Most general aviation wings stall between 16° and 18°. This is known as the critical angle of attack. As you increase the angle of attack the centre of pressure (bottom of the wing) moves forward. As this happens the boundary layer is unable to follow the contour of the wing and begins to separate from the top of the wing starting at the trailing edge. As the angle of attack continues to increase the centre of pressure continues to move forward and as subsequently the boundary layer continues to separate from the top of the wing moving toward the leading edge. When the maximum lift coefficient is reached the stall occurs and the center of pressure rapidly moves backwards. This causes the nose to pitch down.