Retreating blade stall is a hazardous condition sometimes experienced by helicopters and can greatly increase the chances of a crash. The main cause of this particular type of stall has to do with the rotor speed versus the overall speed of the helicopter itself. A differential in these speeds may cause a stall at various altitudes, speeds or when certain maneuvers are performed that exceed the capabilities of the craft. There are ways to avoid this from occurring, however, and correcting it when it does, although a thorough knowledge of the causes and symptoms of retreating blade stall must be understood.
Simply put, retreating blade stall occurs because the speed of the helicopter in a forward direction has exceeded the speed at which the rotors are turning, or when the angle of attack is too great. It is called retreating blade stall because the speed that is measured is the speed at which the rotors are retreating from the forward direction of the helicopter. This is also a factor that affects the overall speed of the helicopter and limits it to around 200 knots. Causes of this condition usually occur at high speeds and include turbulent air, steep or sharp turns, high weight, high altitude and low rotor rpm.
The first signs that a helicopter is experiencing retreating blade stall are an abnormal vibration, pitching up of the nose of the craft and a tendency for the helicopter to want to roll in the direction of the stalled side of the rotors. This rolling tendency is caused by the retreating side of the rotors lacking sufficient lift to keep the craft in the air at high speed, making the helicopter incredibly hard to control and greatly reducing maneuvering capability.
It is possible to prevent retreating blade stall, however, and to stop it from progressing into an unmanageable condition once it has begun. A pilot must be familiar with the controls and operation of the helicopter, though, and be aware of what is happening and why. To prevent this condition from leading to a crash the pilot may try reducing the pitch and power, reduce forward airspeed, during turns reduce the G forces that the craft experiences, check the pedal trim or increase the rpm to the allowable limit. During a high degree of retreating blade stall the helicopter may roll violently and pitch up in such a way as to take the pilot by surprise. In such circumstances the pilot must react quickly and follow the necessary steps in order to delay the stall until they regain control.