Ground effect is like a cushion of air that is felt in an airplane just before touch down, it feels as if the airplane does not what to go any lower. This is due to the air that is trapped between the wing and the landing surface (ground or water).
When an aircraft in flight comes within several feet of the surface, usually about a wingspan to feel the full effect of ground effect. A change occurs in the three-dimensional flow pattern around the aircraft because the vertical component of the airflow around the wing is restricted by the surface. This alters the wings up wash, downwash, and wingtip vortices.
As the aircraft gets closer to the ground, the downwash and wing tip vortices become reduced. Which means the induced drag is also reduced.
Pilots should be aware, and caution should be taken, the airplane may become airborne initially at a low airspeed then settle back down to the runway.
Pilots should not attempt to force an aircraft to become airborne with a deficiency of speed. The POH provides the recommended takeoff speed necessary to provide adequate initial climb performance. In fact, it is best to lower the nose a little to allow the airspeed to climb before slowly climbing out of ground effect.
If flying an aircraft with retractable landing gear, never retract the landing gear or flaps prior to establishing a positive rate of climb and only after achieving a safe altitude. If the landing gear and flaps are retracted while still in ground effect, the airplane will sick and hit the runway.
Remember:
Usually about a wingspan to fell the full effect of ground effect.
If flying an aircraft with retractable landing gear, never retract the landing gear or flaps prior to establishing a positive rate of climb and only after achieving a safe altitude.
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