Final answer:
The time taken for N1 to revert to ground idle after main gear touchdown varies but typically occurs within 3 to 10 seconds, depending on aircraft and engine control systems.
Step-by-step explanation:
The question pertains to the engine behavior of aircraft after landing, specifically the time taken for the N1 to revert back to ground idle after the main gear touchdown. N1 is a term used to refer to the low-pressure compressor speed in a jet engine, and ground idle is the lowest engine speed at which the airplane can operate safely on the ground. Once the aircraft has touched down and the weight-on-wheels system has been activated, the engine control systems typically start to reduce the thrust to idle levels. However, the exact timing can vary depending on the aircraft type and its engine control system. On average, the N1 rotation speed will reach ground idle within several seconds (typically from 3 to 10 seconds) after touchdown.