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However, as the current methodology is based on total fuel use for aviation, most of the weaknesses presented
above will not influence the accuracy of the reported direct greenhouse gas emission figures to a large extent.
2 . 3 Other aircraft methodologies
More detailed methodologies have been developed to better estimate fuel use and NOx emissions from aircraft
based on more exact data on flying conditions (e.g. ANCAT/EC2). As mentioned, this level of sophistication is
not really necessary for the current reporting requirements.
A “high tier” methodology has been developed for the UNECE Emission Inventory Guidebook that enables
emission estimates to be made from flight movement data only. This will be a simplification of the ANCAT/EC2
methodology. This methodology requires data on cruise flight distances and gives fuel use factors for individual
aircraft per mile cruised in addition to LTO. This methodology may help countries to estimate their fuel use for
domestic and international air traffic directly, if other sources of data not are available. The methodology may
also be used for verification of other data.
The UNECE Emission Inventory Guidebook also provides a methodology for estimating emissions from military
aircraft, helicopters and propelled aircraft. Emissions of water vapour, may, if required, be estimated from the
CO2 emissions, assuming a stoichiometric equation.
2 .4 Selection of good practice methods
The current guidelines form the background for selecting good practice methodologies based on the available
data in the reporting country. Use of Tier 2 rather than Tier 1 will increase flexibility, transparency and
comparability more than the accuracy. However, good practice may also be used to estimate the emissions from
more advanced approaches not mentioned in the current guidelines. Such methodologies may involve modelling
the emissions for specific aircraft from various flight modes, depending on altitudes, cruise distances etc.
However, it should be emphasised that such advanced methodologies will, in particular, improve the estimates of
nitrogen oxides emissions, but will not have any effect on the carbon dioxide estimates and only small effect on
methane and nitrous oxide estimates.
2.4.1 Activity Data
The options for determining good practice activity data will depend on the available data and options for data
collection in the reporting country. The most important activity data are the consumption of kerosene (and
aviation gasoline) used for domestic and international aviation, respectively. When using Tier 2, data on the
number of LTOs are also needed. It is assumed that total fuel used for aviation is available from the fuel statistics
in all countries.
Good practice methodologies for determining the domestic fuel used include:
• Determining the domestic fuel use from surveys: In most countries there will be just a few airline companies
involved in domestic air transport. Consequently, it will be easy to request annual data from these companies.
This will be difficult if there are very many airline companies in the country, too few (due to confidentiality)
or the airline companies themselves may find it difficult to distinguish their domestic fuel usage from the
international one, and
Background Paper
100 Energy Sector
• Determining the domestic fuel use from sales statistics: In a lot of countries, sales statistics may give a split
between domestic use and international use since they may be taxed at different rates. This is easier, but less
transparent, than option 1.
If options 1 and 2 not are possible, the domestic fuel use has to be estimated from aircraft movement data and
fuel use factors. This may be time consuming and not always very accurate.
2.4.2 Emission factors
The choice of emission factors for aircraft will depend on the national aircraft fleet:
• Are data on national type of aircraft in use available? If not the default data have to be used;
• Are the emission factors for national aircraft fleet close to the default values in Table 2? If this is so the
default values are appropriate, if not, the use of aircraft specific default values are encouraged;
• Are the aircraft types used in the country included in Table 1? If yes, the table is directly useful. If not, data for
missing aircraft have to be supplemented or use of data for similar aircraft type in the table is necessary, and
• Is there a high proportion of flights with helicopters and small aircraft in the country? If yes, the emissions
from these will have to be estimated separately.
As aircraft have a fixed technology, it is usually not appropriate to use country-specific emission factors unless
documented very thoroughly and are found to be better than the default values. Reasons for such deviations may be
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