System Malfunctions Antitorque System Failure 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.  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. 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. 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.  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 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. 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  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  11-16