Aviation and the Global Atmosphere

7.1. Introduction

This chapter considers how aircraft technologies influence emissions at altitude today and how that may change in the future. This introductory section sets the scene by briefly sketching the development and size of today's industry, outlines the approach the authors have adopted to their task, and describes the structure and content of the rest of the chapter.

Major advances in aircraft technology have been achieved in the past 40-50 years. Over that period, principal methods of propulsion have changed: Propeller aircraft were succeeded by jet-powered aircraft of the 1950s; these jets, in turn, were superseded by today's turbofan-powered aircraft from 1970 onwards. The fleet has expanded rapidly. So too has the capacity of jet aircraft-rising from typical 150-seat versions of the late 1950s to the largest 525-seat variant of the 747-400 aircraft in service today. The performance and capability of aircraft has also changed greatly. Cruise speeds of propeller aircraft have trebled from the 100 knots typical of the 1940s. At the start of the commercial jet age, speeds rose to 450 knots. Today's turbofans cruise at average speeds of around 500 knots, and the Concorde reaches 1350 knots. The search for efficient cruise performance with greater range, particularly for long-haul aircraft, has also resulted in higher flying aircraft-a key factor in determining where most aircraft emissions occur and their resulting impact (as discussed in Chapters 2 to 6). Average cruise altitudes for propeller aircraft rose from about 3 to 7.5 km; today's jets cruise primarily between 10.5 and 11.5 km, with some operating at up to 13 km.

Aviation designers and regulators must address complex challenges, given the operating conditions encountered by aircraft and the correspondingly stringent technical and airworthiness requirements that must be met. Factors that have been and will continue to be considered include the following:
Passenger safety must be assured for all phases of aircraft operations.
Aircraft are more severely constrained by volume and weight considerations than ground-based forms of transportation, placing more stringent limits on available technology choices.
Aircraft systems are typically more complex than other transport modes, and many physical and chemical effects associated with them are closely coupled and interdependent. As such, changes in technology aimed at improving one aspect of performance, (e.g., a particular pollutant, or passenger safety) may have adverse effects on other aspects of performance (e.g., fuel efficiency).
Time scales for technology development and product life are on the order of decades.
Costs to develop, purchase, and operate aircraft are high relative to many other forms of transportation (aviation costs are typically counted in millions and billions of dollars). As with any other commercial product, the impact of technology changes on cost and customer satisfaction must be carefully assessed.

Considering the breadth and complexity of the technology base supporting today's aircraft, this chapter cannot hope to provide more than an overview of the subject. Emphasis has therefore been placed on the following key questions:

What are the principal technological factors that determine the nature and scale of emissions from aircraft at altitude?
What progress has been made to date in reducing emissions, and how may new advances in aircraft and engine technology help reduce them further in the future?
What data exist about actual emissions from aircraft? What is being done, and what needs to be done, to improve our understanding of and our ability to predict the scale and nature of these emissions?
How are emissions from aircraft currently regulated, and how do these regulations influence emissions at altitude?
What performance might we expect from fleets operating in 2015 and 2050 and in setting the scenarios discussed in Chapter 9?

The structure and balance of the chapter have been developed to reflect the fact that advances in technology that influence the impact of aircraft on the environment fall broadly into two categories:

Innovations that improve fuel efficiency, thus reduce the amount of fuel burned (and mass of emissions) per passenger-km flown
Developments that may alter the percentage concentration of a particular exhaust gas (e.g., reduce NOx for a given mass of fuel burned).

Broadly, advances that reduce the weight and drag of the aircraft fall into the first of these two categories. These advances are covered in Sections 7.2 and 7.3, which provide background material and a review of current development themes most relevant to the fuel efficiency of modern aircraft.

Engine technology is more complex. Fuel efficiency is closely linked to engine type (e.g., high bypass ratio) and choice of thermodynamic cycles (e.g., pressure and temperature ratios), but changes in the design of the engine's combustion system can also have a significant effect on the composition of the exhaust plume. These two aspects of engine design are dealt with in Sections 7.4 and 7.5.

Section 7.4 introduces the principal performance and design constraints that designers of new engines face and comments on future trends. Section 7.5 takes account of engine cycle trends on the design requirements of new low-emissions combustors. This section is a key part of the chapter because it deals with the component-the combustor-that has the greatest potential for design changes that may reduce the concentrations of some emissions that are of concern. In particular, this section addresses some of the issues raised in Chapter 2. The challenges are complex, and additional background material is included to describe the many fundamental conflicting characteristics of combustion processes that must be reconciled in emissions reduction technology programs.

Trace species in engine emissions are also considered. Potentially important physical and chemical changes to these species occur in the engine as the gas travels rapidly downstream from the combustor and through the turbine stages, where they undergo sudden changes in pressure and temperature. Section 7.6 discusses the present state of knowledge in this field.

Section 7.7 provides background information about work to date in developing the international engine emissions database. It also reports on progress in using data gathered from ground-based tests to predict corresponding cruise altitude emissions levels. These methods are used in the development of predicted inventories for future scenarios in Chapter 9.

Aircraft fuel continues to be a subject of considerable interest. Kerosene-type fuels are in widespread use today and are likely to remain so in the foreseeable future. This factor inhibits the prospect of further reducing CO2 by changing fuels. The use of kerosene is addressed in some detail in Section 7.8, as is the question of fuel effects on emissions. Looking further ahead, Section 7.8 also considers briefly the use of alternative fuels in the longer term-beyond 2050.

The later sections of the chapter concentrate on the smaller numbers of special category aircraft in the global fleet. The first of these categories is "small aircraft" as used in the regional sector of air transport. Generally, these aircraft fly at lower altitudes than their larger counterparts and therefore have a much lower potential impact on the climate. However, growth in this sector is expected to continue; for completeness, therefore, Section 7.9 reports on the particular problems and differences of these aircraft now and in the future. Section 7.10 is concerned with the significant technical and operational issues that differentiate supersonic aircraft from the subsonic fleet. These aircraft are small in numbers now and are likely to remain so until 2015 or later. Beyond that time frame, a significant rise in the numbers of such aircraft could present a more challenging environmental problem because of the altitude at which they fly-a matter discussed in Chapter 2. This prospect has spawned research programs addressing the particular problems arising from high-speed, high-altitude operations.

Section 7.11 discusses the effects of military priorities that influence trends in the technology applied to aircraft. Although operational effectiveness in combat will continue to be the key consideration in engine design, Section 7.11 also points out why military operators' interest in the composition of exhaust is similar to that for civil engines. This section has links with Chapter 9, which shows how, in relative terms, the impact of military fleets will fall partly because of the anticipated slight reduction in their numbers but mainly because of the predicted steep rise of global civil fleets over the next 50 years.

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