What indicates the initiation of fixed-wing flight?

The initiation of fixed-wing flight is marked by the application of takeoff power. This moment signifies that the aircraft is transitioning from a stationary position on the ground to the active flight phase. During the takeoff roll, applying power to the engines generates thrust, allowing the aircraft to accelerate down the runway.

Once the required speed (takeoff speed, or V1) is reached, the aircraft can lift off the ground and enter flight. This process is critical because it sets in motion the flight dynamics of the aircraft, where lift generated by the wings will overcome the weight of the aircraft, allowing it to ascend into the air.

Taxiing, being on the ground, and retracting landing gear do not indicate the beginning of flight, as the aircraft remains stationary or still operates on the ground during those actions. Thus, the application of takeoff power is a clear and definitive sign of the start of flight.