Finding 2.5). The findings presented above are referenced in the chapters that follow when examining options for reducing aviation lead emissions at airports through changes in the operations of aircraft at airports, to the fuels available to GA pilots, and in the aircraft themselves. They indicate that GAâs critically important functions, particularly in (but not exclusive to) rural and remote communities, mean that measures aimed at reducing aviation lead need to be carefully considered so as not to create undesirable side effects. The distribution and magnitude of those potential effects may differ significantly by region. The relatively low value of most existing piston-engine aircraft and the fleetâs low annual turnover warrant consideration when considering lead mitigation measures focused on changing the mix and types of aircraft in the fleet as a means of reducing reliance on leaded avgas. The disproportionately large portion of avgas consumed by the working segment of the fleet has implications on the extent to which the supply of an unleaded avgas can impact total lead emissions, especially if that supply cannot be used by this segment. The findings are also important for assessing the potential for making changes in airport layouts as a lead mitigation strategy or for making unleaded avgas widely available for use by portions of the piston-engine fleet, which operates across thousands of airports, including many with limited capacity to invest in new fueling infrastructure or airport modifications. REFERENCES ADTPF (Alaska Department of Transportation and Public Facilities). 2009. The Economic Contribution of the Aviation Industry to Alaskaâs Economy. ADTPF, Juneau. March. Available at: https://www.alaskaasp.com/admin/Docs/Economic%20contribution%20of%20the_Aviati on%20industry%20report--compiled.pdf. FAA (Federal Aviation Administration). 2016. Alaskan Region Aviation Fact Sheet. FAA, Washington, D.C. January. Available at: https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/air_traffic_s ervices/artcc/anchorage/media/Alaska_Aviation_Fact_Sheet.pdf. FAA. 2019a. FAA aerospace forecast. Fiscal Years 2019-2039. TC19-0002. FAA, Washington, DC. Available at: https://www.faa.gov/data_research/aviation/aerospace_forecasts/media/FY2019- 39_FAA_Aerospace_Forecast.pdf. FAA. 2019b. Report to Congress National Plan of Integrated Airport Systems (NPIAS) 2019- 2023 FAA, Washington, DC. Available at: https://www.faa.gov/airports/planning_capacity/npias/current/historical.
PREPUBLICATION COPY â Uncorrected Proofs 32 FAA. 2020a. General Aviation and Part 135 Activity Surveys, CY 2019. FAA, Washington, DC. Available at: https://www.faa.gov/data_research/aviation_data_statistics/general_aviation/CY2019/. FAA. 2020b. Section 5. Unit replacement and restoration costs of damaged aircraft. In FAA Airport Benefit-Cost Analysis Guidance. FAA, Washington, DC. September 16. Available at: https://www.faa.gov/regulations_policies/policy_guidance/benefit_cost. GAMA (General Aviation Manufactures Association). 2020. 2019 Databook. GAMA, Washington, DC. Available at: https://gama.aero/wp- content/uploads/GAMA_2019Databook_ForWebFinal-2020-02-19.pdf. NASEM (National Academies of Sciences, Engineering, and Medicine). 2019. Airport Management Guide for Providing Aircraft Fueling Services. Washington, DC: The National Academies Press. https://doi.org/10.17226/25400.