Ground effect occurs when an aircraft is within approximately how many wingspans of the ground?

• A. Two wingspans
• B. Three wingspans
• C. Five wingspans
• D. One wingspan

Answer: (D)

Ground effect is the increase in wing efficiency that occurs when the wing is very close to the surface. When you’re within about one wingspan of the ground, the air can’t flow downward as freely because the ground interferes with the wing’s tip vortices and downwash. That interference creates a cushion of air under the wing, which increases lift for a given angle of attack and reduces induced drag. In practical terms, the airplane feels lighter and can fly slower without losing lift, which is why takeoff and landing can seem more “floaty” near the ground. As you rise higher, the ground’s influence fades, and performance returns to normal. This is why the distance near the ground is described as about one wingspan.