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The UNFCCC Secretariat has concluded that the method to estimate bunker (ships and aircraft) emissions is
incomplete and not consistent between Parties, and that most parties have not specified the methodology used.
Reported data also indicate that, for shipping and aircraft bunkers, CO2 accounts for 98 percent of the CO2
equivalent emissions.
The effect of emissions from aircraft at high altitudes (especially nitrogen oxides (NOx) and water vapour) is of
particular concern. In the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC
Guidelines), emissions from aircraft are to be reported in an equal manner to emissions from other sources, not
specifying the altitude of the emissions and applying the same Global Warming Potential (GWP) values.
Nitrogen oxides and water vapour are not included in the Kyoto Protocol.
1 . 1 Nature, magnitude, and distribution of
sources
Emissions from aircraft originate from fuel burned in aircraft engines. Greenhouse gas emissions are the
combustion products and by-products. CO2 and NOx are most important, but also methane, nitrous oxide and
other by-product gases are emitted. The fuel use and emissions will be dependent on the fuel type, aircraft type,
engine type, engine load and flying altitude.
Two types of fuels are used. Gasoline is used in small piston engined aircraft only. Most aircraft run on kerosene,
and the bulk of fuel used for aviation is kerosene.
In general, there exist two types of engines; reciprocating piston engines, and gas turbines (Olivier (1991) and
EEA (2000)). In piston engines, energy is extracted from a combustion chamber by means of a piston and crank
mechanism that drives the propellers to give the aircraft momentum. In gas turbines compressed air is heated by
combustion in a combustion chamber and the major part of this is used for propulsion of the aircraft. A part of
the energy contained in the hot air flow is used to drive the turbine that in turn drives the compressor. Turbojet
engines use only energy from the expanding exhaust stream for propulsion, whereas turbofan and turboprop
engines use energy from the turbine to drive a fan or propeller for propulsion.
The air traffic is often divided into four categories (EEA 2000):
• Civil IFR (Instrumental Flight Rules) flights;
• Civil VFR (Visual Flight Rules) flights, also called general aviation;
• Civil Helicopters, and
• Operational Military flights.
Most emissions originate from the first category, which covers the scheduled flights of “ordinary” aircraft.
Operations of aircraft are usually divided into two main parts (Figure 1) (EEA 2000):
• The Landing/Take-off (LTO) cycle which includes all activities near the airport that take place below the
altitude of 3000 feet (1000 m). This therefore includes taxi-in and out, take-off, climb-out, and approachlanding.
The LTO is defined in ICAO (1993), and
• Cruise which here is defined as all activities that take place at altitudes above 3000 feet (1000 m). No upper
limit of altitude is given. Cruise, in the inventory methodology, includes climb to cruise altitude, cruise, and
descent from cruise altitudes.
Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories
Aircraft Emissions 95
F i g u r e 1 S t a n d a r d f l y i n g c y c l e
3000 feet
(ca. 1000 m)
Landing
Taxi / idle
Take-off
Taxi / idle
Descent
Climb
LTO-cycle
Cruise
1 . 2 The current state of inventory methodologies
The current methodology proposes two Tiers. The most simple methodology (Tier 1) is based on knowledge of
fuel use only, while the Tier 2 is also based on information of the number of LTOs. In both Tiers emissions from
domestic and international air traffic are to be estimated separately. The fuel used for international air traffic is
defined as all fuel sold for aviation in the reporting country not used for domestic aviation.
1.2.1 Tier 1
The simplest methodology is based on an aggregate figure of fuel consumption for aviation to be multiplied with
average emission factors. The emission factors have been averaged over all flying phases based on an assumption
that 10 percent of the fuel is used in the LTO phase of the flight.
The following are the default emission factors:
CO2 : 19.5 tonne C/PJ;
CH4 : 0.5 kg/PJ, and
N2O : 2 kg/PJ
Aggregate emission factors are also given for NOx, carbon monoxide (CO), sulphur dioxide (SO2) and nonmethane
volatile organic compounds (NMVOCs).
1.2.2 Tier 2
In the Tier 2 methodology, a distinction is made between emissions below and above 1000 m (3000 feet). The
emissions in these two flying phases are estimated separately.
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