What indicates the initiation of fixed-wing flight?

The initiation of fixed-wing flight is indicated when takeoff power is applied. This moment represents the transition from a stationary or taxied position to the point where the aircraft is actively engaging its engines to gain the necessary speed for flight. Applying takeoff power enables the aircraft to accelerate along the runway, which is vital for achieving lift.

Before this point, when the aircraft is safely on the ground, it is still stationary, and there is no movement significant enough to initiate flight. While taxiing is a part of the pre-flight operations, it does not imply that flight has started; the aircraft is still on the ground and maneuvering without significant speed. Similarly, once the landing gear is retracted, the aircraft is already airborne, thus marking a phase of flight rather than its initiation. Therefore, the critical action that marks the start of flight is the application of takeoff power, setting the stage for takeoff and subsequent ascent into the air.