Ground Effect

     The term ground effect might be new to you, but many pilots have encountered the term a few times. Ground effect is the positive influence on the lifting characteristics of the horizontal surfaces of an aircraft wing when it is close to the ground. What causes this you ask? It comes from a reduction in the amount of induced drag generated. The reduction in induced drag when in close proximity to the ground causes the pressure air that is supposed to be circular around the wing becomes elliptical as the airflow is pushed outwards. This causes the effective aspect ratio of the wing to become greater than the geometric aspect ratio which reduces the induced drag. 

    So you're saying to yourself why not just keep the aircraft away from the ground, hence we don't have to worry about the ground effect. Well we have to take off and land the flight, that is when ground effect can become a problem. On December 16th, 2004, a cargo carrier SD3-60 attempted to land at Oshawa at night on a runway that was covered in snow and was shorter then the required distance to safely land. The aircraft had a poor deceleration coming in and the aircraft became airborne in ground effect. It failed to achieve sufficient airspeed to sustain a climb and an aerodynamic stall took place. The carrier impacted the runway and the two pilots sustained serious injuries. 

http://www.skybrary.aero/bookshelf/books/3359.pdf. Helicopter Flying Handbook, FAA, 2012.

http://www.skybrary.aero/bookshelf/books/1146.pdf. Floating following the landing flare linked to ground effect

http://www.skybrary.aero/bookshelf/books/1960.pdf. Potential for ‘bounce’ of wing tip vortices in ground effect