Governor or Fuel Control Failure Hydraulic Failure Abnormal Vibration Low-Frequency Vibrations consideration. If the drive shaft or belt breaks, the lack of any  load on the engine results in an overspeed. In this case, the  throttle must be closed in order to prevent any further damage.  In some helicopters, the tail rotor drive system continues to  be powered by the engine even if the main drive shaft breaks.  In this case, when the engine unloads, a tail rotor overspeed  can result. If this happens, close the throttle immediately and  enter an autorotation. The pilot must be knowledgeable of the  specific helicopter’s system and failure modes. Governors and fuel control units automatically adjust engine  power to maintain rotor rpm when the collective pitch  is changed. If the governor or fuel control unit fails, any  change in collective pitch requires manual adjustment of  the throttle to maintain correct rpm. In the event of a high  side failure, the engine and rotor rpm tend to increase above  the normal range due to the engine being commanded to  put out too much power. If the rpm cannot be reduced and  controlled with the throttle, close the throttle and enter an  autorotation. If the failure is on the low side, the engine  output is allowed to go below the collective and normal  rpm may not be attainable, even if the throttle is manually  controlled. In this case, the collective has to be lowered to  maintain rotor rpm. A running or roll-on landing may be  performed if the engine can maintain sufficient rotor rpm. If  there is insufficient power, enter an autorotation. As stated  previously in this chapter, before responding to any type of  mechanical failure, pilots should confirm that rotor rpm is  not responding to flight control inputs. If the rotor rpm can  be maintained in the green operating range, the failure is in  the instrument, and not mechanical.  Pilots should keep in mind that when there is any suspected  mechanical malfunction, first and foremost they should  always attempt to maintain rotor rpm. If the rotor rpm is at the  normal indication with normal power settings, an instrument  failure might be occurring, and it would be best to fly the  helicopter to a safe landing area. If the rotor rpm is in fact  decreasing or low, then there is a drive line failure.  Many helicopters incorporate the use of hydraulic actuators to  overcome high control forces. A hydraulic system consists of  actuators, also called servos, on each flight control; a pump,  which is usually driven by the main rotor transmission;  and a reservoir to store the hydraulic fluid. A switch in the  cockpit can turn the system off, although it is left on during  normal conditions. A pressure indicator in the cockpit may  be installed to monitor the system. With the many rotating parts found in helicopters, some  vibration is inherent. A pilot needs to understand the  cause and effect of helicopter vibrations because abnormal  vibrations cause premature component wear and may even  result in structural failure. With experience, a pilot learns  what vibrations are normal and those that are abnormal  and can then decide whether continued flight is safe or not.  Helicopter vibrations are categorized into low, medium, or  high frequency. An impending hydraulic failure can be recognized by a  grinding or howling noise from the pump or actuators,  increased control forces and feedback, and limited control  movement. The required corrective action is stated in detail  in the RFM. In most cases, airspeed needs to be reduced in  order to reduce control forces. The hydraulic switch and  circuit breaker should be checked and recycled. If hydraulic  power is not restored, make a shallow approach to a running  or roll-on landing. This technique is used because it requires  less control force and pilot workload. Additionally, the  hydraulic system should be disabled by placing the switch  in the off position. The reason for this is to prevent an  inadvertent restoration of hydraulic power, which may lead  to overcontrolling near the ground. Low-frequency vibrations (100–500 cycles per minute) usually  originate from the main rotor disk. The main rotor operational  range, depending on the helicopter, is usually between 320  and 500 rpm. A rotor blade that is out of track or balance will  cause a cycle to occur with every rotation. The vibration may  be felt through the controls, the airframe, or a combination  of both. The vibration may also have a definite direction  of push or thrust. It may be vertical, lateral, horizontal, or  even a combination of these. Normally, the direction of the  vibration can be determined by concentrating on the feel of  the vibration, which may push a pilot up and down, backwards  and forwards, or in the case of a blade being out of phase, from  side to side. The direction of the vibration and whether it is  felt in the controls or the airframe is important information for  the mechanic when he or she troubleshoots the source. Out- of-track or out-of-balance main rotor blades, damaged blades,  In those helicopters in which the control forces are so high  that they cannot be moved without hydraulic assistance, two  or more independent hydraulic systems are installed. Some  helicopters use hydraulic accumulators to store pressure that  can be used for a short time while in an emergency if the  hydraulic pump fails. This gives enough time to land the  helicopter with normal control. 11-22