flat spin, is the most hazardous of all spins. In a flat spin, the glider yaws around the vertical axis at a pitch attitude nearly level with the horizon. A flat spin often has a very high rate of rotation; the recovery is difficult, and sometimes impossible. If a glider is properly loaded within its CG limits, entry into a flat spin should not occur. Erect spins and flat spins can also be inverted. The entry, wing form, and CG usually determine the type of spin resulting from an uncoordinated wing stall. 

Since spins normally occur when a glider is flown in an uncoordinated manner at lower airspeeds, coordinated use of the flight controls is important. It is critical that pilots learn to recognize and recover from the first sign of a stall or spin. Entering a spin near the ground, especially during the landing pattern, is usually fatal. [Figure 3-33] A pilot must learn to recognize the warning signs, especially during the approach and landing phase in a crosswind. A crosswind resulting in a tailwind on the base leg may lead the pilot to tighten the turn using rudder, or too steep a turn for the airspeed. An uncoordinated turn could lead to the upper wing exceeding its critical AOA before the lower wing, which could result in a very high rate of roll towards the upper wing as the upper wing stalls. If an excessive steep turn is attempted, the glider may roll towards the inside wing or the outside wing depending on the exact trim state at the instant of the stall. Situational awareness of position to final approach should be part of a before-landing routine. 

## Ground Effect 

Ground effect is a reduction in induced drag for the same amount of lift produced. Within one wingspan above the ground, the decrease in induced drag enables the glider to fly at a lower airspeed. In ground effect, a lower AOA is required to produce the same amount of lift. Ground effect enables the glider to fly near the ground at a lower airspeed and causes the glider to float as it approaches the touchdown point. 

During takeoff and landing, the ground alters the three- dimensional airflow pattern around the glider. The result is a decrease in downwash and a reduction in wingtip vortices. Upwash and downwash refer to the effect an airfoil has on the free airstream. Upwash is the deflection of the oncoming airstream upward and over the wing. Downwash is the downward deflection of the airstream as it passes over the wing and past the trailing edge. 

During flight, the downwash of the airstream causes the relative wind to be inclined downward in the vicinity of the wing. This is called the average relative wind. The angle between the free airstream relative wind and the average relative wind is the induced AOA. In effect, the greater the downward deflection of the airstream, the higher the induced AOA and the higher the induced drag. Ground effect restricts the downward deflection of the airstream, decreasing both induced AOA and induced drag. 

Ground effect, in addition to the decrease in wind due to surface friction and other terrain features upwind of the landing area, can greater increase the landing distance of a glider. A glider pilot, especially a visiting pilot, should inquire about local effects from local pilots to enhance flight planning and safe landings. 

