5. Be aware that if a considerable amount of left pedal is being maintained, a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw. 6. Be alert to changing wind conditions, which may be experienced when flying along ridge lines and around buildings. RECOVERY TECHNIQUE MAIN DRIVE SHAFT FAILURE HYDRAULIC FAILURES GOVERNOR FAILURE ABNORMAL VIBRATIONS If a sudden unanticipated right yaw occurs, the follow- ing recovery technique should be performed. Apply full left pedal while simultaneously moving cyclic control forward to increase speed. If altitude permits, reduce power. As recovery is effected, adjust controls for  normal forward flight. Collective pitch reduction aids in arresting the yaw rate but may cause an excessive rate of descent. Any large, rapid increase in collective to prevent ground or  obstacle contact may further increase the yaw rate and decrease rotor r.p.m. The decision to reduce collective must be based on your assessment of the altitude  available for recovery. If the rotation cannot be stopped and ground contact is imminent, an autorotation may be the best course of action. Maintain full left pedal until the rotation stops, then adjust to maintain heading. The main drive shaft, located between the engine and the main rotor gearbox, transmits engine power to the main rotor gearbox. In some helicopters, particularly those with piston engines, a drive belt is used instead of a drive shaft. A failure of the drive shaft or belt has the same effect as an engine failure, because power is no longer provided to the main rotor, and an autorotation has to be initiated. There are a few differences,  however, that need to be taken into 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 hap- pens, close the throttle immediately and enter an autorotation.  Most helicopters, other than smaller piston powered 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 gearbox; and a reservoir to store the hydraulic fluid. A switch in the cockpit can turn the system off, although it is left on under normal conditions. A pressure indicator in the cockpit may be installed to monitor the system. 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 corrective action required is stated in detail in the appropriate rotorcraft flight  manual. However, 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 either pulling the circuit breaker and/or 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. In those helicopters where the control forces are so high that they cannot be moved without hydraulic assistance, two or more independent hydraulic systems may be installed. Some helicopters use hydraulic accu- mulators to store pressure that can be used for a short time while in an emergency if the hydraulic pump fails. This gives you enough time to land the helicopter with normal control. Governors automatically adjust engine power to main- tain rotor r.p.m. when the collective pitch is changed. If the governor fails, any change in collective pitch requires you to manually adjust the throttle to maintain correct r.p.m. In the event of a high side governor  failure, the engine and rotor r.p.m. try to increase above the normal range. If the r.p.m. cannot be reduced and controlled with the throttle, close the throttle and enter an autorotation. If the governor fails on the low side, normal r.p.m. may not be attainable, even if the throttle is manually controlled. In this case, the collective has to be lowered to maintain r.p.m. A running or roll-on landing may be performed if the engine can maintain sufficient rotor r.p.m. If there is insufficient power, enter an autorotation.  With the many rotating parts found in helicopters, some vibration is inherent. You need 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, you learn what vibrations are normal versus 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.  11-14