During which flight condition would the wing's spar predominantly experience bending loads?

The wing's spar predominantly experiences bending loads during flight, primarily when the aircraft is in level flight or during maneuvers. In this condition, the aerodynamic forces acting on the wings create significant lift, which naturally leads to bending moments being transferred to the wing structure, especially the spar.

During landing and takeoff, while there are still loads on the wing, the nature of these loads varies. For instance, during landing, there are additional vertical forces acting due to descent and touchdown, but the primary focus on bending loads occurs during sustained flight. In taxiing, the loads on the wings diminish significantly since the aircraft is operating at low speeds, primarily supported by the ground rather than aerodynamic lift. Thus, the most pronounced bending loads on the wing spar occur during the aircraft's in-flight condition when lift is actively generated.