airflow through the rotor disk, and the resulting AOA is so high that even full down collective will not restore normal airflow. In the helicopter when the rotor stalls, it does not do so symmetrically because any forward airspeed will produce a higher airflow on the advancing side than on the retreating side. This causes the retreating blade to stall first, and its weight makes it descend as it moves aft while the advancing blade is climbing as it goes forward. The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disc sometimes referred to as rotor “blow back” or “flap back.” As the helicopter begins to descend, the upward flow of air acting on the bottom surfaces of the tail boom and any horizontal stabilizers tend to pitch the aircraft nose down. These two effects, combined with any aft cyclic by the pilot attempting to keep the aircraft level, allow the rotor blades to blow back and contact the tail boom, in some cases actually severing the tail boom. Since the tail rotor is geared to the main rotor, in many helicopters the loss of main rotor rpm also causes a significant loss of tail rotor thrust and a corresponding loss of directional control. 

Rotor stalls in helicopters are not recoverable. At low altitude, rotor stall will result in an accident with significant damage to the helicopter, and at altitudes above approximately 50 feet the accident will likely be fatal. Consequently, early recognition of the low rotor rpm condition and proper recovery technique is imperative. 

Low rotor rpm can occur during power-off and power-on operations. During power-off flight, a low rpm situation can be caused by the failure to quickly lower the collective after an engine failure or by raising the collective at too great a height above ground at the bottom of an autorotation. However, more common are power-on rotor stall accidents. These occur when the engine is operating normally but the pilot demands more power than is available by pulling up too much on the collective. Known as “overpitching,” this can easily occur at higher density altitudes where the engine is already producing its maximum horsepower and the pilot raises the collective. The corresponding increased AOA of the blades requires more engine horsepower to maintain the speed of the blades; however, the engine cannot produce any additional horsepower, so the speed of the blades decreases. A similar situation can occur with a heavily loaded helicopter taking off from a confined area. Other causes of a power-on low rotor rpm condition include the pilot rolling the throttle the wrong way in helicopters not equipped with a governor or a governor failure in helicopters so equipped. 

As the rpm decreases, the amount of horsepower the engine can produce also decreases. Engine horsepower is directly proportional to its rpm, so a 10 percent loss in rpm due to overpitching, or one of the other scenarios above, will 

result in a 10 percent loss in the engine’s ability to produce horsepower, making recovery even slower and more difficult than it would otherwise be. With less power from the engine and less lift from the decaying rotor rpm, the helicopter will start to settle. If the pilot raises the collective to stop the settling, the situation will feed upon itself rapidly leading to rotor stall. 

There are a number of ways the pilot can recognize the low rotor rpm situation. Visually, the pilot can not only see the rotor rpm indicator decrease but also the change in torque will produce a yaw; there will also be a noticeable decrease in engine noise, and at higher airspeeds or in turns, an increase in vibration. Many helicopters have a low rpm warning system that alerts the pilot to the low rotor rpm condition. 

To recover from the low rotor rpm condition the pilot must simultaneously lower the collective, increase throttle if available and apply aft cyclic to maintain a level attitude. At higher airspeeds, additional aft cyclic may be used to help recover lost rpm. Recovery should be accomplished immediately before investigating the problem and must be practiced to become a conditioned reflex. 

## System Malfunctions 

By following the manufacturer’s recommendations regarding operating limits and procedures and periodic maintenance and inspections, many system and equipment failures can be eliminated. Certain malfunctions or failures can be traced to some error on the part of the pilot; therefore, appropriate flying techniques and use of threat and error management may help to prevent an emergency 

## Antitorque System Failure 

Antitorque failure usually falls into one of two categories. One is failure of the power drive portion of the tail rotor disk resulting in a complete loss of antitorque. The other category covers mechanical control failures prohibiting the pilot from changing or controlling tail rotor thrust even though the tail rotor may still be providing antitorque thrust. 

Tail rotor drive system failures include driveshaft failures, tail rotor gearbox failures, or a complete loss of the tail rotor itself. In any of these cases, the loss of antitorque normally results in an immediate spinning of the helicopter’s nose. The helicopter spins to the right in a counterclockwise rotor disk and to the left in a clockwise system. This discussion is for a helicopter with a counterclockwise rotor disk. The severity of the spin is proportionate to the amount of power being used and the airspeed. An antitorque failure with a high-power setting at a low airspeed results in a severe spinning to the 

