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A Takeoff/Go-around switch (TO/GA) is a switch, on the autothrottle of modern large aircraft, with two modes, takeoff (TO) and go around (GA). The mode is dependent on the phase of flight; usually, on approach to land the autopilot will be set to approach mode, therefore if the TO/GA switch is pressed it will activate the Go Around mode of the Autothrottle; conversely, when take-off is set on the autopilot the switch activates Take Off mode of the Autothrottle. On Boeing aircraft TO/GA modes are selected by a separate switch near the throttle levers, but on Airbus aircraft it is activated by pushing the thrust levers fully forward to the TOGA detent.
Once an aircraft has lined up, the pilots increase the engine power. The level reached is 40-60% RPM of Low Pressure Turbine on Boeings, and 50% on Airbuses. This is done to prevent flame-out on takeoff due to compressor stall. The pilots then press the TO/GA switch, by pushing the thrust levers to TOGA on Boeings or to FLEX on Airbuses, and the engines then increase to their computed take off power. Modern aircraft flight management computers will determine the power needed by the engines to take off, based on a number of factors such as runway length, wind speed, temperature, and most importantly the weight of the aircraft. In older aircraft these calculations were performed by the pilots before a takeoff. The advantage of having such a system is the ability to reduce wear and tear on the engines by only using as much power as is actually required to ensure the aircraft reaches a safe take off speed.
 Go around
The go around setting is used when an approach is taking place. If a pilot finds that they are unable to land, activating this switch (pushing thrust levers to TOGA detent) will increase the power to go-around thrust. Most importantly, the TO/GA switch disables the autopilot, so it does not follow the ILS glideslope anymore and it overrides any autothrottle mode which would keep the aircraft in landing configuration. On Airbus planes it doesn't disengage AP, but causes it to stop following the ILS and perform Go Around maneuver automatically. In an emergency situation, using a TO/GA switch is often the quickest way of increasing thrust to abort a landing. On Airbus planes pushing throttles to TOGA detent does all regarding flight path and speed.
Some aircraft make the TO/GA switch to command the autopilot to fly a missed approach pattern, if the autopilot is programmed to do so. The TO/GA switch can be used even after touchdown if reverse thrust has not been selected.