includes the landing roll, which is the movement of an aircraft from touchdown through  deceleration to taxi speed or full stop.  APPENDIX A1$_{ }$ • Approach  begins when an aircraft descends below the atmospheric mixing height and  ends when an aircraft touches down on a runway.  • Takeoff  begins when full power is applied to an aircraft and ends when an aircraft reaches  approximately 500 to 1,000 feet.  • Climb out  begins when an aircraft powers back from the takeoff mode and ascends above  the atmospheric mixing height.  Engine startup operations are also often included in the LTO cycle to account for VOC emissions  that occur during this mode occurring within the terminal gate area prior to the aircraft departure.  Similarly, a Touch-and-Go (TGO) cycle consists of the approach mode, followed immediately  by the takeoff and climb out modes. TGO operations are generally performed for purposes, usually occur at military bases or smaller civilian airports, and generally have a flight  profile that starts and ends at a much lower altitude than a regular LTO cycle.  Importantly, aircraft emissions vary by operational mode because at power settings other than  cruise, engine performance and emission levels are less than optimal. For example, when aircraft  are idling, CO and VOC emissions are highest. Alternatively, during take -off and climbout, CO  and VOC emissions decrease and NO$_{x }$emissions increase significantly.  This methodology for calculating aircraft engine emissions provides insight into the framework and application used by EDMS/AEDT and is adapted from EPA’s  Procedures for  Emission Inventory Preparation, Volume IV . 1  Following this approach, engine emissions are computed for one complete LTO cycle of each  aircraft type. Within the LTO cycle, emissions are mainly a function of the number of operations  (i.e., arrivals and departures); the aircraft fleet characteristics (i.e., aircraft type, number engines, and take-off weight); the engine operating features (i.e., fuel flow rates emission factors); and the time the aircraft spends in each of the operating modes (i.e., takeoff,  climb out, approach, and taxi/idle), also known as the Time-In-Mode (TIM).  For this approach,  Equation A1-1  ( Emission Calculation for Each Aircraft Type for One LTO ) is  used to compute NO$_{x}$, CO, and VOC emissions for each aircraft/engine combination and is  repeated for each aircraft/engine type and LTO TIM. If an aircraft has multiple engines, then the  equation is multiplied by the number of engines used on that specific aircraft type.  $^{ }$  EPA,  Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources , December 1992 [EPA-420- R-92-009], http://www.epa.gov/otaq/models/nonrdmdl/r92009.pdf.  1  A1.2  EPA Methodology  2$_{ }$