Multiengine Emergency Operations Single-Engine Failure  Medium- and High-Frequency Vibrations Dual-Engine Failure  Lost Procedures Tracking and Balance When one engine has failed, the helicopter can often maintain  altitude and airspeed until a suitable landing site can be  selected. Whether or not this is possible becomes a function  of such combined variables as aircraft weight, density  altitude, height above ground, airspeed, phase of flight,  and single-engine capability. Environmental response time  and control technique may be additional factors. Caution  must be exercised to correctly identify the malfunctioning  engine since there is no telltale yawing as occurs in most  multiengine airplanes. Shutting down the wrong engine  could be disastrous! Even when flying multiengine powered helicopters, rotor rpm  must be maintained at all costs, because fuel contamination has  been documented as the cause for both engines failing in flight. worn bearings, dampers out of adjustment, or worn parts are  possible causes of low frequency vibrations. Medium-frequency vibrations (1,000–2,000 cycles per  minute) range between the low frequencies of the main rotor  (100–500 cycles per minute) and the high frequencies (2,100  cycles per minute or higher) of the engine and tail rotor.  Depending on the helicopter, medium-frequency vibration  sources may be engine and transmission cooling fans, and  accessories such as air conditioner compressors, or driveline  components. Medium-frequency vibrations are felt through  the entire airframe, and prolonged exposure to the vibrations  will result in greater pilot fatigue. The flight characteristics and the required crew member  control responses after a dual-engine failure are similar to  those during a normal power-on descent. Full control of the  helicopter can be maintained during autorotational descent.  In autorotation, as airspeed increases above 70–80 KIAS, the  rate of descent and glide distance increase significantly. As  airspeed decreases below approximately 60 KIAS, the rate  of descent increases and glide distance decreases.  Most tail rotor vibrations fall into the high-frequency range  (2,100 cycles per minute or higher) and can be felt through  the tail rotor pedals as long as there are no hydraulic actuators  to dampen out the vibration. This vibration is felt by the pilot  through his or her feet, which are usually “put to sleep” by  the vibration. The tail rotor operates at approximately a 6:1  ratio with the main rotor, meaning for every one rotation  of the main rotor the tail rotor rotates 6 times. A main  rotor operating rpm of 350 means the tail rotor rpm would  be 2,100 rpm. Any imbalance in the tail rotor disk is very  harmful as it can cause cracks to develop and rivets to work  loose. Piston engines usually produce a normal amount of  high-frequency vibration, which is aggravated by engine  malfunctions, such as spark plug fouling, incorrect magneto  timing, carburetor icing and/or incorrect fuel/air mixture.  Vibrations in turbine engines are often difficult to detect as  these engines operate at a very high rpm. Turbine engine  vibration can be at 30,000 rpm internally, but common  transmission speeds are in the 1,000 to 3,000 rpm range for  the output shaft. The vibrations in turbine engines may be  short lived as the engine disintegrates rapidly when damaged  due to high rpm and the forces present.  Pilots become lost while flying for a variety of reasons, such  as disorientation, flying over unfamiliar territory, or visibility  that is low enough to render familiar terrain unfamiliar. When  a pilot becomes lost, the first order of business is to fly the  aircraft; the second is to implement lost procedures. Keep  in mind that the pilot workload will be high, and increased  concentration will be necessary. If lost, always remember to  look for the practically invisible hazards, such as wires, by  searching for their support structures, such as poles or towers,  which are almost always near roads. If lost, follow common sense procedures.  Modern equipment used for tracking and balancing the main  and tail rotor blades can also be used to detect other vibrations  in the helicopter. These systems use accelerometers mounted  around the helicopter to detect the direction, frequency, and  intensity of the vibration. The built-in software can then  analyze the information, pinpoint the origin of the vibration,  and suggest the corrective action. The use of a system such as a health and usage monitoring  system (HUMS) provides the operator the ability to record  engine and transmission performance and provide rotor track  and balance. This system has been around for over 30 years  and is now becoming more affordable, more capable, and  more commonplace in the rotorcraft industry. •  Try to locate any large landmarks, such as lakes, rivers,  towers, railroad tracks, or Interstate highways. If a  landmark is recognized, use it to find the helicopter’s  location on the sectional chart. If flying near a town or  city, a pilot may be able to read the name of the town  on a water tower or even land to ask for directions. •  If no town or city is nearby, the first thing a pilot should  do is climb. An increase in altitude increases radio and  navigation reception range as well as radar coverage. 11-23