Definition
A temporary increase in lift and reduction in induced drag that occurs when an airplane is flying within approximately one wingspan of the ground. The proximity of the surface alters the airflow around the wing, reducing the downward and outward flow of air behind it and weakening the wingtip vortices, which lowers induced drag and allows the wing to produce the same lift at a lower angle of attack.
Plain English
When an airplane gets very close to the ground -- within about one wingspan -- the ground changes the airflow around the wing in a way that makes the wing more efficient. The airplane feels lighter and slipperier through the air for a short time, just during takeoff and landing.
Context Anchor
Encountered during takeoff just after liftoff and during landing as the airplane gets close to the runway.
Derivation
The name is literal: it is the effect produced by the ground itself on the airflow around the wing. The phrase is worth pausing on because the ground does not lift the airplane mechanically -- it changes how the air behaves under and behind the wing.
Why Pilots Care
Ground effect can cause an airplane to become airborne before it has reached safe climb speed or to float during landing, leading to runway overrun or loss of directional control if the pilot does not anticipate the sudden loss of this benefit once the airplane climbs out of ground effect.
Grounding Statement
A few feet above the runway, the airplane may feel like it wants to keep flying or floating, but that benefit fades quickly as it climbs higher.
Intuition Check
Ground effect is not a magic cushion of air holding the airplane up. It is a real airflow change near the ground that reduces drag, and it does not mean the airplane is ready to climb safely.
Example Sentence 1
The airplane lifted off in ground effect at a low airspeed, but the pilot held it level just above the runway to accelerate before beginning the climb.
Example Sentence 2
During the landing flare the airplane floated farther than expected because it remained in ground effect longer than anticipated in the calm air.