At what height above the surface is an airplane usually affected by ground effect?

Ground effect occurs when an aircraft is flying close to the surface, typically within a height that is influenced by its wingspan. When an airplane is less than half of its wingspan above the surface, the ground effect becomes significant. This phenomenon enhances lift and reduces drag as the airflow under the wing is interrupted by the ground, leading to a cushion of air that supports the aircraft.

At this proximity to the ground, the pressure differential created under the wings results in an increase in lift, which can aid in takeoff and landing performance. This effect diminishes as the aircraft ascends above this height, making it less relevant once the aircraft is operating at altitudes greater than half the wingspan. Therefore, the correct answer accurately reflects the typical height at which ground effect has a pronounced influence on the aircraft's performance.