When an aircraft slows to below its stall speed, it is unable to generate enough lift in order to cancel out the forces that act on the aircraft (such as weight and centripetal force). This will cause the aircraft to drop in altitude. The drop in altitude may cause the pilot to increase the angle of attack by pulling back on the stick, because normally increasing the angle of attack puts the aircraft in a climb. However, when the wing exceeds its critical angle of attack, an increase in angle of attack will lead to a loss of lift and a further loss of airspeed – the wing stalls. The reason why the wing stalls when it exceeds its critical angle of attack is that the airflow over the top of the wing separates.