cyclic control input (as determined by the rotation of the rotor) up to a degree. However, the rapid rate of change of blade flex and angle of attack causes uncontrolled longitudinal twist and severe vibration in later stages, resulting in total loss of cyclic control if left unchecked. Assuming no rotor damage, recovery from the condition is possible by using the procedure described below under Flight performance during a retreating blade stall. Failure These compensations can only do so much. Increasing angle of attack to compensate for reduced blade airspeed has the effect of maintaining lift only until the point where critical angle of attack is reached, after this point lift sharply decreases. All airfoils have a critical angle of attack (also called a stall angle of attack) which is the angle of attack that produces most lift. Above this angle flow over the airfoil becomes detached and lift decreases, this is commonly called a stall. When a fixed-wing aircraft exceeds its critical angle of attack the entire aircraft loses lift and enters a condition called a stall. The usual results of a fixed-wing stall are a sharp drop in aircraft altitude and a dive. Stalls in fixed-wing aircraft are virtually always a recoverable event. In a retreating-blade stall, however, only the retreating half of the helicopter's rotor disc experiences a stall. The advancing blade continues to generate lift, but the retreating blade enters a stall condition, usually resulting in an uncommanded increase in pitch of the nose and a roll in the direction of the retreating side of the rotor disc. In counter-clockwise rotating rotor systems (as in most American-made types) this is the left side. In clockwise rotating systems it is a roll to the right. Flight performance during a retreating blade stall As the aircraft approaches retreating blade stall conditions, it will shudder and the nose will begin to pitch up. The resultant upward pitching of the nose will naturally begin to correct the situation as it results in slowing the aircraft. If forced to continue the acceleration via flight controls (forward cyclic + collective), it may roll to the side of the retreating blade. Recovery involves lowering the collective pitch, relieving forward pressure on the cyclic or more commonly, both. Either of these control movements should restore the proper attached airflow over the retreating blade thus generating lift again. This is normally an automatically corrected condition