A retreating blade stall is when the retreating blade flaps down so significantly that the blade reaches its critical angle of attack and stalls.
The retreating blade does not produce as much lift as the advancing blade. To equalize the lift across the rotor disc, the retreating blade must increase its angle of attack (AOA) through flapping. However, there is a limit to how much the AOA can increase before the critical angle is reached. Once reached, the blade will stall. Retreating blade stall is a significant factor in determining a helicopter’s VNE speed. Although the blade is at its highest AOA when directly to the left (270 degrees), because of gyroscopic precession, the stall is felt as a tail down/nose up movement. In addition, the pilot will likely feel low frequency vibrations.
To recover from a retreating blade stall, the pilot should lower collective to reduce the AOA. After lowering collective, aft cyclic can be used to slow the helicopter’s forward speed. Using aft cyclic without first lowering collective will increase the problem as aft cyclic produces a flare effect and increases the AOA. The flare reduces the induced flow as some of the air is received from under the disc. When induced flow is reduced, the AOA is increased as the resultant relative wind is influenced more by the forward movement and rotation of the blades, if other factors remain the same. Forward cyclic as an initial response also increases the problem. With forward cyclic, changes in blade angles are most significant on the retreating blade. The retreating blade angle is increased due to forward cyclic, further aggravating the problem. Because of the stalled condition, the blade flapping does not occur to equalize lift.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-11
Principles of Helicopter Flight, 2nd Edition, pg. 137
FM 3-04.203-2007 Fundamentals of Flight pg. 1-66