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when possible. Approach and climbout times in mode vary based on the local mixing height.
D-7
The methodology for determining approach and climbout is described in EPA’s Procedures for
Emission Inventory Preparation, Volume IV, Chapter 5 Taxi time is highly site- and situationspecific.
Potential sources of site-specific taxi/idle times include aircraft operators and the FAA.
The FAA compiles monthly taxi data received from aircraft operators for many airports. These
aircraft taxi time data are available from FAA’s Office of Aviation Policy, Plans and
Management Analysis (Reference 66). On-site monitoring of taxi times over an extended period
also is a feasible means of estimating this data. The time spend in takeoff mode is fairly standard
and will not vary much from location to location. Therefore, a standard default time usually is
used in emission calculations. Default takeoff times are provided for several aircraft categories in
EPA’s Procedures for Emission Inventory Preparation, Volume IV, Chapter 5. If reverse thrust
is used on aircraft landing, it typically is applied for approximately 15 seconds. Unless sitespecific
data is available, 15 seconds should be used as a default reverse thrust time in mode for
LTOs that include reverse thrust on landing.
D3. GENERAL AVIATION AND AIR TAXI AIRCRAFT
D3.1 Methodology
The following discusses the procedures for calculating standard and non-standard LTO exhaust
emission from general aviation and air taxi aircraft, as well as general aviation aircraft
evaporative emissions.
There are two approaches for calculating exhaust emissions from standard operations of general
aviation and air taxi aircraft. The approach used above in Section D2.1 to calculate emissions
from standard operations of commercial air carrier aircraft (i.e., presented in EPA’s Procedures
for Emission Inventory Preparation, Volume IV, Chapter 5) also can be used for standard
operations of general aviation and air taxi aircraft. Due to the nature and tracking of general
aviation and air taxi operations and operators, often it is difficult to find the detailed information
on aircraft mix and activity needed to estimate emission using this detailed approach. Where
detailed information on specific aircraft mix and activity is unavailable, the EPA alternative
methodology can be used. The alternative methodology uses “generalized” emission indices, in
pounds per LTO, to estimate emissions. Separate generalized emission indices are provided in
EPA’s Procedures for Emission Inventory Preparation, Volume IV, Chapter 5 for general
aviation aircraft and air taxi aircraft. The generalized emission indices are based on default
values for key data inputs and a representative general aviation or air taxi fleet mix.
Not all aircraft operations follow the standard LTO cycle. The more detailed EPA procedure can
be adjusted for the non-standard conditions and used to calculate emissions from non-standard
operations of general aviation and air taxi aircraft. An example of non-standard operations are
practice touch-and-goes. In a touch-and-go, the taxi/idle mode is eliminated, the approach and
climbout modes shortened, and a “return flight” mode is added (to allow the pilot to turn around
and repeat the procedure). To calculate the emissions from a touch-and-go, the taxi/idle mode
should be eliminated from the calculations. Rather than reducing the approach and climbout
times and then adding additional time for circling the airfield, the full approach and climbout
times should be used (assuming this will account for the additional flight time within the mixing
zone).
Most general aviation aircraft are powered by piston engines, which are fueled by aviation
gasoline (AvGas). Aviation gasoline has a much higher volatility than jet fuel and the fuel tanks
are vented to the atmosphere resulting in significant HC evaporation. Evaporative emissions are
associated with refueling, pre-flight safety procedures, and fuel venting due to diurnal
D-8
temperature changes. The EPA methodology for calculating refueling losses is provided in an
EPA Office of Air and Radiation memorandum from Mary Manners to Susan Willis dated
October 20, 1996; Subject: Revised Methodology for Calculating the Refueling Losses for
General Aviation Aircraft. The following equations, Error! Reference source not found. and
Error! Reference source not found. (Mariano this should be Equation D-2) , developed for
the EPA, should be used for quantifying HC evaporative emissions from general aviation aircraft
pre-flight safety procedures and fuel venting. [Note: These calculations are not to be used for
turbine engines.]
ET = 0.20 lb/LTOL x LTO
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