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distinguished in the energy statistics (different qualities), but there may be countries where these data are aggregated.
The emission factors of CO2 per fuel basis are well determined as aviation fuel has a well-defined quality. This
means that country-specific values of CO2 emission factors (on a per fuel basis) should never deviate much from
default values. The emission factors of N2O and CH4 must be considered to be highly uncertain. However, as the
latter pollutants do not contribute much to total emissions in the overall inventory, this is not of a great concern.
Countries should be encouraged to determine better emission factors for CH4 and N2O to improve the default
values in the future.
2 .1 Tier 1
The Tier 1 methodology is very simple, but is usually appropriate for estimating CO2, CH4 and N2O emissions
with the current knowledge of emission factors.
The main reasons for encouraging the use of higher tiers are as follows:
• Increased transparency with respect to reporting of other pollutants in other inventories (e.g. CORINAIR);
• Gives the possibility to obtain time series reflecting changes in technology;
• Gives the possibility to verify the estimates;
• Allows (possible future) reporting of emissions from cruise and LTO separately, and
• Gives more accurate NOx estimates.
Background Paper
98 Energy Sector
F i g u r e 2 T i e r 2 m e t h o d o l o g y f o r e s t ima t i n g e m i s s i o n s f r o m
a i r c r a f t
2 .2 Tier 2
Data to be used in the Tier 2 methodology (the aggregated number of LTOs) are likely to be available in most
countries. However, the LTO per aircraft type is not always available. In both cases, the methodology will require
that the number of LTOs be available for domestic and international aviation separately, which may not always
be the case.
There are some weaknesses in the Tier 2 methodology:
• The aggregate emission factors and fuel use factors are based on fuel use of average aircraft. If the average
aircraft is different in a particular country the estimated fuel use may be wrong. This may be the case in
countries with a higher proportion of small or large aircraft than assumed in Table 2. The default aircraft are,
in fact, quite large and will overestimate LTO emissions in most countries;
• If the estimation is based on Table 1, some common aircraft will not be found in the table. These may be new
aircraft with presumably lower fuel consumption than average (e.g. MD80, MD90 and new generation 737s)
and perhaps also some special type of aircraft that may be in use outside USA and Western Europe, and
x EF x EF
x EF x EF
Emissions for
Domestic
LTO
Emissions for
International
LTO
Domestic Cruise
Fuel Use
Emissions for
Domestic
Cruise
Emissions for
International
Cruise
International Cruise
Fuel Use
Fuel Sold for
International
Use
Fuel Sold for
Domestic
Use
Number of
LTO’s for
Domestic
Flights
Domestic LTO
Fuel Use
International LTO
Fuel Use
Fuel Consumption
per Domestic
LTO
Fuel Consumption
per International
LTO
Number of
LTO’s for
International
Flights
Total National Number
of LTO’s
Total National Fuel Sold for
Aircraft
Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories
Aircraft Emissions 99
• LTO data are available for scheduled flights only (category 1). Flights with e.g. helicopters, taxi flight,
pleasure flights etc. will not be covered by the methodology. In most countries these emissions are
insignificant, but in some countries they will have to be taken into account.
The emission factors given in the Tier 2 methodology reflect the state-of-the-art knowledge (ICAO (1995) and
ANCAT/EC (1998)). The knowledge on emissions during the LTO part of the flight is, however, far better than
the knowledge on emissions during cruise. The emission factors for methane and nitrous oxide are generally
highly uncertain. The methane data are taken from Olivier (1991), but the source of the nitrous oxide data in the
IPCC Guidelines is not quite clear. The default value is, however, consistent with AERONOX (1995). Nitrous
oxide emission per TJ oil is presented to be 3.5 times higher for aviation than for other forms of oil combustion.
As almost all types of aircraft are widely used all over the world, the default fuel use factors and emission factors
for individual aircraft should in principle be applicable to all countries. The national average may vary from
country to country depending on the average aircraft fleet and average distance flown.
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