APPENDIX A2 - Auxiliary Power Units Emissions  Inventory for Criteria Pollutants A2.1  Overview  A2.2  Methodology  10$_{ }$ 14  EPA,  Procedures for Emission Inventory Preparation, Volume IV: Mobile Source s, December 1992 [EPA-420- R-92-009], http://www.epa.gov/otaq/models/nonrdmdl/r92009.pdf.  APU emissions are estimated for one complete landing-takeoff (LTO) cycle (i.e., during arrival  and departure) of each aircraft type and are based on emission factors, fuel flow for the aircraft’s  specific APU model and the amount of APU usage during the course of the LTO  cycle.  Equation A2-1  ( APU Emissions from a Single LTO ) can be used to estimate emissi ons from a  single APU. If a particular aircraft has multiple APUs, then the equation must be multiplied by  the number of APUs on the aircraft. Using  Equation A2-1  ( APU Emissions from a Single LTO ),  APU emissions per LTO can be calculated for multiple aircraft types and any period of time (i.e.,  day, month, and year).  APU engines burn jet fuel and create exhaust emissions similar to aircraft engines. emissions include hydrocarbons (HC), carbon monoxide (CO), nitrogen oxide (NO$_{x}$), and sulfur  dioxide (SO$_{2}$). The methodology for calculating emissions  from APUs is similar to that of an  aircraft engine operating in one power setting only. The methodology  is adapted from EPA’s  Procedures for Emission Inventory Preparation, Volume IV 14  .  APUs operating practices largely are determined by individual airlines and vary considerably  among aircraft types and airlines. When an aircraft arrives at a terminal gate, the pilot has the  option of shutting off power to the main jet engine (s) and operating the on-board APU. Some  airlines start the APU when the aircraft is on approach and keep it on during the entire taxi -in  phase as a precaution to insure its availability if needed for engine restart. T ypically APUs are  turned on while taxiing to the gate  or parking apron and remain in use while the aircraft is  parked. Alternately, an aircraft can receive 400 Hertz (Hz) gate power and pre-conditioned air  (PCA) from ground power unit (GPU) and air conditioning (A/C) equipment from connections at  the gate. On departure, the APU is used to start the  main engines. Once the main engine(s) are  started APU can also  provide the electric power and ventilation to the aircraft. Again, some  airlines prefer to keep the APU running during taxi-out as a back-up.  This appendix details the methodology for computing emissions from auxiliary power (APUs). APUs consist of small turbine engines used by many commercial jet aircraft to start the  main engine(s); provide electrical power to aircraft radios, lights, and other equipment; and to  power the onboard air conditioning (heating and cooling) system s. There are different models  and series of APUs to meet the needs of various aircraft and generally APUs are not common on  smaller aircraft.  APPENDIX A2$_{ }$