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22 fuel associated with those flights could be similarly overstated at Approach--portion of flight from the time the aircraft the airports where the flights originated (Airports Y and Z). This reaches the mixing height or 3,000 ft altitude and exits might be illustrated when comparing the fuel or emissions re- the runway; sults of Method 3 with those of Methods 1 or 2. The leakage of Taxi/idle-in, taxi/idle-out (often combined into taxi/idle/ emissions (i.e., the consequence of policies that result in a local delay)--time aircraft is moving on the taxiway system until decrease due to the policy but increase emissions elsewhere) reaching the gate, and on departure from the gate until tax- caused by the fuel tax policy could only be captured through a ied on to the runway; comparison of the fuel required by actual flight (as estimated by Takeoff--the roll down the runway through lift-off up to Method 3) as compared to the fuel dispensed at an airport. about 1,000 ft; and Climbout--the departure segment from takeoff until exit- 3.1.2 Aircraft Method 2 ing the mixing height or 3,000 ft. This method involves using the same fuel sales data as in The LTO cycle has been used extensively in modeling cri- Method 1, but improving the resolution of the data by calcu- teria pollutants (e.g., CO, NOx, etc.). Therefore, the same lating LTO emissions separately, as shown in Figure 3-2. Fig- methods for each of the pollutants apply in this method. ure 3-2 shows the cruise derivation being performed with Unless other methods are available, the FAA's AEDT/EDMS emissions, but it can also be conducted with fuel consump- (FAAa 2007) can be used to estimate fuel consumption dur- tion before computing emissions. ing the LTO cycle, as well as to estimate the criteria pollutants In air quality evaluations, flight operations in the local envi- emitted in this operating phase. The LTO-based fuel con- ronment are referred to as the LTO cycle. One aircraft LTO sumption information can be used with the appropriate emis- cycle is equivalent to two aircraft operations (one landing and sion factors noted in Method 1 to calculate emissions for CO2, one takeoff). The standard LTO cycle begins when the air- CH4, and N2O. AEDT/EDMS (on the layer titled "Aircraft by craft crosses into the mixing zone (about 3,000 ft altitude) as Mode") reports the fuel burn by each aircraft type in units it approaches the airport on its descent from cruising alti- that can be selected by the user, ranging from grams to short tude, lands, and taxis to the gate. The cycle continues as the tons. Thus, the fuel sales data and the LTO aircraft fuel burn aircraft taxis back out to the runway for takeoff and climbs from AEDT/EDMS must be converted to common units-- out to cross the mixing zone. The operating modes in a stan- either gallons of fuel or pounds are preferred, as noted in the dard LTO cycle are as follows: Method 1 discussion. Using the LTO-based fuel consumption produced by AEDT/ Aircraft Method 1 EDMS (or derived LTO emissions) and the total fuel sales data (or derived total emissions), cruise fuel consumption (or Emission Factors Airport Fuel and Other Data as AEDT/ the derived cruise) emissions can be determined as follows Sales Data EDMS through difference: Necessary Cruise emissions = (total emissions) (LTO emissions) or Aircraft LTO Fuel Cruise fuel consumption = (total fuel sales) (LTO fuel Calculate Consumption consumption) For example, if the fuel sales data noted that 20,000 gallons of Jet A fuel were sold at the airport, and the AEDT/EDMS Total Aircraft GHG Calculate run for the airport indicated aircraft consumed 1,670 gal- Emissions lons (reported as 5,181 kg) in the LTO mode, this calcula- tion would indicate 18,330 gal were consumed during cruise (20,000 - 1,670 = 18,330). As a result, 175.4 metric tons of Subtract Aircraft LTO GHG CO2 (18,330 gal 21.095 lbs CO2/gal fuel 0.0004536 metric Emissions ton/lb) emissions would be attributed to cruise. Aircraft Cruise GHG Emissions Disaggregating the emissions into LTO and cruise allows for better tracking of emissions over time. Figure 3-2. Overview of Aircraft Method 2.