9.3.2. NASA, ANCAT/EC2, and DLR 2015 Emissions Forecasts

The first NASA subsonic aircraft emissions inventory for 2015 was created as part of an assessment of the effects of a future HSCT (Baughcum et al., 1994); it has now been superseded by a new study (Baughcum et al., 1998; Mortlock and Van Alstyne, 1998) that includes new emissions technology assumptions and more detailed fleet mix and route system calculations. The NASA 2015 forecast inventory was calculated using methods similar to those used for NASA's historical and present-day inventories. Separate forecasts were created for scheduled operations (flights shown in the OAG database), charter operations, cargo operations, domestic operations in the FSU and China, military operations, and general aviation.

The forecast for scheduled traffic was based on the 1996 Boeing Current Market Outlook (Boeing, 1996), which projects separate traffic growth rates by region. Growth in worldwide demand for air travel was expected to average about 5% per year to the year 2015, with international travel growing at a slightly faster rate than domestic travel (Figure 9-7). By 2015, demand for air travel is projected to be 2.5 times greater than in 1996.

The total projected demand for scheduled air travel in the year 2015 was assigned to actual aircraft on a projected city-pair schedule derived from the schedules for 1995 published in the OAG. Individual city-pair service schedules for 1995 within each of the traffic flow regions were grown to 2015 by using the consolidated regional growth rate applicable for that region. Aircraft types were assigned to routes by using a market share forecast model. The turboprop market (for which there was no detailed forecast) was projected for 2015 by assuming that city pairs not served by the smallest turbojet category (50-90 seats) after demand growth to 2015 will continue to be served by small, medium, or large turboprops.

The result of the fleet assignment task was a detailed city-pair flight schedule by aircraft type required to satisfy forecast scheduled passenger demand in 2015. This schedule was used to calculate the 3-D emissions inventory for scheduled passenger service. Simplifying assumptions were the same as those used in calculating the historical and present-day inventories.

Projections of engine and aircraft technology levels for the 2015 scheduled fleet with regard to fuel efficiency and NOx emissions were made by assuming a continuation of present trends. In general, engines in the 2015 scheduled fleet represent the state-of-the-art in engine technology available either in production or in the final stages of development at the time the assignments were made (1997). These engines include low-emissions derivatives of previously existing engines. It is unlikely that any radical changes in airframe or engine design-even if such designs were acceptable-would have much of an effect on the 2015 fleet, given the time required to bring new designs into service. The combined effects of 2015 fleet mix and technology projections on the NOx technology level of the projected 2015 fleet appear in Figure 9-8, which shows the percentage of total fleet fuel burned by aircraft having landing/take-off cycle (LTO) emissions at a given level relative to the CAEP/2 NOx limit. (CAEP is chartered to propose worldwide certification standards for aircraft emissions and noise. The CAEP/2 designation refers to emissions certification standards adopted at the second meeting of the CAEP in December 1991.) Much more of the fleet consists of low-NOx aircraft-engine combinations in 2015, with ~70% of fuel burned in engines with NOx emission levels between 20 and 40% below the CAEP/2 certification limit.

DTI has developed a traffic and fleet forecast model for civil aviation, which was adapted under the direction of ANCAT and EIDG to produce an estimate of fuel burned and NOx emitted by civil aviation for the forecast year of 2015 (Gardner, 1998). Fuel and NOx growth factors-base to forecast-were calculated and applied to the ANCAT/EC2 city-pair gridded 1992 base year inventory to produce a gridded 2015 forecast.

DTI's top-down regional traffic demand forecasting model has a horizon of 25 years. Traffic coverage in the model includes all scheduled civil operations but excludes the former Soviet Union, Eastern Europe, freight, military, non-European charter traffic, business jets, and general aviation. Factors were developed to account for these traffic sectors in the forecast. The traffic forecast assumes a relationship between traffic [available seat-kilometers (ASK)] and GDP growth, and is assessed on a regional and flow basis (i.e., traffic flow between specific regions). The relationship is modified by assumptions on airline yields-a surrogate for fares price-and by a market maturity term that modifies demand as a function of time. Future fleets are estimated from traffic forecasts in terms of size and composition.

The concept of "traffic efficiency" was used to estimate fuel consumption from traffic values. Traffic efficiency is defined as the amount of traffic or capacity (ASK) per unit of fuel consumed. Aircraft manufacturers' traffic efficiency data for current aircraft types and projections for future aircraft types were used to develop efficiency trends for the eight categories of generic aircraft adopted for forecasting purposes, over a range of flight sector lengths. This approach permitted estimation of fuel consumption on the basis of regional and global traffic forecasts. Average efficiency figures were also calculated for the eight generic aircraft types in the 1992 base year fleet; a fleet average value of about 24.0 seat-km per liter was

found. This figure compares well with those in Greene (1992) and Balashov and Smith (1992) for the years 1989 and 1990, respectively, which gave traffic efficiencies of 20.5 seat-km per liter.

Greene (1992) and Balashov and Smith (1992) forecast an annual improvement in commercial air fleet fuel efficiency (see Table 9-2). These efficiencies include improvements arising from the introduction of new aircraft into the fleet and changes to operating conditions and passenger management. For the DTI work, the Greene (1992) forecasts were used to 2010. Annual improvements in fuel efficiency was assumed to decrease to 1% per year beyond 2010.

Using this efficiency trend, traffic efficiencies were calculated for the future aircraft fleet. The base year fleet average was estimated to increase to 31.8 seat-km per liter by 2015.

The same trends in fuel efficiency were applied to all size and technology classes. This approach represents a simplification because improvement figures are really a fleet average and would be influenced strongly by the rate of introduction of new aircraft. Given the much smaller contribution of older aircraft to global traffic performance, however, this factor will be only a second-order effect.

The emission performance of the forecast fleet was determined in part by the assumed response of the engine manufacturing industry to an assumed regulatory scenario. An emissions certification stringency regime was proposed for the forecast period, and compliance with the tighter limits was achieved by modifying the emissions performance of engines as they became noncompliant. This calculation was assessed from a base year engine fleet, comprising engines typical of and representing those found in the fleet (and compatible with the aircraft generic types described above). Performance improvements were applied only to new fleet entrants and were appropriate for staged and ultra-low NOx control technology in some cases.

This process results in an estimate of fuel burn and NOx emissions for the base year and forecast fleet using the same methodology; 1992-2015 fuel and NOx growth factors are thereby calculated. The growth factors were applied to the ANCAT/EC2 base year gridded fuel and NOx estimates to provide a 2015 gridded forecast.

The methods used to project civil aviation traffic demand for the DLR 2015 inventory were based on regional growth factors calculated by DTI. Thus, the DLR 2015 forecast differs from the ANCAT/EC2 forecast only in that the base year inventory is slightly different because of the different fuel and profiling methodology and the aircraft generic types. Thus, in the comparison of results, ANCAT and DLR 2015 forecasts are not assumed to be different because the DLR forecast is essentially an application of the DTI/ANCAT forecast.

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