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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 4 Analysis of Small Aircraft Transportation System Concept As explained in Chapter 1, the statement of task asks the study committee to answer the following two questions: Do the relative merits of the Small Aircraft Transportation System (SATS) concept, in whole or in part, contribute to addressing travel demand in the coming decades with sufficient net benefit to warrant public investment in technology and infrastructure development and deployment? What are the most important steps that should be taken at the national, state, and local levels in support of SATS deployment? Under the SATS concept, highly advanced small aircraft would be operated as a means of personal transportation in airspace and at airports that are now used only lightly. The committee interprets the first question as asking broadly whether the concept is sufficiently plausible and desirable to serve as a guide for general aviation (GA) research and technology programs and as a basis for government investments in the development and deployment of supporting infrastructure. The committee interprets the second question as a request for specific advice on what, if any, steps the National Aeronautics and Space Administration (NASA) and other public agencies ought to take to further the development of such a system or its constituent capabilities and technologies. The committee’s answers to these questions and its advice on NASA’s GA research and technology program are offered in Chapter 5. The supporting analyses are presented below. First, the likelihood that many novel and advanced aviation technologies can be developed, integrated, tested, and adopted in a manner that ensures safe performance and user affordability is examined. Consideration is then given to the ability of the nation’s small airports and uncontrolled airspace to accommodate such a system, which will depend in part on the kinds of aircraft envisioned for SATS and their utility and performance characteristics. The potential for significant user interest in such a system is then examined given what is known about travel demand trends, patterns, and the factors that influence them. While such analyses are helpful in judging the likelihood of such a system emerging as anticipated, they do not address its social desirability. Important considerations deserving closer scrutiny are the safety and environmental effects of SATS. For instance, how will a shift from travel in larger airplanes to smaller airplanes affect overall transportation safety, energy use, and emissions? How will a shift in traffic
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 from large to smaller airports affect aviation safety, environmental compatibility, and overall settlement patterns? By serving mainly small airports in nonmetropolitan areas, how will SATS affect traffic volumes, capacity, and congestion in the larger commercial air transportation system? Before promoting SATS as a socially desirable outcome, it is important to consider these and other issues. PROSPECTS FOR TECHNOLOGY DEVELOPMENT AND DEPLOYMENT For the small aircraft transportation system that NASA envisions to emerge, many new technologies and systems would have to be developed, validated, and integrated for production and use. While future technologies cannot be predicted with certainty, it is possible to surmise the overall magnitude of the technological challenge facing SATS given a general understanding of factors that influence product development and deployment in aviation. The success of SATS would require many significant advances in small aircraft propulsion, flight control, communications, navigation, surveillance, landing, and manufacturing systems; no attempt is made here to explore the various technical challenges in each of these areas. What must be considered, however, is whether so many coordinated advances could be planned for and achieved in an aviation environment in which safety assurance and affordability are key, and often conflicting, constraints. Safety Assurance Nearly all aspects of aviation are subject to extensive government and industry standards, advisories, and procedures aimed at ensuring safety—from pilot training and proficiency requirements to criteria for aircraft and airport design, maintenance, and inspection. The public’s expectations for safety are high because the consequences of mistakes can be fatal, so emphasis is placed on avoiding accidents through exacting design, material, and manufacturing standards; multiple backup systems and redundancies; standard operating procedures; and training and qualification standards for pilots, maintenance personnel, air traffic controllers, and many others involved in the aviation sector. To ensure these high levels of performance, the introduction of new aviation technologies, components, and systems must be preceded by extensive analysis and evaluation. Through this multilayered process, most new technologies and procedures are incorporated into the aviation system gradually. The expeditious and orchestrated development and introduction of many new aviation technologies and systems on the scale required by SATS is unprecedented. The current safety assurance system, though deliberate and slow-paced, has been accompanied by continual improvements in aviation safety over the course of many decades. Because of the overriding importance of safety, the prospect that SATS would emerge quickly and with sufficient user, regulator, and industry confidence appears highly questionable. Moreover, given that SATS is envisioned to appeal to a new kind of small aircraft user—including many novice pilots with limited experience and skills—the magnitude of this safety challenge appears to have been greatly underestimated. Affordability Central to the SATS vehicle concept is that technological advances can reduce the cost of owning and operating small aircraft even as the attributes of these aircraft—
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 including safety—are enhanced greatly. The idea is that a combination of lower aircraft costs and increased performance and utility will spur user interest. The history of technological development is replete with examples of large reductions over time in the cost of products, even as their performance and capabilities increase. However, this has seldom been the case for high-performing production aircraft. Indeed, the increasingly sophisticated small jet aircraft produced in the GA sector since the 1960s (with the introduction of the six-place Learjet) have become more expensive over time relative to consumer purchasing power (see Figure 4-1). In 1998, the 415 jet airplanes produced by GA manufacturers cost an average of $11.5 million each, which is about twice as much per unit as in 1980, when 326 new jets sold for an average price of $5.1 million each, adjusted for inflation. The number of small jets produced in any given year is only in the hundreds, and they are highly customized and individually crafted. Under the SATS concept, however, high user demand is assumed to prompt large reductions in unit price, primarily because of increases in the number of aircraft manufactured and distributed. Some prospective aircraft manufacturers, such as Eclipse Aviation and Safire Aircraft Company (see Chapter 2), are anticipating levels of jet aircraft production several times higher than that of all GA manufacturers today. These companies anticipate the emergence of volume-related production economies and improved manufacturing methods. Such developments, the companies believe, will allow them to sell their aircraft (with mostly conventional avionics) for about $1 million per unit—a price that can make small jets more practical and economical to a significant portion of the public. To date, there is little evidence of the potential for such economies in small-aircraft manufacturing, and such an outcome would run counter to recent industry trends. The U.S. GA industry today produces fewer than 3,000 aircraft per year, a figure that represents the aggregate output of more than a dozen manufacturers for all types of GA aircraft (GAMA 1999). Even at its peak during the 1970s, the industry never produced more than 20,000 aircraft of all types per year, and it has not produced more than 4,000 aircraft in any year since 1982. Fewer than 500 of the most sophisticated jet aircraft are manufactured each year, and no single GA manufacturer produces more than 150 to 200 small jets per year. Since the SATS concept rests on assumptions—so far undemonstrated—about the manufacture of increasingly sophisticated and safe small aircraft at a cost substantially less than that of conventional aircraft today, it appears to be mostly speculative. AIRPORT AND AIRSPACE COMPATIBILITIES Another central feature of the SATS concept is that advanced small airplanes will be able to make much better use of the country’s thousands of small GA airports and vast amounts of lightly used airspace. NASA’s SATS Program Plan maintains that “most of the U.S. population lives within a 30-min. drive of over 5,000 public-use airports. … an untapped natural resource for mobility.”1 It also identifies the “non-radar airspace below 6,000 ft and the en route structure below 18,000 ft” as underutilized airspace that the airborne technologies of SATS aircraft can put to better use. These elements 1 Small Aircraft Transportation System Program Plan (Version 6), NASA Office of Aerospace Technology, p. 3.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 Figure 4-1 Inflation-adjusted index of changes in average price of new GA aircraft, 1980–1998. Note: price adjusted using the Consumer Price Index (Census Bureau 1998). Value of new GA aircraft shipments derived from GAMA (1999, p. 6).
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 of the nation’s airspace system are seen as offering a ready infrastructure for the use of advanced small aircraft in transportation, requiring little additional public investment. The assumption of available infrastructure needs to be examined, which requires an understanding of the basic kinds of aircraft postulated for such a system. So far, NASA’s SATS program plan simply envisions “fixed-wing aircraft applications.”2 It is possible that multiple vehicle types, including piston-engine, turboprop, and turbofan jet aircraft, would be accommodated in such a system. However, piston-engine and turbine aircraft have differing capabilities, from the speeds and altitudes at which they cruise most efficiently to their landing and takeoff characteristics. These differences have implications not only for the kinds of technological advances needed to make small aircraft more useful and desirable for transportation, but also for the extent to which airport and airway infrastructure can accommodate SATS vehicles. Some of the implications for just two of many possible aircraft types are considered next.3 SATS Jet Aircraft Jet aircraft require longer runways than propeller aircraft with comparable passenger (or payload) capacity. In general, the runways must be well maintained and have a paved surface. As reported in Chapter 2, 3,027 public- and private-use airports in the United States have at least one runway that is at least 4,000 feet long, and 1,749 of these airports have a runway that is at least 5,000 feet long. Today’s small jet aircraft operate mostly from the latter 1,749 airports with longer runways and night lighting. In particular, they operate in the roughly 1,200 of these airports that have a precision instrument landing system (ILS) on at least one runway. ILS allows for dependable operations during periods of low visibility. Radar coverage, which is not always coupled with ILS, allows for the safe separation of aircraft during instrument flight rule (IFR) operations—a capability that is indispensable in airports with a large amount of traffic. However, ILS is expensive to install and maintain. As discussed in Chapter 2, new technologies such as ADS-B and WAAS have the potential to substitute for or supplement ILS in the near future, reducing the cost of low-visibility operations substantially. All jet aircraft are equipped for ILS approaches, and as a practical matter, all jet operators are IFR-qualified to use these systems. The 1,200 ILS-equipped airports include nearly all of the country’s 526 commercial-service airports and about 258 metropolitan reliever airports. Advanced technologies that allow for precision approaches without the installation and maintenance of ILS will expand the number of airports available for low-visibility jet operations by instrument-rated pilots.4 2 Small Aircraft Transportation System Program Plan (Version 6), NASA Office of Aerospace Technology, p. 2. 3 The SATS program plan does not mention the potential role of other types of aircraft apart from fixed-wing airplanes, such as STOL/VTOL, tilt-wing, and even autogyros. Technological advances over the 25-year period envisioned for SATS deployment could presumably improve the performance and reliability of these aircraft to make them more acceptable and practical as a means of short- to medium-distance transportation. 4 Although it is conceivable that advanced technologies will eventually allow lesser-trained (e.g., non-IFR and single-engine rated) operators to fly jet aircraft under low-visibility conditions, it is not possible to know what infrastructure would be needed for such operations. Longer runways might be needed, for instance.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 Airports with paved 5,000-foot runways not equipped with ILS would be initial candidates for jet operations, although some airports with smaller runways, 3,000 feet long or longer, might be usable by the smallest jet aircraft. To illustrate the implications for small jet access, Figure 4-2 shows the location of commercial-service airports in and around Georgia, as well as the location of other GA airports having ILS and those having 5,000-foot runways without ILS. Of the state’s 109 public-use airports, Figure 4-2 Geographic distribution of all public-use airports in Georgia and major commercial-service airports in other states near Georgia border.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 46 have at least one runway that is 5,000 feet long or more, including 30 that do not have ILS or offer commercial service. Assuming that most current runways that are now equipped with ILS are at least 5,000 feet long, advanced technologies would give jet operators dependable access to about 550 more airports that have 5,000-foot runways but no ILS. If additional technological advances allowed more small jet aircraft to operate safely on 4,000-foot runways, an additional 1,300 non-ILS-equipped airports would become candidates for jet operations. Conceivably, further advances in the short-field takeoff and landing capabilities of small jet aircraft could expand access to another 2,600 public-and private-use airports with 3,000- to 3,900-foot runways. In practice, however, the number of airports that could handle small jet aircraft on a regular basis is sure to be smaller than these figures suggest, especially in the absence of significant airport investments. The need to control noise and pollutant emissions would undoubtedly compel many of these airports to purchase additional land around the airport and take other actions to control noise and air quality impacts. Many would also have to improve the condition of their runways and upgrade their runway maintenance programs to handle jet aircraft. The low-altitude structure (below 18,000 feet) envisioned for SATS may be incompatible with the usual design of jet aircraft to fly fast at high altitudes. Designing jet aircraft to fly at lower altitudes would offset the important speed advantages offered by jet aircraft, as well as the critical advantage of flying above inclement weather. However, at the higher altitudes, all airspace is highly controlled (Class A). Whether SATS jet aircraft would be used in the existing controlled jetways or in other lightly used portions of the controlled, high-altitude structure is unclear. The intended pattern of use, however, would have important implications for the technologies needed to ensure compatibility with other non-SATS air traffic. SATS Propeller Aircraft NASA’s SATS Program Plan does not explicitly identify propeller (piston-engine or turboprop) aircraft as the main type of aircraft envisioned for SATS. However, this can be inferred through the emphasis placed on the ability of SATS users to access 5,000 more public-use airports with little or no infrastructure modification, affordability of SATS aircraft by a large segment of the public, and confinement of SATS operations mainly to the uncontrolled low-altitude airspace structure. Propeller aircraft, and especially piston-engine aircraft, have advantages in each of these areas. Most can land on short, unpaved runways and therefore can land and take off at all small airports without modification to runways. A conventional, high-performance piston-engine aircraft can be produced today at a fraction of the cost of even the least expensive turbine-engine aircraft. Piston-engine aircraft, and to a lesser degree turboprops, routinely operate in the lower-altitude uncontrolled airspace (Class G) and at airports without ILS, and cabin pressurization could increase the altitudes at which piston-engine aircraft can regularly fly (e.g., from 12,000 to perhaps 18,000 feet; however, such a capability adds significantly to an aircraft’s production cost). While many IFR-rated pilots operate these aircraft, large numbers of private pilots without an IFR rating also fly them. Piston-engine aircraft are easier to learn to fly and maintain proficiency in than higher-performance, higher-cost turbine-
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 engine aircraft; hence, the potential exists for training more pilots at reasonable expense. The deployment of piston-engine or even turboprop aircraft as the primary SATS vehicle raises a number of obstacles. Propeller aircraft flying at the level of weather present significant challenges in meeting the public expectations of safety (both real and perceived), ride comfort, reliability, and travel speed. Moreover, the extent to which the public would benefit from more reliable (low-visibility) access to 5,000 public-use airports is questionable. ASSESSING USER DEMAND Forecasting long-range user demand for any mode of transportation is difficult because demographics, preferences, technologies, and other factors affecting travel demand and mode choice can change over time. This difficulty is compounded when the characteristics of the mode of transportation lack definition. Central to the SATS concept is the idea that advanced small airplanes will have reliable access to many more small airports in the country. As discussed above, however, the SATS concept does not define the type of small aircraft that will predominate, apart from an emphasis on fixed-wing, small aircraft. The committee has been asked to estimate demand for SATS. Without more specific information on vehicle characteristics, it is difficult to begin estimating the pool of potential users, since the general aircraft type affects, among other things, the speed, comfort, reliability, and cost of service; the mix of locations that can be served; and operator training and proficiency requirements. Given the many technical, demographic, and economic uncertainties, the committee questions the ability of anyone to offer such detailed and definitive information on future vehicle characteristics; yet, it is difficult to gauge the prospective demand for SATS without it. The ability to attract travelers to jet and propeller aircraft is likely to vary markedly because of large differences in the cost and performance characteristics of each aircraft type. Moreover, there is an implicit, but unexamined, assumption that small airports, if made more accessible, would divert large numbers of users from highway travel to air travel. This assumption is critical not only in assessing potential user demand, but also in evaluating the desirability of SATS from safety and environmental standpoints. Demand for Travel by SATS Jet and Propeller Aircraft As detailed above, jet aircraft differ fundamentally from piston-engine aircraft in their operating characteristics and requirements. Not only are the former much more expensive to produce, they require more extensive pilot training and proficiency and longer, better-maintained runways. Aircraft equipped with jet engines may produce more noise, and therefore they are often subject to restrictions on where they can fly. Small jets may produce more air pollutant emissions than small piston-engine aircraft, raising air quality concerns in the vicinity of the airports they operate from. At the same time, travelers have shown a much stronger preference to travel by jet than by propeller aircraft. Jets, which fly above most weather, are more reliable and more comfortable to fly in (passengers experience less turbulence and interior noise and vibration). Jets are much faster, are designed to have greater range than propeller aircraft, require more skilled pilots, and have achieved a better safety record.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 Over the past 20 years, the use of propeller aircraft, including turboprops, in business aviation and commercial airline transportation has declined and has shown little potential for significant growth in intercity transportation applications. The persistence and magnitude of this trend suggest that any anticipated SATS emphasizing propeller aircraft would have limited user demand. As discussed in Chapter 2, about 70 percent of GA airports are located within 75 miles of one or more of the country’s 525 commercial-service airports, many of which have commercial jet service or can accommodate private jet operations. In addition, most of the 260 GA reliever airports can accommodate jet aircraft. As estimated, 1,800 to 3,000 public-and private-use airports could accommodate small jet aircraft. Therefore, it is reasonable to question whether the greater accessibility of propeller aircraft, which can operate on shorter airfields, is likely to generate any additional user demand, particularly given the apparent reluctance of many travelers to fly in such aircraft. Although preference for travel in jet aircraft is clear, constraints on the large-scale production and deployment of new small jet aircraft remain. Small jet aircraft are expensive to produce and operate and may raise substantial environmental concerns in communities exposed to the effects of increased jet operations. In a transportation system oriented toward small jets, fewer airports would be accessible than in one oriented toward propeller airplanes. Nevertheless, limitations on the use of some small airports may be offset by the performance attributes of jet aircraft that appeal to travelers. In contrast, the challenge with regard to propeller aircraft, particularly piston-engine aircraft, is in making them more comfortable, faster, reliable, and safer—characteristics closer to those of jet aircraft. Given the dissimilarities between jet and propeller aircraft, it is surprising that the SATS program emphasizes GA aircraft and thus far has made little distinction between aircraft types. The program has supported equally the development of both kinds of GA aircraft and their use in such a transportation system without acknowledging the significantly different challenges and opportunities each presents. Without large reductions in the cost of producing and operating jet aircraft and large gains in the ride quality, speed, and safety of propeller aircraft, whether either type of aircraft would attract significant user demand is questionable. Traveler Demand in Small Cities and Nonmetropolitan Markets An important consideration in assessing demand for SATS is the extent to which expanded access to small, nonradar airports is likely to attract large numbers of users. For the most part, airports with radar are located in metropolitan areas. Radar contributes to the safe separation of IFR traffic, allowing reliable operations under poor visibility. Large commercial-service airports require such controlled separation of traffic, and even small airports within large metropolitan areas are located under controlled airspace. The emphasis in the SATS concept on providing access to radarless small-city and nonmetropolitan airports results from NASA’s recognition of the challenge of integrating SATS operations with those of commercial airlines in about 175 urban areas under Class B and C airspace in the United States. This limitation on the scope of SATS is understandable because of the complexity of urban air traffic patterns, but it raises questions about the likelihood of SATS generating much user demand. The most recent American Travel Survey conducted for the U.S. Department of Transportation (DOT) indicates that in 1995 Americans made more than 1 billion
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 domestic intercity trips of 100 miles or longer (from point of origin to final destination). Eighty-three percent of these trips began or ended in one of the nation’s 160 largest metropolitan areas with populations of more than 250,000 (see Figure 4-3). These urban areas contain about two-thirds of the country’s population. Thus, only 17 percent of trips both began and ended in one of the country’s 170 smaller metropolitan or nonmetropolitan areas, despite the fact that about one-third of the population resides in these areas. For the most part, Americans travel to or from large urban areas. Indeed, 60 percent of all trips involve one of the country’s 50 most populated metropolitan areas, where about 55 percent of U.S. population lives. These data suggest that if SATS aircraft do not access airports in large metropolitan areas, the potential of that system to attract significant numbers of users will be greatly limited. A key promise of SATS is on-demand transportation service. This capability can be especially important to business travelers, who place a high value on time and schedule flexibility. Yet business travelers—who make about one-fifth of all intercity trips—are even more likely than others to be traveling between large metropolitan areas: 65 percent of all business trips involve a metropolitan area among the 50 most populous as point of origin or destination, and 86 percent involve one of the country’s largest 160 metropolitan areas. If the emphasis in the SATS concept is on serving small cities and nonmetropolitan areas, then significant demand for SATS services is required from leisure travelers, who account for 87 percent of the trips taken in small-city and nonmetropolitan markets. Yet, leisure travelers—who plan their trips relatively far in advance—are usually more concerned about the price of travel than the schedule flexibility permitted by on-demand service of the type that SATS vehicles might provide. Larger metropolitan areas also account for a disproportionately high share of intercity trips because they contain important business locations and because their residents tend to have higher incomes than do residents of smaller metropolitan and nonmetropolitan areas. The 50 largest metropolitan areas in the United States have average household incomes that are 10 to 25 percent higher than those in the rest of the country (Census Bureau 1998, Table 729). Intercity travel increases as household incomes rise, and travel by air is highly correlated with income (see Figure 4-4). Hence, the travelers having the highest propensity for air travel, urban travelers, may have the least to gain from a SATS that emphasizes nonmetropolitan service. It is reasonable to question whether these recent travel patterns are reliable indicators of future travel trends, especially if Americans move farther away from metropolitan areas as communications and transportation systems continue to enhance personal mobility. The notion that innovations in communications technology have fostered a population shift from urban to rural areas is not confirmed by demographic data. While central cities have lost residents and businesses over the past half-century, their suburbs have boomed, as most metropolitan areas have gained population overall. According to Census Bureau data, nearly 80 percent of the U.S. population lived in the country’s 330 metropolitan areas in 1998 (see Figure 4-5). Another 11 percent lived in nonmetropolitan areas that are adjacent to metropolitan areas. Only 9 percent lived in other nonmetropolitan areas. By comparison, in 1970, 73 percent of the population lived in metropolitan areas, 14 percent lived in adjacent metropolitan areas, and nearly 13 percent lived in nonmetropolitan areas far from cities. Most population growth has occurred in the suburbs of metropolitan areas as incomes have risen.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 Figure 4-3 Intercity person trips involving metropolitan and nonmetropolitan areas by all modes, American Travel Survey, 1995.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 or observed, data. However, if many variables affecting demand are dramatically changed because of, say, changes in technology, observed responses may have to be supplemented by stated preference surveys. To be reliable, such surveys must be carefully designed so that respondents are placed as much as possible in realistic choice situations and asked to enumerate their preferences after being given information on all the attributes of all their choice alternatives, including other transportation options. It is important that the surveyer not influence respondent choices by promoting one option over another, for example, by providing more information on the new option than on current options. Finally, even demand models that are based on sound empirical and survey data must be applied with caution in forecasting user interest in a transportation mode that is not likely to emerge for several decades and that has attributes that can only be partially defined. A particular problem with predicting user demand for transportation systems that are not expected to be operational for several decades is that both observed and stated preference data reflect current consumer choices affected by existing conditions and technologies. Over time, one can expect—but not necessarily anticipate—changes in income levels, demographics, consumer preferences, availability of other transportation options, and other factors that can influence the demand for any given mode of transportation. However, in such cases, demand modeling can be useful in illuminating the many assumptions that must be examined in planning a system. Central to most transportation demand studies are observations of how people behave when presented with various travel options. In this regard, the SATS concept is difficult to examine, because there is no real-world SATS experience to observe. Nevertheless, more general observations of travel patterns and preferences can provide some idea of the scale of potential demand. For instance, observations on demand gleaned from the commuter airline, air taxi, and business aviation sectors indicate that a large portion of the public prefers travel by jet aircraft over travel by propeller aircraft, for reasons cited earlier. This information suggests that a small aircraft transportation system dominated by propeller aircraft would require major improvements in vehicle ride quality, travel speed, reliability, and safety performance to generate user interest. Small jet aircraft are more likely to appeal to many travelers, but these vehicles have proved to be too expensive for use by most travelers, few of whom fly in private jets or charter publicly available air taxis. Small jets, to attract significant demand, would need to become much less expensive to produce and operate than they are today, especially for short-range application. Regardless of vehicle type, a SATS oriented toward short- to medium-range intercity trips would need to compete for travelers, especially leisure travelers, not only with airlines but also with the automobile. Air taxi services have had little suc-
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 cess in attracting these travelers, largely because the automobile offers so much utility at a relatively low cost of operations. The greatest appeal of SATS would therefore appear to be among time-sensitive business travelers who use commercial air taxis on an occasional to regular basis. As discussed earlier, however, most small airports and most people are located within areas served by commercial airlines. Even a cursory review of these data, commonly used in transportation demand studies, suggests that SATS would need to serve large markets in order to have more than a niche role in the nation’s transportation system. DESIRABILITY OF A SMALL AIRCRAFT TRANSPORTATION SYSTEM In the previous section, consideration was given to the plausibility of the SATS concept emerging as planned. Many uncertainties were exposed, including a questionable potential for significant user demand. Yet, even if its plausibility could be affirmed, the desirability of such a transportation system would warrant closer scrutiny. The promise of SATS is that it will help reduce congestion and delay in commercial aviation and extend air service to more communities throughout the country. The potential for SATS to achieve these two goals and the possible effects of such a system on aviation safety and environmental quality are examined in this section. Whether the envisioned SATS is indeed desirable as an outcome warranting government promotion will depend to a large extent on its ability to meet its anticipated goals without having counteracting safety and environmental effects. SATS and Decongestion An anticipated benefit of the SATS concept is that full-scale deployment will help alleviate congestion and flight delays at commercial airports and in the nation’s controlled airspace by diverting some passenger traffic and flights to smaller GA airports. The idea is that this system, in addition to inducing new travel, would absorb a substantial portion of air travel that would otherwise have been accommodated by airlines. This shift could free up additional capacity in the commercial aviation system and, at a minimum, keep the system from becoming more congested. The public would benefit from such an outcome not only because of reduced congestion and associated flight delays, but also because of the reduced need to invest in more conventional airport and air traffic control capacity. This assumes that SATS deployment would require only limited public investment in supporting infrastructure. The most far-reaching SATS vision postulates that this new transportation system will combine with other advances in communications and information technology to cause a growing number of people and businesses to move outside large metropolitan areas, often referred to as “exurbia.” If SATS facilitates demographic changes that limit urban-oriented growth, it would also ease congestion pressures on the commercial aviation sector at major metropolitan airports. In this regard, SATS is viewed as a potential contributor to increasingly dispersed, or scattered, settlement patterns in the United States. Yet, demographic trends do not point to the emergence of such patterns, despite repeated predictions of exurban growth for the past two or three decades.7 As noted earlier, the trend in the United States, as in all 7 See, for instance, Naisbitt (1982).
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 developed countries, has been toward increased urbanization, leading to the expansion of metropolitan areas in both population and land area. Another anticipated outcome is that SATS will shift passenger traffic out of the hub-and-spoke system by diverting connecting passengers from these systems. The idea is that many passengers from small cities could use SATS to fly directly to their intended destination without making a transfer at a large hub airport; for instance, by flying straight from Erie to Allentown, Pennsylvania, without having to change planes in Pittsburgh. The decongestion effects of this expected outcome are open to question for several reasons. For one, the number of travelers in small-city airline markets—that is, those who begin and end their trips in small commercial-service airports—accounts for a very small percentage of all airline passengers, as shown in Figure 4-8 and in Table 4-3. Most passengers originate or end trips in large airports. A system that has little influence on passenger volumes in these markets has limited ability to affect congestion in the national airspace system. In this regard, it is important to note that flights from small cities to the hub airports of larger cities carry many passengers making connections to other cities, often on jet airliners. Since SATS service is not a substitute for these trips, to the extent that SATS vehicles are used at all they may not reduce the number of commuter flights from small cities to hubs, but only the size of the aircraft flying from these cities. The replacement of larger, faster aircraft by smaller, slower aircraft, including turboprops for regional jets, could result in poorer service in some smaller cities. Thus, the use of such aircraft could even reduce capacity at hub airports by increasing congestion (e.g., because of the need to increase the spacing of smaller aircraft and larger jet aircraft in the traffic streams of terminal areas). Another reason to question the decongestion promise of SATS is that capacity constraints are not a problem throughout the commercial air transportation system, but mainly at a few key airports that contribute to delays elsewhere in the system. If SATS has little effect on passenger traffic and flight volumes in these bottleneck airports, as appears likely, then SATS holds only limited potential to relieve congestion and reduce flight delays in the entire system. Moreover, many delay episodes are caused by severe weather, such as thunderstorms. The incidence and severity of weather-related problems, as well as delays caused by other factors such as aircraft mechanical problems, are affected only indirectly by the volume of passengers passing through the system. Finally, another anticipated means by which SATS might shift passenger traffic out of the hub-and-spoke system is by allowing some travelers who normally fly from large commercial airports to fly from smaller reliever airports located in the suburbs of large metropolitan areas. Some travelers might be attracted by the convenience of these airports, some of which have passenger amenities and are generally less crowded and served by relatively uncongested roadways. For SATS to serve these satellite airports, however, the aircraft must be able to function within the already heavy and complex air traffic over large metropolitan areas. As described in Chapter 2, the airspace over the country’s largest metropolitan areas, the origin and destination points for most air travelers, is closely controlled. The proximity of reliever airports to major metropolitan areas raises the possibility that SATS activity at relievers will have the unintended effect of changing the mix and increasing the
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 Figure 4-8 Share of daily passenger trips on scheduled airlines by size of origin and destination airports, 1999, second quarter. Note: “Large-Large” means that the airports on both ends of the trip (origin and final destination) are large airports (handling 5,000 or more outbound trips per day). See definitions, data, and sources in accompanying Table 4-3.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 Table 4-3 Domestic Airline Passenger Trips by Market Size and Distance, 1999 Second Quarter Passengers per Day Each Way for Trip Length (Miles) Airport Pair Market Number of Markets 75 to 200 201 to 400 401 to 600 601 to 900 901 to 1,200 1,201 to 3,000 Total Percentage Large to large 1,892 15,387 64,682 50,271 69,257 68,085 125,499 393,181 68.43 Large to medium 6,076 8,882 39,490 18,071 24,659 22,500 34,043 147,645 25.70 Large to small 7,128 1,557 2,207 1,748 2,722 3,013 5,170 16,417 2.86 Large to very small 3,377 190 323 191 294 315 466 1,779 0.31 Medium to medium 4,194 84 1,454 2,271 3,149 1,614 2,127 10,699 1.86 Medium to small 6,560 276 790 759 721 425 837 3,808 0.66 Medium to very small 1,572 84 96 65 76 53 72 446 0.08 Small to small 1,744 12 78 80 60 70 118 418 0.07 Small to very small 477 137 10 9 13 19 19 207 0.04 Total 33,020 26,609 109,130 73,465 100,951 96,094 168,351 574,600 100.00 Percentage 4.63 18.99 12.79 17.57 16.72 29.30 100.00 NOTE: Passenger data are based on 10 percent ticket sample by DOT. Connecting passenger enplanements are not included in origin and destination (O&D) counts. Only large carriers are required to report these data to DOT; however, because of code sharing agreements between large and small carriers, many trips on commuter carriers are included. Large airport = 50,000 to 5,001 outbound domestic passengers (O&D) per day—from Los Angeles (LAX), Atlanta, and Chicago O’Hare to Omaha, Tucson, and Oklahoma City. About 65 airports in total. Medium airport = 5,000 to 501 outbound domestic passenger trips (O&D) per day—from Buffalo, Tulsa, and Anchorage to Bozeman, MT; Lafayette, LA; and Traverse City, MI. About 110 airports in total. Small airport = 500 to 51 outbound domestic passenger trips (O&D) per day—from Erie, PA; Charlottesville, VA; and Fayetteville, AR, to Clarksburg, WV; Dubois, PA; and Brainerd, MN. About 175 airports in total. Very small airport = 50 to 5 outbound domestic passenger trips (O&D) per day—from Staunton, VA; Pierre, SD; and Manhattan, KS, to Jonesboro, AR; Sidney, MT; and Bluefield, WV. About 200 aiports in total.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 complexity of the airspace around major metropolitan airports, thus further taxing capacity in the commercial aviation sector. SATS and Air Service in Small Communities As discussed in Chapter 2, there are about 550 commercial-service airports in the United States, and more than half of these airports are in small cities. The smallest 200 small-city airports have 4 to 16 departures per day, usually on turboprop aircraft flown by scheduled commuter airlines. The commuter airlines fly mainly to large hub airports, where travelers from small markets can make connections to their final destinations. In being connected to large hub airports, travelers in small markets gain access to hundreds of city-pair markets to a degree that is not possible through direct, point-to-point service. For reasons explained in Chapter 2, the introduction of hub-and-spoke systems over the past two decades has been especially beneficial to travelers in lightly traveled markets. The consolidation of traffic flows at hub airports allows for more scheduled flights and the use of larger aircraft, since travelers originating from and headed to many different locations can share aircraft for parts of their trips. A problem with this system is that travelers in small markets often must make time-consuming connections, even for short trips. Airlines have a strong economic incentive to add markets to their hub networks to maximize passenger flows. The number of city-pairs served increases with the square of the number of spokes,8 adding more potential customers. Although many of the individual city-pairs created may have only a few passengers per year, the sheer number of such markets created increases the volume of traffic heading to and from the small city. At the same time, airlines recognize the need to avoid duplicative operations in small markets that would result from serving multiple airports in the region. The goal is to provide convenient scheduled service without spreading passenger flows at any one airport so thinly that reasonable flight frequencies cannot be supported, increasingly smaller aircraft must be used, and basic airport services and amenities cannot be sustained. To the extent that SATS aircraft are desirable to travelers (that is, they have characteristics that are generally acceptable) and can be produced and operated at low cost, commuter airlines could use them to provide more service to small airports, including some that are not served today. Commuter airlines serve mostly business travelers, who place a high value on airport convenience and frequent flights. The network of cities that commuter airlines serve represents a balance of business traveler demands for frequent flights, fast and comfortable service, and access to convenient locations. The economic constraints that govern the type of aircraft that are cost-efficient for the number of passengers and the extent to which airports can afford user-desired services and amenities are also taken into consideration. Although commuter airline service is geared to the time-sensitive and high-fare business traveler, leisure travelers often benefit from this service by filling unused seats at marginal cost. Introducing small jet aircraft that are easier to fly, perhaps requiring only one pilot, and much less expensive to produce than small jets today could alter this 8 The number of city-pairs created in a hub-and-spoke network is equivalent to ½(x + x2), where x is the number of spokes (Wheeler 1989).
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 balance, making more small airports economical to serve in the airlines’ hub-and-spoke networks. The idea that SATS aircraft could efficiently serve short- to medium-range small-city markets on a nonstop (point-to-point) basis presents a far more significant economic challenge for commercial airline service. For a scheduled commuter airline, or any airline operating a hub-and-spoke network, there is little to be gained from providing service that bypasses the network. The diversion of passengers depresses load factors on network flights, which could lead to service cutbacks. Such an outcome could be especially problematic for small cities if SATS aircraft were to siphon passengers from the hub-and-spoke system, leaving only those small-city passengers connecting for longer-distance flights. The efficiency of the hub-and-spoke system is contingent on the mixing of passengers headed to numerous places, both near and far. Air taxi operators, rather than network airlines, are the most likely candidates to adapt such aircraft to their operations. Air taxis currently serve many small cities; however, most air taxi service is in large markets, including the nation’s largest commercial-service airports and busy metropolitan reliever airports (see Chapter 2). In many respects, such a pattern is to be expected, since air taxis primarily serve business travelers, and most business is conducted in large urban areas. SATS could make air taxi service more economical for more travelers where it is in demand today; whether SATS would generate any significant demand outside the large business markets is unclear. As discussed above, small communities have relatively few business travelers, and leisure travelers are highly sensitive to the price of travel, especially for short or medium-distance trips that can be accomplished with an automobile. An aircraft that is easier and less costly to fly for on-demand service would have utility to some business travelers in small markets; however, its potential to compete with the automobile and efficient hub-and-spoke airline operations appears to be much more limited. SATS and Air Transportation Safety More than any other mode of transportation, air travel is highly regulated for safety assurance. Nearly all aspects of aviation, from the design and maintenance of individual aircraft parts to the training and retraining of pilots, are subject to stringent regulation aimed at ensuring and progressively improving aviation safety. One of the goals of the SATS research and technology program is to improve the safety of small aircraft operations, a long-standing concern. Many of the individual capabilities and technologies being pursued under the SATS umbrella could confer safety benefits on the conventional GA sector; for instance, by reducing pilot workload and improving the quality and timeliness of information for pilot decision making. Whether the full-scale SATS concept could improve the overall safety of air transportation is a more complicated question. To predict net safety effects, it is first necessary to understand where the users are likely to come from—for instance, whether they would otherwise have flown on the larger aircraft of scheduled airlines, driven automobiles, used other modes of transportation, or not traveled at all. The anticipated role of SATS in alleviating congestion in the commercial aviation sector suggests that diverting passengers from larger airliners is an intended outcome. Whether this outcome would confer net safety benefits on the public is question-
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 able, since the larger jets flown by airlines have safety records that are many times superior to those of GA (as discussed in Chapter 3). The replacement of a smaller number of larger airline-operated aircraft many smaller, albeit advanced, aircraft operated by nonprofessional pilots raises the possibility of a safety decrement, given the comparative safety performance of small and large aircraft. Highway travel in general is considered less safe than air travel.9 To the extent that SATS users would otherwise have driven automobiles on their intercity trips, transportation safety might be expected to improve for these travelers. However, for reasons discussed earlier, SATS appears to offer limited potential for traffic diversion from the automobile for short- to medium-range trips, largely because of the automobile’s flexibility and low out-of-pocket costs. SATS and Environmental Compatibility As described in detail in Chapter 3, environmental concerns have important influences on the use and expansion of airports, both large and small. Communities near airports are often vocal and politically influential opponents of airport expansions, particularly because of noise concerns. Because aviation noise has proved to be so problematic (and widespread), the assumption that small airports could readily handle many more SATS aircraft without investing in costly noise mitigations (such as land purchases to create noise buffer zones) warrants more careful consideration. Moreover, as noted, residents near airports are known to object to increased aircraft flight activity for other reasons, including concerns over congestion on local roads leading to and from the airport and over the safety of aircraft flying over homes and other structures. Whatever the source of concern, such adverse responses should be expected and not underestimated. Likewise, air quality concerns are certain to arise in connection with implementation of the SATS concept. Even if SATS aircraft engines emit fewer air pollutants than conventional small aircraft engines, increases in total operations and shifts in aircraft fleet mixes from piston-engine GA aircraft to more turbofan SATS aircraft may increase total pollutant emissions at small airports. Increases in these emissions are likely to compel assessment and action by public agencies and may prompt public opposition to SATS deployment, as well as the need for costly mitigations. Without more information on the specific circumstances (e.g., adjacent land uses, environmental sensitivities) of individual airports, it is reasonable to assume that prevailing use patterns are compatible with existing runway configurations, location, and physical infrastructure. Fundamentally different traffic mixes and levels would create other needs, including further environmental controls. Noise constraints on remote and rural airports might be less restrictive; however, such airports are least likely to have utility. Finally, additional thousands, or even hundreds of thousands, of small aircraft cruising in the nation’s airways are bound to have environmental effects that are not yet understood. One area of uncertainty is resultant changes in energy use and 9 Although Evans et al. (1990) estimated, on the basis of data from the 1980s, that for trips up to 600 miles a business traveler (typified as a middle-aged male, sober, and wearing a seat belt) had a lower risk of fatality driving on a rural Interstate highway in daylight than flying in a piston-engine commuter airplane.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 environmental implications. Small aircraft use less fuel per mile than large aircraft, but they are less fuel-efficient on a passenger-mile basis, particularly in the case of jets. As an example, a small 5-seat jet that consumes 100 gallons of fuel per flight hour covering 400 miles consumes 1 gallon of fuel for every 20 seat-miles. In comparison, a 50-seat regional jet that consumes about 500 gallons of fuel per hour consumes 1 gallon of fuel for every 40 seat-miles. Hence, the use of many more small aircraft to carry the same number of people once carried in larger aircraft could bring about much higher fuel usage. The environmental effects of such an increase in fuel consumption, including the atmospheric effects, warrant explicit examination. KEY FINDINGS FROM ANALYSES It is important to distinguish the individual capabilities and technologies being pursued under the SATS umbrella from the SATS concept itself. The specific capabilities and technologies can have merit individually or collectively—for instance, in improving aspects of GA safety—even if the full-scale SATS concept does not. The justification for promoting and planning a full-scale system is that it will confer large public benefits, primarily by helping to alleviate congestion in the national airspace system and extend much-needed air service to more communities. The SATS concept is thus being used to guide decisions about the various kinds of technologies that should be furthered through NASA research and development and, conversely, those that should not. It is also being offered as a guide for public investment decisions about airport and airspace infrastructure development and deployment. As such, the SATS concept deserves examination. The analyses in this chapter raise many important questions about the SATS concept. For the concept to be plausible—that is, credible enough to promote and plan for—the following assumptions must hold: Many major technological advances in propulsion, flight control, communications, navigation, surveillance, and manufacturing techniques can be achieved and coordinated to occur at about the same time. They can be validated by producers and regulators to ensure a high degree of safety when used in a new operating environment and by operators having piloting skills and training that differ markedly from those of today’s pilots. Much larger numbers of people will be both willing and able to serve as pilots. Growth in demand for SATS aircraft will prompt, and be propelled by, large reductions in the cost of producing advanced, high-performance small aircraft, primarily as a result of improvements in aircraft manufacturing and certification processes and scale economies not previously exhibited in the GA industry. Large numbers of travelers will accept propeller aircraft, including piston-engine airplanes, as a mode of intercity transportation, and these aircraft can be made much more comfortable to fly in, more reliable, faster, safer, and more affordable. Small jet aircraft can be produced in mass quantities at much lower cost than today’s jet aircraft; designed to operate on shorter, lightly maintained runways; and made capable of operating efficiently on short-range trips and in lower-altitude, uncontrolled airspace that will not interfere with commercial flights.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 A small aircraft transportation system oriented toward short- to medium-range intercity trips (200 to 1,000 miles) can compete for travelers with the low-cost, adaptable automobile, particularly among price-sensitive leisure travelers who make most trips under 1,000 miles. Counter to current demographic trends, increasing numbers of people and businesses with a propensity for travel and a high value of time will locate outside metropolitan areas, causing GA airports in small and rural communities to become more convenient to more people. Alternatively, SATS aircraft can be made capable of operating in the complex and congested airspace in and around the nation’s metropolitan centers, which is currently where most people live, most businesses are located, and most intercity travel demand occurs. Scheduled airlines, including commuter carriers that serve small cities, will not adapt these same advanced technologies effectively enough to make SATS less advantageous. This is a long list of improbabilities. Failure of any of them puts the SATS concept at risk. Moreover, the total SATS concept would not address the causes of aviation congestion and delay because it would have little, if any, effect on capacity and operations in the nation’s busiest and most congestion-prone airports and airways. Whether SATS would improve air service in small communities, to the benefit of the public and travelers in small markets, is likewise unclear. Scheduled commuter airlines now serve most small communities, though frequently at regional airports and with service oriented toward hub-and-spoke systems. The on-demand, nonstop, short-range service envisioned in the SATS concept would be a niche service, unlikely to be competitive with network carriers, which are themselves likely to adopt many of the advanced technologies to enhance their own service offerings. The prospect of diverting passengers from larger commercial airliners to small aircraft operated by private pilots and to airports with limited safety services should be examined in light of long-standing goals to enhance transportation safety. The safety record of small aircraft has been improving but remains poor compared with that of aircraft operated by scheduled airlines. A high degree of utility from the use of small aircraft for transportation and the introduction of technologies that improve small aircraft safety might justify an emphasis on SATS; however, only the latter is evident. Likewise, the prospects of environmental gains from a SATS oriented toward more fuel-intensive vehicles flying with fewer occupants at low altitudes are not apparent. The net effect on the environment could be deleterious. REFERENCES Abbreviation GAMA General Aviation Manufacturers Association Brand, D. 1996. The Values of Time Savings for Intercity Air and Auto Travelers for Trips Under 500 Miles in the U.S. Office of the Secretary of Transportation, U.S. Department of Transportation, June.
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Future Flight: A Review of the Small Aircraft Transportation System Concept - Special Report 263 Census Bureau. 1998. Statistical Abstract. Washington, D.C. Evans, L., M. Frick, and R. Schwing. 1990. Is It Safer to Fly or to Drive?—A Problem in Risk Communication. Risk Analysis, Vol. 10, No. 2, pp. 239–246. GAMA. 1999. 1999 General Aviation Statistical Databook. Washington, D.C. Levinson, D. M., and D. Gillen. 1998. The Full Cost of Intercity Highway Transportation. Transportation Research D, Vol. 3, No. 4, July, pp. 207–223. Naisbitt, J. 1982. Megatrends: Ten New Directions Transforming Our Lives. Warner Books, New York. Wheeler, C. F. 1989. Strategies for Maximizing the Profitability of Airline Hub-and-Spoke Networks. In Transportation Research Record 1214, TRB, National Research Council, Washington, D.C., pp. 1–9.
Representative terms from entire chapter: