prior to main engines start up on departure and the run time after  the aircraft engines are shut  down on arrival and, if available, connected to ground power at an airport. Notably, if ground  power and pre-conditioned air is not available, an APU may be used the entire time an aircraft is  at an airport’s gate.  The required input data for obtaining an estimate of HAP emission for an APU includes: (i) the  fuel flow rate for the specific APU model, (ii) the APU run time (for a complete landing-takeoff  cycle), (iii) a THC emission index specific to the APU model,  (iv) a factor that converts the  estimated level of  THC to TOG, and  (v) a HAPs speciation profile.  Equation B-5  ( HAP  Emissions – APU ) demonstrates how to derive HAP emissions for an APU.  GSE provide service to aircraft while at an airport terminal and are generally classified as either  “on-road” vehicles or trucks (e.g., an airline employee shuttle bus) and other similar vehicles or  “non-road” vehicles/equipment such as tugs, tractors, loaders, etc. The fleet of GSE utilized at an  airport, their operating times and fuel types (gasoline or diesel) varies by aircraft (commercial, commuter, GA, military), by airline, and by airport.  GSE emissions are calculated either using a total GSE population- based approach (i.e., an  accounting of all the equipment and vehicles on the airport) or using an aircraft LTO approach  (i.e., based on the type and number of GSE servicing  each aircraft type for an LTO) (see  Appendix A.3 ,  Ground Support Equipment Emissions Inventory for Criteria Pollutants ).  For GSE, the input data  used determine the amount of  HAP emission include: (i) the brake  horsepower of the equipment,  (ii) the load factor,  (iii) equipment usage (equipment operating  time), (iv) THC emission indices, (v) a THC to TOG conversion factor specific to the equipment  fuel type, and (vi) a HAPs speciation profile that is also specific to the fuel type.  B.2.3.1 Methodology  B.2.3.2 Emission Factors  B.2.3  Ground Support Equipment  B.2.2.2 Conversion Factors  Equation B-5. HAP Emissions - APU  Where:  A = Fuel flow rate (kg/sec)  B = Time in operation (sec)  D = THC emission index (g/kg fuel consumed)  F = THC to TOG conversion factor (unitless)  H= Speciation profile for HAP of interest (mass fraction)  1/1000 = Converstion factor from grams to kilograms  I= Mass of HAP of interest (kg)  A x B x D x F x H x 1/1000 = I  APPENDIX B$_{ }$ 57$_{ }$