What causes ground effect?

• A. Interference of the Earth's surface with airflow around the airplane
• B. Increase in air density
• C. Turbulence from the surface
• D. Decreased wind speed

Answer: (A)

When an airplane flies close to the ground, the Earth's surface acts as a boundary that changes how the air flows around the wing. The nearby surface interferes with the wingtip vortices and the downwash that normally trails behind the wing, so the downwash is reduced. With less downward flow, the wing produces more lift for the same angle of attack, and induced drag drops. This is why lift appears more efficient and the aircraft can feel “floatier” near the ground, especially within about one wingspan. The other ideas don’t describe this boundary‑layer interference caused by the surface: it’s not about a general rise in air density, surface turbulence, or a simple decrease in wind speed.