engines, power settings, amount of fuel burned, and other variables (e.g., temperature, altitude,  etc.).  Computational methods and modeling t echniques for assessing aircraft engine emissions are  discussed in  Section 6  ( Preparing an Emissions Inventory ) and  Section 7  ( Conducting  Dispersion Modeling ). For detailed instructions and guidance on how to compute aircraft engine  emissions refer to  Appendix A1  ( Aircraft Emission Inventory for Criteria Pollutant s ) of this  Handbook.  Auxiliary Power Units (A PUs) are smaller turbine engines that provide power to an aircraft  during approach, while taxiing, or positioned at the terminal gate. APUs power the aircraft 's  instruments, lights, heat and air conditioning, and other equipment  and are used for starting the  main aircraft’s engines. APUs burn jet fuel and is common on both  large commercial and  military aircraft and some air taxi and GA aircraft.  APUs typically remain in use while the aircraft is parked and serviced at the gate, unless an  alternative source of electricity and preconditioned air is made available. In such cases, APUs are  reactivated at least  five to ten minutes before the aircraft leaves the gate or parking space.  Normally, APUs are turned off after the main engines have been started, prior to takeoff.  Computational methods and modeling techniques for assessing APU emissions are discussed in  Section 6  ( Preparing an Emissions Inventory ) and  Section 7  ( Conducting Dispersion Modeling ).  For detailed instruction and guidance on how to compute APU emissions refer to  Appendix A2  (Auxiliary Power Units Emission Inventory for Criteria Pollutants ).  Ground Support Equipment (GSE) is a term used to describe the equipment that service aircraft  while loading and unloading passengers and freight at an airport. The types of GSE and their use  depend on the aircraft type and the designated category of an aircraft operation (e.g., passenger,  cargo, etc.). GSE consist of aircraft tugs, air start units,  forklifts, tractors, air-conditioning units,  ground power units (GPUs), baggage tugs, belt loaders, fuel or hydrant trucks, catering trucks,  cabin trucks, deicer trucks, water trucks, lavatory trucks, and cargo loaders, among others.  Air emissions resulting from the  operation of GSE vary depending on the number and type of  equipment used to service each aircraft, along with the duration of equipment operation aircraft landing-takeoff (LTO) cycle and the fuel type (e.g., gasoline, diesel, propane, electric,  etc.).  Computational methods and modeling techniques for assessing GSE emissions are discussed in  Section 6  ( Preparing an Emissions Inventory ) and  Section 7  ( Conducting Dispersion Modeling ).  For detailed instruction and guidance on how to compute GSE emissions refer to  Appendix A3  ( Ground Support Equipment Emission Inventory for Criteria Pollutants ).  Ground access vehicles encompass motor vehicles traveling  upon on- and off-airport roadways,  within airport parking facilities, and idling along terminal curbsides.  These include airport  passenger vehicles ( e.g., private autos, taxis/limos, shuttles, vans, buses, rental cars, etc.);  3.1.4.  Ground Access Vehicles  3.1.3.  Ground Support Equipment  3.1.2.  Auxiliary Power Units  Aviation Emissions and Air Quality Handbook  11$_{ }$