Final answer:
When the N1 MODE 1(2) pushbutton is turned on, the aircraft's autothrottle system maintains the engine's thrust by controlling its fan speed to a set target N1 level.
Step-by-step explanation:
When the N1 MODE 1(2) pushbutton is selected ON, it commands the aircraft's autothrottle system to control the engine's fan speed to a pre-set target N1 percentage. When the N1 MODE 1(2) pushbutton is turned on, the aircraft's autothrottle system maintains the engine's thrust by controlling its fan speed to a set target N1 level. N1 refers to the rotational speed of the engine's fan and low-pressure compressor, measured as a percentage of a predetermined maximum speed.
By activating this mode, the autothrottle system will maintain the selected N1 level for the engine's thrust, which is essential for certain flight phases, such as takeoff, climb, or cruise. This function is prominently used in airplanes that have a digital engine control system, ensuring both fuel efficiency and adherence to noise abatement procedures.