Choice of method

Three methodological tiers for estimating emissions of CO2, CH4 and N2O from aviation are presented. Tier 1 and Tier 2 methods use fuel consumption data. Tier 1 is purely fuel based, while Tier 2 method is based on the number of landing/take-off cycles (LTOs) and fuel use. Tier 3 uses movement16 data for individual flights. All tiers distinguish between domestic and international flights. However, energy statistics used in Tier 1 often do not accurately distinguish between domestic and international fuel use or between individual source categories, as defined in Table 3.6.1. Tiers 2 and 3 provide more accurate methodologies to make these distinctions.

The choice of methodology depends on the type of fuel, the data available, and the relative importance of aircraft emissions. For aviation gasoline, though country-specific emission factors may be available, the numbers of LTOs are generally not available. Therefore, Tier 1 and its default emission factors would probably be used for aviation gasoline. All tiers can be used for operations using jet fuel, as relevant emission factors are available for jet fuel. Table 3.6.2 summarizes the data requirements for the different tiers:

The decision tree shown in Figure 3.6.1 should help to select the appropriate method. The resource demand for the various tiers depends in part on the number of air traffic movements. Tier 1 should not be resource intensive. Tier 2, based on individual aircraft, and Tier 3A, based on Origin and Destination (OD) pairs, would use incrementally more resources. Tier 3B, which requires sophisticated modelling, requires the most resources.

Given the current limited knowledge of CH4 and N2O emission factors, more detailed methods will not significantly reduce uncertainties for CH4 and N2O emissions. However, if aviation is a key category, then it is recommended that Tier 2 or Tier 3 approaches are used, because higher tiers give better differentiation between domestic and international aviation, and will facilitate estimating the effects of changes in technologies (and therefore emission factors) in the future.

The estimates for the cruise phase become more accurate when using Tier 3A methodology or Tier 3B models. Moreover because Tier 3 methods use flight movement data instead of fuel use, they provide a more accurate separation between domestic and international flights. Data may be available from the operators of Tier 3 models (such as SAGE, (Kim, 2005a and b; Malwitz, 2005) and AERO2K (Eyers, 2004). Other methods for differentiating national and international fuel use such as considering LTOs, passenger-kilometer data, a percentage split based on flight timetables (e.g., OAG data, ICAO statistics for tonne-kilometres performed by countries) are shortcuts. The methods may be used if no other methods or data are available.

16 Movement data refers to, at a minimum, information on the origin and destination, aircraft type, and date of individual flights.

Table 3.6.1 Source categories

Source category

Coverage

1 A 3 a Civil Aviation

Emissions from international and domestic civil aviation, including take-offs and landings. Comprises civil commercial use of airplanes, including: scheduled and charter traffic for passengers and freight, air taxiing, and general aviation. The international/domestic split should be determined on the basis of departure and landing locations for each flight stage and not by the nationality of the airline. Exclude use of fuel at airports for ground transport which is reported under 1 A 3 e Other Transportation. Also exclude fuel for stationary combustion at airports; report this information under the appropriate stationary combustion category.

1 A 3 a i International aviation (International Bunkers)

Emissions from flights that depart in one country and arrive in a different country. Include take-offs and landings for these flight stages. Emissions from international military aviation can be included as a separate sub-category of international aviation provided that the same definitional distinction is applied and data are available to support the definition.

1 A 3 a ii Domestic

Aviation

Emissions from civil domestic passenger and freight traffic that departs and arrives in the same country (commercial, private, agriculture, etc.), including take-offs and landings for these flight stages. Note that this may include journeys of considerable length between two airports in a country (e.g. San Francisco to Honolulu). Exclude military, which should be reported under 1 A 5 b.

1 A 5 b Mobile (aviation component)

All remaining aviation mobile emissions from fuel combustion that are not specified elsewhere. Include emissions from fuel delivered to the country's military not otherwise included separately in 1 A3 a i as well as fuel delivered within that country but used by the militaries of other countries that are not engaged in multilateral operations.

1.A.5 c Multilateral Operations (aviation component)

Emissions from fuels used for aviation in multilateral operations pursuant to the Charter of the United Nations. Include emissions from fuel delivered to the military in the country and delivered to the military of other countries.

Table 3.6.2 Data requirements for different tiers

Data, both Domestic and International

Tier 1

Tier 2

Tier 3A

Tier 3B

Aviation gasoline consumption

X

Jet Fuel consumption

X

X

Total LTO

LTO by aircraft type

X

Origin and Destination (OD) by aircraft type

X

Full flight movements with aircraft and engine data

X

Other reasons for choosing to use a higher tier include estimation of emissions jointly with other pollutants (e.g. NOx) and harmonisation of methods with other inventories. In Tier 2 (and higher) the emissions for the LTO and cruise phases are estimated separately, in order to harmonise with methods that were developed for air pollution programmes that cover only emissions below 914 meters (3000 feet). There may be significant discrepancies between the results of a bottom-up approach and a top-down fuel-based approach for aircraft. An example is presented in Daggett et al. (1999).

TIER 1 METHOD

The Tier 1 method is based on an aggregate quantity of fuel consumption data for aviation (LTO and cruise) multiplied by average emission factors. The methane emission factors have been averaged over all flying phases based on the assumption that 10 percent of the fuel is used in the LTO phase of the flight. Emissions are calculated according to Equation 3.6.1:

Equation 3.6.1 (aviation equation 1)

Emissions = Fuel Consumption • Emission Factor

Tier 1 method should be used to estimate emissions from aircraft that use aviation gasoline which is only used in small aircraft and generally represents less than 1 percent of fuel consumption from aviation. Tier 1 method is also used for jet-fuelled aviation activities when aircraft operational use data are not available.

Domestic and international emissions are to be estimated separately using the above equation, using one of the methods discussed in section 3.6.1.3 to allocate fuel between the two.

TIER 2 METHOD

Tier 2 method is only applicable for jet fuel use in jet aircraft engines. Operations of aircraft are divided into LTO and cruise phases. To use Tier 2 method, the number of LTO operations must be known for both domestic and international aviation, preferably by aircraft type. In Tier 2 method a distinction is made between emissions below and above 914 m (3000 feet); that is emissions generated during the LTO and cruise phases of flight.

Tier 2 method breaks the calculation of emissions from aviation into the following steps:

1. Estimate the domestic and international fuel consumption totals for aviation.

2. Estimate LTO fuel consumption for domestic and international operations.

3. Estimate the cruise fuel consumption for domestic and international aviation.

4. Estimate emissions from LTO and cruise phases for domestic and international aviation. Tier 2 approach uses Equations 3.6.2 to 3.6.5 to estimate emissions:

Equation 3.6.2 (aviation equation 2)

Total Emissions = LTO Emissions + Cruise Emissions

Where:

Equation 3.6.3 (aviation equation 3)

LTO Emissions = Number of LTOs •Emission Factor LTO

Equation 3.6.4 (aviation equation 4)

LTO Fuel Consumption = Number of LTOs •Fuel Consumption per LTO

Equation 3.6.5 (aviation equation 5)

Cruise Emissions = (Total Fuel Consumption - LTO Fuel Consumption)

• Emission Factor Cruise

Figure 3.6.1 Decision tree for estimating aircraft emissions (applied to each greenhouse gas)

Figure 3.6.1 Decision tree for estimating aircraft emissions (applied to each greenhouse gas)

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees.

The basis of the recommended Tier 2 methodology is presented schematically in Figure 3.6.2.

In Tier 2 method, the fuel used in the cruise phase is estimated as a residual: total fuel use minus fuel used in the LTO phase of the flight (Equation 3.6.5). Fuel use is estimated for domestic and international aviation separately. The estimated fuel use for cruise is multiplied by aggregate emission factors (average or per aircraft type) in order to estimate the CO2 and NOx cruise emissions.17 Emissions and fuel used in the LTO phase are estimated from statistics on the number of LTOs (aggregate or per aircraft type) and default emission factors or fuel use factors per LTO cycle (average or per aircraft type).

Current scientific understanding does not allow other gases (e.g., N2O and CH4) to be included in calculation of cruise emissions. (IPCC,1999).

Tier 2 method considers activity data at the level of individual aircraft types and therefore needs data on the number of domestic LTOs by aircraft type and international LTOs by aircraft type. The estimate should include all aircraft types frequently used for domestic and international aviation. Table 3.6.3 provides a way of mapping actual aircraft to representative aircraft types in the database. Cruise emission factors for emissions other than NOx are not provided in Tier 2 method; either national emission factors or the tier default emission factors must be used to estimate these cruise emissions.

TIER 3 METHODS

Tier 3 methods are based on actual flight movement data, either: for Tier 3A origin and destination (OD) data or for Tier 3B full flight trajectory information. National Tier 3 approaches can be used if they are well documented and have been reviewed following the guidance provided in Volume 1, Chapter 6 (QA/QC). To facilitate data review, countries that use Tier 3 methodology could separately report emissions for Commercial Scheduled Aviation and Other Jet Fuelled Activities.

Tier 3A takes into account cruise emissions for different flight distances. Details on the origin (departure) and destination (arrival) airports and aircraft type are needed to use Tier 3A, for both domestic and international flights. In Tier 3A, inventories are modelled using average fuel consumption and emissions data for the LTO phase and various cruise phase lengths, for an array of representative aircraft categories.

The data used in Tier 3A methodology takes into account that the amount of emissions generated varies between phases of flight. The methodology also takes into account that fuel burn is related to flight distance, while recognizing that fuel burn can be comparably higher on relatively short distances than on longer routes. This is because aircraft use a higher amount of fuel per distance for the LTO cycle compared to the cruise phase.

The EMEP/CORINAIR Emission inventory guidebook (EEA 2002) provides an example of Tier 3A method for calculating emissions from aircraft. The EMEP/CORINAIR Emission inventory guidebook is continually being refined and is published electronically via the European Environment Agency Internet web site. EMEP/CORINAIR provides tables with emissions per flight distance.

(Note that there are three EMEP/CORINAIR methods for calculating aircraft emissions; but, only the Detailed CORINAIR Methodology equates to Tier 3A.)

Tier 3B methodology is distinguished from Tier 3A by the calculation of fuel burnt and emissions throughout the full trajectory of each flight segment using aircraft and engine-specific aerodynamic performance information. To use Tier 3B, sophisticated computer models are required to address all the equipment, performance and trajectory variables and calculations for all flights in a given year. Models used for Tier 3B level can generally specify output in terms of aircraft, engine, airport, region, and global totals, as well as by latitude, longitude, altitude and time, for fuel burn and emissions of CO, hydrocarbons (HC), CO2, H2O, NOx, and SOx. To be used in preparing annual inventory submissions, Tier 3B model must calculate aircraft emissions from input data that take into account air-traffic changes, aircraft equipment changes, or any input-variable scenario. The components of Tier 3B models ideally are incorporated so that they can be readily updated, so that the models are dynamic and can remain current with evolving data and methodologies. Examples of models include the System for assessing Aviation's Global Emissions (SAGE), by the United States Federal Aviation Administration (Kim, 2005 a and b; Malwitz, 2005), and AERO2k, (Eyers, 2004), by the European Commission.

Figure 3.6.2 Estimating aircraft emissions with Tier 2 method

Figure 3.6.2 Estimating aircraft emissions with Tier 2 method

Table 3.6.3

Correspondence between representative aircraft and other aircraft types

Generic

ICAO

IATA

Generic

ICAO

IATA

Generic

ICAO

IATA

aircraft type

aircraft in group

aircraft type

aircraft in group

aircraft type

aircraft in group

A30B

AB3

Boeing

B737

73G

DC9

DC9

AB4

737-700

73 W

DC91

D91

Airbus A300

AB6

Boeing

B738

738

DC92

D92

A306

ABF

737-800

73H

Douglas DC-9

DC93

D93

ABX

Boeing

B739

739

DC94

D94

ABY

737-900

D95

310

B741

74T

DC95

D9C

312

Boeing

N74S

74L

D9F

Airbus A310

A310

313

747-100

B74R

74R

D9X

31F

B74R

74V

L10

31X

Boeing 747-200

742

Lockheed

L101

L11

31Y

B742

74C

L-1011

L15

Airbus A319

A319

319

74X

L1F

A318

318

Boeing

B743

743

McDonnell

M11

Airbus A320

A320

320

747-300

74D

Douglas

MD11

M1F

32S

747

MD11

M1M

Airbus A321

A321

321

744

MD80

M80

Airbus

A330

330

Boeing 747-400

74E

McDonnell Douglas MD80

MD81

M81

A330-200

A332

332

B744

74F

MD82

M82

Airbus

A330

330

74J

MD83

M83

A330-300

A333

333

74M

MD87

M87

74Y

MD88

MD88

Airbus

A3 42

342

McDonnell

A340-200

757

Douglas

MD90

M90

Boeing 757-200

MD90

Airbus

A3 40

340

B752

75 F

Tupolev Tu134

T134

TU3

A340-300

A3 43

343

75M

Tupolev Tu154

T154

TU5

Airbus

A3 45

345

Boeing

B753

753

Avro RJ85

RJ85

AR8

A340-500

757-300

ARJ

Airbus

A3 46

346

Boeing

B762

762

B461

141

A340-600

767-200

76X

B462

142

703

767

143

Boeing 707

B703

707

Boeing

B763

76F

BAe 146

146

70F

767-300

763

B463

14F

70M

76Y

14X

Boeing 717

B712

717

Boeing

B764

764

14Y

Boeing

B721

721

767-400

14Z

727-100

72M

Boeing

B772

777

Embraer

E145

ER4

722

777-200

772

ERJ145

ERJ

727

Boeing

B773

773

F100

100

Boeing

B722

72C

777-300

F70

F70

727-200

72B

D10

Fokker 100/70/28

F21

72F

D11

F22

72S

Douglas DC-10

D1C

F28

F23

Boeing

B731

731

DC10

D1F

F24

737-100

D1M

F28

Boeing 737-200

732

D1X

B11

B732

73M

D1Y

B12

73X

DC85

D8F

BAC 111

BA11

B13

737

DC86

D8L

B14

Boeing

B733

73F

Douglas DC-8

D8M

B15

737-300

733

D8Q

Donier

D328

D38

73Y

DC87

D8T

Do 328

Boeing

B734

737

D8X

Gulfstream

GRJ

737-400

734

D8Y

IV/V

Boeing

B735

737

Yakovlev

YK42

YK2

737-500

735

Yak 42

Boeing 737-600

B736

736

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