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situation where the r.p.m. is low even though you areusing maximum throttle. This is usually the result ofFigure 11-9. In a low G condition, improper corrective actioncould lead to the main rotor hub contacting the rotor mast.The contact with the mast becomes more violent with eachsuccessive flapping motion. This, in turn, creates a greaterflapping displacement. The result could be a severelydamaged rotor mast, or the main rotor system could separate from the helicopter.11-11the main rotor blades having an angle of attack that hascreated so much drag that engine power is not sufficient to maintain or attain normal operating r.p.m.If you are in a low r.p.m. situation, the lifting power ofthe main rotor blades can be greatly diminished. As soonas you detect a low r.p.m. condition, immediately applyadditional throttle, if available, while slightly loweringthe collective. This reduces main rotor pitch and drag. Asthe helicopter begins to settle, smoothly raise the collective to stop the descent. At hovering altitude you mayhave to repeat this technique several times to regain normal operating r.p.m. This technique is sometimes called“milking the collective.” When operating at altitude, thecollective may have to be lowered only once to regainrotor speed. The amount the collective can be lowereddepends on altitude. When hovering near the surface,make sure the helicopter does not contact the ground asthe collective is lowered.Since the tail rotor is geared to the main rotor, low mainrotor r.p.m. may prevent the tail rotor from producingenough thrust to maintain directional control. If pedalcontrol is lost and the altitude is low enough that alanding can be accomplished before the turning rateincreases dangerously, slowly decrease collective pitch,maintain a level attitude with cyclic control, and land.SYSTEM MALFUNCTIONSThe reliability and dependability record of modernhelicopters is very impressive. By following themanufacturer’s recommendations regarding periodicmaintenance and inspections, you can eliminate mostsystems and equipment failures. Most malfunctions orfailures can be traced to some error on the part of thepilot; therefore, most emergencies can be averted beforethey happen. An actual emergency is a rare occurrence.ANTITORQUE SYSTEM FAILUREAntitorque failures usually fall into two categories.One focuses on failure of the power drive portion of thetail rotor system resulting in a complete loss of antitorque. The other category covers mechanical controlfailures where the pilot is unable to change or controltail rotor thrust even though the tail rotor may still beproviding antitorque thrust.Tail rotor drive system failures include driveshaft failures, tail rotor gearbox failures, or a complete loss ofthe tail rotor itself. In any of these cases, the loss ofantitorque normally results in an immediate yawing ofthe helicopter’s nose. The helicopter yaws to the rightin a counter-clockwise rotor system and to the left in aclockwise system. This discussion assumes ahelicopter with a counter-clockwise rotor system. Theseverity of the yaw is proportionate to the amount ofpower being used and the airspeed. An antitorquefailure with a high power setting at a low airspeedresults in a severe yawing to the right. At low powersettings and high airspeeds, the yaw is less severe. Highairspeeds tend to streamline the helicopter and keep itfrom spinning.If a tail rotor failure occurs, power has to be reduced inorder to reduce main rotor torque. The techniquesdiffer depending on whether the helicopter is in flightor in a hover, but will ultimately require an autorotation.If a complete tail rotor failure occurs while hovering,enter a hovering autorotation by rolling off thethrottle. If the failure occurs in forward flight,enter a normal autorotation by lowering the collectiveand rolling off the throttle. If the helicopter hasenough forward airspeed (close to cruising speed) whenthe failure occurs, and depending on the helicopterdesign, the vertical stabilizer may provide enough directional control to allow you to maneuver the helicopter toa more desirable landing sight. Some of the yaw may becompensated for by applying slight cyclic control opposite the direction of yaw. This helps in directionalcontrol, but also increases drag. Care must be taken notto lose too much forward airspeed because the streamlining effect diminishes as airspeed is reduced. Also,more altitude is required to accelerate to thecorrect airspeed if an autorotation is entered into at alow airspeed.A mechanical control failure limits or prevents control of tail rotor thrust and is usually caused by astuck or broken control rod or cable. While the tailrotor is still producing antitorque thrust, it cannot becontrolled by the pilot. The amount of antitorquedepends on the position where the controls jam orfail. Once again, the techniques differ depending onthe amount of tail rotor thrust, but an autorotation isgenerally not required.LANDING—STUCK LEFT PEDALBe sure to follow the procedures and techniquesoutlined in the FAA-approved rotorcraft flight manual for the helicopter you are flying. A stuck leftpedal, such as might be experienced during takeoff orclimb conditions, results in the helicopter’s noseyawing to the left when power is reduced. Rolling offthe throttle and entering an autorotation only makesmatters worse. The landing profile for a stuck leftpedal is best described as a normal approach to amomentary hover at three to four feet above thesurface. Following an analysis, make the landing. Ifthe helicopter is not turning, simply lower the
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helicopter to the surface. If the helicopter is turningto the right, roll the throttle toward flight idle theamount necessary to stop the turn as you land. If thehelicopter is beginning to turn left, you should beable to make the landing prior to the turn ratebecoming excessive. However, if the turn ratebecomes excessive prior to the landing, simplyexecute a takeoff and return for another landing.11-12LANDING—STUCK NEUTRAL OR RIGHT PEDALThe landing profile for a stuck neutral or a stuck rightpedal is a low power approach or descent with arunning or roll-on landing. The approach profile canbest be described as a steep approach with a flare at thebottom to slow the helicopter. The power should be lowenough to establish a left yaw during the descent. Theleft yaw allows a margin of safety due to the fact thatthe helicopter will turn to the right when power isapplied. This allows the momentary use of power at thebottom of the approach. As you apply power, the helicopter rotates to the right and becomes aligned with thelanding area. At this point, roll the throttle to flight idleand make the landing. The momentary use of powerhelps stop the descent and allows additional time foryou to level the helicopter prior to closing the throttle.If the helicopter is not yawed to the left at the conclusionof the flare, roll the throttle to flight idle and use thecollective to cushion the touchdown. As with anyrunning or roll-on landing, use the cyclic to maintain theground track. This technique results in a longer groundrun or roll than if the helicopter was yawed to the left.UNANTICIPATED YAW / LOSS OF TAILROTOR EFFECTIVENESS (LTE)Unanticipated yaw is the occurrence of an uncommanded yaw rate that does not subside of its ownaccord and, which, if not corrected, can result in theloss of helicopter control. This uncommanded yaw rateis referred to as loss of tail rotor effectiveness (LTE)and occurs to the right in helicopters with a counterclockwise rotating main rotor and to the left in helicopters with a clockwise main rotor rotation. Again, thisdiscussion covers a helicopter with a counter-clockwiserotor system and an antitorque rotor.LTE is not related to an equipment or maintenance malfunction and may occur in all single-rotor helicoptersat airspeeds less than 30 knots. It is the result of the tailrotor not providing adequate thrust to maintain directional control, and is usually caused by either certainwind azimuths (directions) while hovering, or by aninsufficient tail rotor thrust for a given power setting athigher altitudes.For any given main rotor torque setting in perfectlysteady air, there is an exact amount of tail rotor thrustrequired to prevent the helicopter from yawing eitherleft or right. This is known as tail rotor trim thrust. Inorder to maintain a constant heading while hovering,you should maintain tail rotor thrust equal to trim thrust.The required tail rotor thrust is modified by the effectsof the wind. The wind can cause an uncommanded yawby changing tail rotor effective thrust. Certain relativewind directions are more likely to cause tail rotor thrustvariations than others. Flight and wind tunnel testshave identified three relative wind azimuth regions thatcan either singularly, or in combination, create an LTEconducive environment. These regions can overlap,and thrust variations may be more pronounced. Also,flight testing has determined that the tail rotor does notactually stall during the period. When operating inthese areas at less than 30 knots, pilot workloadincreases dramatically.MAIN ROTOR DISC INTERFERENCE(285-315°)Refer to figure 11-10. Winds at velocities of 10 to 30knots from the left front cause the main rotorvortex to be blown into the tail rotor by the relativewind. The effect of this main rotor disc vortex causesthe tail rotor to operated in an extremely turbulent environment. During a right turn, the tail rotor experiencesa reduction of thrust as it comes into the area of themain rotor disc vortex. The reduction in tail rotor thrustcomes from the airflow changes experienced at the tailrotor as the main rotor disc vortex moves across the tailrotor disc. The effect of the main rotor disc vortexinitially increases the angle of attack of the tail rotorblades, thus increasing tail rotor thrust. The increase inthe angle of attack requires that right pedal pressure beadded to reduce tail rotor thrust in order to maintain thesame rate of turn. As the main rotor vortex passes thetail rotor, the tail rotor angle of attack is reduced. Thereduction in the angle of attack causes a reduction inthrust and a right yaw acceleration begins. This acceleration can be surprising, since you were previouslyadding right pedal to maintain the right turn rate. Thisthrust reduction occurs suddenly, and if uncorrected,develops into an uncontrollable rapid rotation about themast. When operating within this region, be aware thatthe reduction in tail rotor thrust can happen quitesuddenly, and be prepared to react quickly to counterthis reduction with additional left pedal input.Figure 11-10. Main rotor disc vortex interference.300°330°285°270°240°210° 150°120°90°60°30°15 Knots20 Knots
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10 Knots0°360°Region of DiscVortex Interference315°11-13WEATHERCOCK STABILITY(120-240°)In this region, the helicopter attempts to weathervaneits nose into the relative wind. Unless aresisting pedal input is made, the helicopter starts aslow, uncommanded turn either to the right or leftdepending upon the wind direction. If the pilot allows aright yaw rate to develop and the tail of the helicoptermoves into this region, the yaw rate can acceleraterapidly. In order to avoid the onset of LTE in thisdownwind condition, it is imperative to maintain positive control of the yaw rate and devote full attention toflying the helicopter.Figure 11-11. Weathercock stability.TAIL ROTOR VORTEX RING STATE(210-330°)Winds within this region cause a tail rotor vortex ringstate to develop. The result is a non-uniform, unsteady flow into the tail rotor. The vortex ringstate causes tail rotor thrust variations, which result inyaw deviations. The net effect of the unsteady flow isan oscillation of tail rotor thrust. Rapid and continuouspedal movements are necessary to compensate for therapid changes in tail rotor thrust when hovering in a leftcrosswind. Maintaining a precise heading in this regionis difficult, but this characteristic presents no significant problem unless corrective action is delayed.However, high pedal workload, lack of concentrationand overcontrolling can all lead to LTE.When the tail rotor thrust being generated is less thanthe thrust required, the helicopter yaws to the right.When hovering in left crosswinds, you must concentrated on smooth pedal coordination and not allow anuncontrolled right yaw to develop. If a right yaw rateis allowed to build, the helicopter can rotate into thewind azimuth region where weathercock stability thenaccelerates the right turn rate. Pilot workload during atail rotor vortex ring state is high. Do not allow a rightyaw rate to increase.Figure 11-12. Tail rotor vortex ring state.LTE AT ALTITUDEAt higher altitudes, where the air is thinner, tail rotorthrust and efficiency is reduced. When operating athigh altitudes and high gross weights, especially whilehovering, the tail rotor thrust may not be sufficient tomaintain directional control and LTE can occur. In thiscase, the hovering ceiling is limited by tail rotor thrustand not necessarily power available. In these conditions gross weights need to be reduced and/oroperations need to be limited to lower density altitudes.REDUCING THE ONSET OF LTETo help reduce the onset of loss of tail rotor effectiveness, there are some steps you can follow.1. Maintain maximum power-on rotor r.p.m. If themain rotor r.p.m. is allowed to decrease, the antitorque thrust available is decreased proportionally.2. Avoid tailwinds below an airspeed of 30 knots. Ifloss of translational lift occurs, it results in anincreased power demand and additional antitorque pressures.3. Avoid out of ground effect (OGE) operations andhigh power demand situations below an airspeedof 30 knots.4. Be especially aware of wind direction and velocitywhen hovering in winds of about 8-12 knots. Thereare no strong indicators that translational lift hasbeen reduced. A loss of translational lift results inan unexpected high power demand and anincreased antitorque requirement.Region Where WeathercockStability Can Introduce Yaw Rates360°0°15 Knots10 Knots5 Knots17 Knots30°60°90°120°150°180°210°240°270°300°330°17 Knots15 Knots10 Knots5 Knots0°180°150°30°120°60°90°210°240°270°300°330°360°Region ofRoughnessDue toTailRotor VortexRing StateF11-145. Be aware that if a considerable amount of leftpedal is being maintained, a sufficient amount ofleft pedal may not be available to counteract anunanticipated right yaw.6. Be alert to changing wind conditions, which maybe experienced when flying along ridge lines and
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around buildings.RECOVERY TECHNIQUEIf a sudden unanticipated right yaw occurs, the following recovery technique should be performed. Apply fullleft pedal while simultaneously moving cyclic controlforward to increase speed. If altitude permits, reducepower. As recovery is effected, adjust controls fornormal forward flight.Collective pitch reduction aids in arresting the yaw ratebut may cause an excessive rate of descent. Any large,rapid increase in collective to prevent ground orobstacle contact may further increase the yaw rate anddecrease rotor r.p.m. The decision to reduce collectivemust be based on your assessment of the altitudeavailable for recovery.If the rotation cannot be stopped and ground contact isimminent, an autorotation may be the best course ofaction. Maintain full left pedal until the rotation stops,then adjust to maintain heading.MAIN DRIVE SHAFT FAILUREThe main drive shaft, located between the engine andthe main rotor gearbox, transmits engine power to themain rotor gearbox. In some helicopters, particularlythose with piston engines, a drive belt is used instead ofa drive shaft. A failure of the drive shaft or belt has thesame effect as an engine failure, because power is nolonger provided to the main rotor, and an autorotationhas to be initiated. There are a few differences,however, that need to be taken into consideration. If thedrive shaft or belt breaks, the lack of any load on theengine results in an overspeed. In this case, the throttlemust be closed in order to prevent any further damage.In some helicopters, the tail rotor drive systemcontinues to be powered by the engine even if the maindrive shaft breaks. In this case, when the engineunloads, a tail rotor overspeed can result. If this happens, close the throttle immediately and enter anautorotation.HYDRAULIC FAILURESMost helicopters, other than smaller piston poweredhelicopters, incorporate the use of hydraulic actuatorsto overcome high control forces. A hydraulic systemconsists of actuators, also called servos, on each flightcontrol; a pump, which is usually driven by the mainrotor gearbox; and a reservoir to store the hydraulicfluid. A switch in the cockpit can turn the system off,although it is left on under normal conditions. Apressure indicator in the cockpit may be installed tomonitor the system.An impending hydraulic failure can be recognized by agrinding or howling noise from the pump or actuators,increased control forces and feedback, and limitedcontrol movement. The corrective action required isstated in detail in the appropriate rotorcraft flightmanual. However, in most cases, airspeed needs to bereduced in order to reduce control forces. The hydraulicswitch and circuit breaker should be checked andrecycled. If hydraulic power is not restored, make ashallow approach to a running or roll-on landing. Thistechnique is used because it requires less control forceand pilot workload. Additionally, the hydraulic systemshould be disabled, by either pulling the circuit breakerand/or placing the switch in the off position. Thereason for this is to prevent an inadvertent restorationof hydraulic power, which may lead to overcontrollingnear the ground.In those helicopters where the control forces are sohigh that they cannot be moved without hydraulicassistance, two or more independent hydraulic systemsmay be installed. Some helicopters use hydraulic accumulators to store pressure that can be used for a shorttime while in an emergency if the hydraulic pump fails.This gives you enough time to land the helicopter withnormal control.GOVERNOR FAILUREGovernors automatically adjust engine power to maintain rotor r.p.m. when the collective pitch is changed. Ifthe governor fails, any change in collective pitchrequires you to manually adjust the throttle to maintaincorrect r.p.m. In the event of a high side governorfailure, the engine and rotor r.p.m. try to increase abovethe normal range. If the r.p.m. cannot be reduced andcontrolled with the throttle, close the throttle and enteran autorotation. If the governor fails on the low side,normal r.p.m. may not be attainable, even if the throttleis manually controlled. In this case, the collective hasto be lowered to maintain r.p.m. A running or roll-onlanding may be performed if the engine can maintainsufficient rotor r.p.m. If there is insufficient power,enter an autorotation.ABNORMAL VIBRATIONSWith the many rotating parts found in helicopters, somevibration is inherent. You need to understand the causeand effect of helicopter vibrations because abnormalvibrations cause premature component wear and mayeven result in structural failure. With experience, youlearn what vibrations are normal versus those that areabnormal and can then decide whether continued flightis safe or not. Helicopter vibrations are categorized intolow, medium, or high frequency.11-15LOW FREQUENCY VIBRATIONSLow frequency vibrations (100-500 cycles per minute)usually originate from the main rotor system. Thevibration may be felt through the controls, the airframe,or a combination of both. Furthermore, the vibrationmay have a definite direction of push or thrust. It may
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be vertical, lateral, horizontal, or even a combination.Normally, the direction of the vibration can be determined by concentrating on the feel of the vibration,which may push you up and down, backwards andforwards, or from side to side. The direction of thevibration and whether it is felt in the controls or theairframe is an important means for the mechanicto troubleshoot the source. Some possible causescould be that the main rotor blades are out of track orbalance, damaged blades, worn bearings, dampers outof adjustment, or worn parts.MEDIUM AND HIGH FREQUENCY VIBRATIONSMedium frequency vibrations (1,000 - 2,000 cycles perminute) and high frequency vibrations (2,000 cyclesper minute or higher) are normally associated with outof-balance components that rotate at a high r.p.m., suchas the tail rotor, engine, cooling fans, and componentsof the drive train, including transmissions, drive shafts,bearings, pulleys, and belts. Most tail rotor vibrationscan be felt through the tail rotor pedals as long as thereare no hydraulic actuators, which usually dampen outthe vibration. Any imbalance in the tail rotor system isvery harmful, as it can cause cracks to develop andrivets to work loose. Piston engines usually produce anormal amount of high frequency vibration, which isaggravated by engine malfunctions such as spark plugfouling, incorrect magneto timing, carburetor icingand/or incorrect fuel/air mixture. Vibrations in turbineengines are often difficult to detect as these enginesoperate at a very high r.p.m.TRACKING AND BALANCEModern equipment used for tracking and balancing themain and tail rotor blades can also be used to detectother vibrations in the helicopter. These systems useaccelerometers mounted around the helicopter to detectthe direction, frequency, and intensity of the vibration.The built-in software can then analyze the information,pinpoint the origin of the vibration, and suggest thecorrective action.FLIGHT DIVERSIONThere will probably come a time in your flight careerwhen you will not be able to make it to your destination.This can be the result of unpredictable weather conditions,a system malfunction, or poor preflight planning. In anycase, you will need to be able to safely and efficientlydivert to an alternate destination. Before any crosscountry flight, check the charts for airports or suitablelanding areas along or near your route of flight. Also,check for navaids that can be used during a diversion.Computing course, time, speed, and distance information in flight requires the same computations usedduring preflight planning. However, because of thelimited cockpit space, and because you must divideyour attention between flying the helicopter, makingcalculations, and scanning for other aircraft, you shouldtake advantage of all possible shortcuts and rule-ofthumb computations.When in flight, it is rarely practical to actually plot acourse on a sectional chart and mark checkpoints anddistances. Furthermore, because an alternate airport isusually not very far from your original course, actualplotting is seldom necessary.A course to an alternate can be measured accuratelywith a protractor or plotter, but can also be measuredwith reasonable accuracy using a straightedge and thecompass rose depicted around VOR stations. Thisapproximation can be made on the basis of a radialfrom a nearby VOR or an airway that closely parallelsthe course to your alternate. However, you mustremember that the magnetic heading associated witha VOR radial or printed airway is outbound fromthe station. To find the course TO the station, it maybe necessary to determine the reciprocal of theindicated heading.Distances can be determined by using a plotter, or byplacing a finger or piece of paper between the two andthen measuring the approximate distance on themileage scale at the bottom of the chart.Before changing course to proceed to an alternate, youshould first consider the relative distance and route offlight to all suitable alternates. In addition, you shouldconsider the type of terrain along the route. If circumstances warrant, and your helicopter is equipped withnavigational equipment, it is typically easier to navigate to an alternate airport that has a VOR or NDBfacility on the field.After you select the most appropriate alternate, approximate the magnetic course to the alternate usinga compass rose or airway on the sectional chart. If timepermits, try to start the diversion over a prominentground feature. However, in an emergency, divertpromptly toward your alternate. To complete allplotting, measuring, and computations involved beforediverting to the alternate may only aggravate anactual emergency.Once established on course, note the time, and thenuse the winds aloft nearest to your diversion point tocalculate a heading and groundspeed. Once you havecalculated your groundspeed, determine a new arrivaltime and fuel consumption.11-16You must give priority to flying the helicopter whiledividing your attention between navigation andplanning. When determining an altitude to use whilediverting, you should consider cloud heights, winds,terrain, and radio reception.LOST PROCEDURESGetting lost in an aircraft is a potentially dangeroussituation especially when low on fuel. Helicopters have
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an advantage over airplanes, as they can land almostanywhere before they run out of fuel.If you are lost, there are some good common senseprocedures to follow. If you are nowhere near or cannotsee a town or city, the first thing you should do is climb.An increase in altitude increases radio and navigationreception range, and also increases radar coverage. Ifyou are flying near a town or city, you may be able toread the name of the town on a water tower or even landto ask directions.If your helicopter has a navigational radio, such as aVOR or ADF receiver, you can possibly determineyour position by plotting your azimuth from two ormore navigational facilities. If GPS is installed, or youhave a portable aviation GPS on board, you can use itto determine your position and the location of thenearest airport.Communicate with any available facility usingfrequencies shown on the sectional chart. If you areable to communicate with a controller, you may beoffered radar vectors. Other facilities may offerdirection finding (DF) assistance. To use thisprocedure, the controller will request you to holddown your transmit button for a few seconds andthen release it. The controller may ask you to changedirections a few times and repeat the transmitprocedure. This gives the controller enough information to plot your position and then give you vectors to a suitable landing sight. If your situationbecomes threatening, you can transmit your problems on the emergency frequency 121.5 MHZ andset your transponder to 7700. Most facilities, andeven airliners, monitor the emergency frequency.EMERGENCY EQUIPMENT ANDSURVIVAL GEARBoth Canada and Alaska require pilots to carry survivalgear. However, it is good common sense that any timeyou are flying over rugged and desolated terrain, consider carrying survival gear. Depending on the size andstorage capacity of your helicopter, the following aresome suggested items:• Food that is not subject to deterioration due toheat or cold. There should be at least 10,000 calo-ries for each person on board, and it should bestored in a sealed waterproof container. It shouldhave been inspected by the pilot or his representative within the previous six months, and bear alabel verifying the amount and satisfactory condition of the contents.• A supply of water.• Cooking utensils.• Matches in a waterproof container.• A portable compass.• An ax at least 2.5 pounds with a handle not lessthan 28 inches in length.• A flexible saw blade or equivalent cutting tool.• 30 feet of snare wire and instructions for use.• Fishing equipment, including still-fishing baitand gill net with not more than a two inch mesh.• Mosquito nets or netting and insect repellentsufficient to meet the needs of all persons aboard,when operating in areas where insects are likelyto be hazardous.• A signaling mirror.• At least three pyrotechnic distress signals.• A sharp, quality jackknife or hunting knife.• A suitable survival instruction manual.• Flashlight with spare bulbs and batteries.• Portable ELT with spare batteries.Additional items when there are no trees:• Stove with fuel or a self-contained means of providing heat for cooking.• Tent(s) to accommodate everyone on board.Additional items for winter operations:• Winter sleeping bags for all persons when thetemperature is expected to be below 7°C.• Two pairs of snow shoes.• Spare ax handle.• Honing stone or file.• Ice chisel.• Snow knife or saw knife.12-1Attitude instrument flying in helicopters is essentiallyvisual flying with the flight instruments substituted forthe various reference points on the helicopter and thenatural horizon. Control changes, required to produce agiven attitude by reference to instruments, are identicalto those used in helicopter VFR flight, and yourthought processes are the same. Basic instrument training is intended as a building block towards attaining aninstrument rating. It will also enable you to do a 180°turn in case of inadvertent incursion into instrumentmeteorological conditions (IMC).FLIGHT INSTRUMENTSWhen flying a helicopter with reference to the flightinstruments, proper instrument interpretation is thebasis for aircraft control. Your skill, in part, depends onyour understanding of how a particular instrument orsystem functions, including its indications and limitations. With this knowledge, you can quickly determinewhat an instrument is telling you and translate thatinformation into a control response.PITOT-STATIC INSTRUMENTSThe pitot-static instruments, which include the airspeedindicator, altimeter, and vertical speed indicator, operate on the principle of differential air pressure. Pitotpressure, also called impact, ram, or dynamic pressure,is directed only to the airspeed indicator, while staticpressure, or ambient pressure, is directed to all threeinstruments. An alternate static source may be includedallowing you to select an alternate source of ambient
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pressure in the event the main port becomes blocked.AIRSPEED INDICATORThe airspeed indicator displays the speed of the helicopter through the air by comparing ram air pressurefrom the pitot tube with static air pressure from thestatic port—the greater the differential, the greater thespeed. The instrument displays the result of this pressure differential as indicated airspeed (IAS).Manufacturers use this speed as the basis for determining helicopter performance, and it may be displayed inknots, miles per hour, or both. When anindicated airspeed is given for a particular situation,you normally use that speed without making a correction for altitude or temperature. The reason no correc-tion is needed is that an airspeed indicator and aircraftperformance are affected equally by changes in air density. An indicated airspeed always yields the sameperformance because the indicator has, in fact, compensated for the change in the environment.INSTRUMENT CHECK—During the preflight, ensurethat the pitot tube, drain hole, and static ports are unobstructed. Before liftoff, make sure the airspeed indicatoris reading zero. If there is a strong wind blowing directlyat the helicopter, the airspeed indicator may read higherPitotHeater SwitchPitotTubeAirspeedIndicatorVerticalSpeedIndicator(VSI) AltimeterDrainOpeningStatic PortONOFFAlternate Static SourceALTSTATIC AIRPULL ONFigure 12-1. Ram air pressure is supplied only to the airspeedindicator, while static pressure is used by all three instruments. Electrical heating elements may be installed to prevent ice from forming on the pitot tube. A drain opening toremove moisture is normally included.DiaphragmStatic Air LineRam AirPitot TubeFigure 12-2. Ram air pressure from the pitot tube is directedto a diaphragm inside the airspeed indicator. The airtightcase is vented to the static port. As the diaphragm expandsor contracts, a mechanical linkage moves the needle on theface of the indicator.12-2than zero, depending on the wind speed and direction.As you begin your takeoff, make sure the airspeed indicator is increasing at an appropriate rate. Keep in mind,however, that the airspeed indication might be unreliable below a certain airspeed due to rotor downwash.ALTIMETERThe altimeter displays altitude in feet by sensing pressure changes in the atmosphere. There is an adjustablebarometric scale to compensate for changes in atmospheric pressure. The basis for altimeter calibration is the InternationalStandard Atmosphere (ISA), where pressure, temperature, and lapse rates have standard values. However,actual atmospheric conditions seldom match the standard values. In addition, local pressure readings withina given area normally change over a period of time, andpressure frequently changes as you fly from one area toanother. As a result, altimeter indications are subject toerrors, the extent of which depends on how much thepressure, temperature, and lapse rates deviate from standard, as well as how recently you have set the altimeter.The best way to minimize altimeter errors is to updatethe altimeter setting frequently. In most cases, use thecurrent altimeter setting of the nearest reporting stationalong your route of flight per regulatory requirements.INSTRUMENT CHECK—During the preflight, ensurethat the static ports are unobstructed. Before lift-off, setthe altimeter to the current setting. If the altimeter indicates within 75 feet of the actual elevation, the altimeteris generally considered acceptable for use.VERTICAL SPEED INDICATORThe vertical speed indicator (VSI) displays the rate ofclimb or descent in feet per minute (f.p.m.) by measuring how fast the ambient air pressure increases ordecreases as the helicopter changes altitude. Since theVSI measures only the rate at which air pressurechanges, air temperature has no effect on this instrument. There is a lag associated with the reading on the VSI,and it may take a few seconds to stabilize when showing rate of climb or descent. Rough control techniqueand turbulence can further extend the lag period andcause erratic and unstable rate indications. Some aircraft are equipped with an instantaneous vertical speedindicator (IVSI), which incorporates accelerometers tocompensate for the lag found in the typical VSI.INSTRUMENT CHECK—During the preflight, ensurethat the static ports are unobstructed. Check to see thatthe VSI is indicating zero before lift-off. During takeoff,check for a positive rate of climb indication.SYSTEM ERRORSThe pitot-static system and associated instruments areusually very reliable. Errors are generally caused whenthe pitot or static openings are blocked. This may becaused by dirt, ice formation, or insects. Check the pitotand static openings for obstructions during the preflight.It is also advisable to place covers on the pitot and staticports when the helicopter is parked on the ground.The airspeed indicator is the only instrument affected by ablocked pitot tube. The system can become clogged in twoAneroidWafersAltimeter
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发表于 2009-3-21 00:04:19
Setting WindowAltitudeIndicationScale10,000 ftPointer1,000 ftPointer100 ft PointerAltimeter SettingAdjustment KnobCrosshatchFlagA crosshatchedarea appearson some altimeterswhen displayingan altitude below10,000 feet MSL.Static PortFigure 12-3. The main component of the altimeter is a stack ofsealed aneroid wafers. They expand and contract as atmospheric pressure from the static source changes. The mechanical linkage translates these changes into pointer movements onthe indicator.DiaphragmDirect StaticPressureCalibratedLeakFigure 12-4. Although the sealed case and diaphragm areboth connected to the static port, the air inside the case isrestricted through a calibrated leak. When the pressures areequal, the needle reads zero. As you climb or descend, thepressure inside the diaphragm instantly changes, and theneedle registers a change in vertical direction. When thepressure differential stabilizes at a definite ratio, the needleregisters the rate of altitude change.12-3ways. If the ram air inlet is clogged, but the drain holeremains open, the airspeed indicator registers zero, regardless of airspeed. If both the ram air inlet and the drain holebecome blocked, pressure in the line is trapped, and theairspeed indicator reacts like an altimeter, showing anincrease in airspeed with an increase in altitude, and adecrease in speed as altitude decreases. This occurs aslong as the static port remains unobstructed.If the static port alone becomes blocked, the airspeedindicator continues to function, but with incorrect readings. When you are operating above the altitude wherethe static port became clogged, the airspeed indicatorreads lower than it should. Conversely, when operatingbelow that altitude, the indicator reads higher than thecorrect value. The amount of error is proportional tothe distance from the altitude where the static systembecame blocked. The greater the difference, the greaterthe error. With a blocked static system, the altimeterfreezes at the last altitude and the VSI freezes at zero.Both instruments are then unusable.Some helicopters are equipped with an alternate staticsource, which may be selected in the event that the mainstatic system becomes blocked. The alternate source generally vents into the cabin, where air pressures are slightlydifferent than outside pressures, so the airspeed andaltimeter usually read higher than normal. Correctioncharts may be supplied in the flight manual.GYROSCOPIC INSTRUMENTSThe three gyroscopic instruments that are required forinstrument flight are the attitude indicator, headingindicator, and turn indicator. When installed in helicopters, these instruments are usually electrically powered.Gyros are affected by two principles—rigidity in space andprecession. Rigidity in space means that once a gyro isspinning, it tends to remain in a fixed position and resistsexternal forces applied to it. This principle allows a gyro tobe used to measure changes in attitude or direction.Precession is the tilting or turning of a gyro in response topressure. The reaction to this pressure does not occur atthe point where it was applied; rather, it occurs at a pointthat is 90° later in the direction of rotation from where thepressure was applied. This principle allows the gyro todetermine a rate of turn by sensing the amount of pressure created by a change in direction. Precession can alsocreate some minor errors in some instruments.ATTITUDE INDICATORThe attitude indicator provides a substitute for the natural horizon. It is the only instrument that provides animmediate and direct indication of the helicopter’spitch and bank attitude. Since most attitude indicatorsinstalled in helicopters are electrically powered, theremay be a separate power switch, as well as a warningflag within the instrument, that indicates a loss ofpower. A caging or “quick erect” knob may beincluded, so you can stabilize the spin axis if the gyrohas tumbled. HEADING INDICATORThe heading indicator, which is sometimes referred toas a directional gyro (DG), senses movement aroundthe vertical axis and provides a more accurate headingreference compared to a magnetic compass, which hasa number of turning errors. .Bank IndexGyroGimbalRotationRollGimbalPitchGimbalHorizonReferenceArmFigure 12-5. The gyro in the attitude indicator spins in thehorizontal plane. Two mountings, or gimbals, are used sothat both pitch and roll can be sensed simultaneously. Due torigidity in space, the gyro remains in a fixed position relativeto the horizon as the case and helicopter rotate around it.Adjustment GearsAdjustmentKnobGimbalRotationGimbal GyroMain
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发表于 2009-3-21 00:04:29
Drive GearCompassCard GearFigure 12-6. A heading indicator displays headings based ona 360° azimuth, with the final zero omitted. For example, a 6represents 060°, while a 21 indicates 210°. The adjustmentknob is used to align the heading indicator with the magneticcompass.12-4Due to internal friction within the gyroscope, precession is common in heading indicators. Precessioncauses the selected heading to drift from the set value.Some heading indicators receive a magnetic north reference from a remote source and generally need noadjustment. Heading indicators that do not have thisautomatic north-seeking capability are often called“free” gyros, and require that you periodically adjustthem. You should align the heading indicator with themagnetic compass before flight and check it at 15-minute intervals during flight. When you do an in-flightalignment, be certain you are in straight-and-level,unaccelerated flight, with the magnetic compass showing a steady indication.TURN INDICATORSTurn indicators show the direction and the rate of turn.A standard rate turn is 3° per second, and at this rateyou will complete a 360° turn in two minutes. A halfstandard rate turn is 1.5° per second. Two types ofindicators are used to display this information. Theturn-and-slip indicator uses a needle to indicate direction and turn rate. When the needle is aligned with thewhite markings, called the turn index, you are in astandard rate turn. A half-standard rate turn is indicated when the needle is halfway between the indexes.The turn-and-slip indicator does not indicate roll rate.The turn coordinator is similar to the turn-and-slipindicator, but the gyro is canted, which allows it tosense roll rate in addition to rate of turn. The turn coordinator uses a miniature aircraft to indicate direction,as well as the turn and roll rate. Another part of both the turn coordinator and the turnand-slip indicator is the inclinometer. The position ofthe ball defines whether the turn is coordinated or not.The helicopter is either slipping or skidding anytimethe ball is not centered, and usually requires an adjustment of the antitorque pedals or angle of bank to correct it. INSTRUMENT CHECK—During your preflight, checkto see that the inclinometer is full of fluid and has noair bubbles. The ball should also be resting at its lowestpoint. Since almost all gyroscopic instruments installedin a helicopter are electrically driven, check to see thatthe power indicators are displaying off indications.Turn the master switch on and listen to the gyros spoolup. There should be no abnormal sounds, such as agrinding sound, and the power out indicator flagsshould not be displayed. After engine start and beforeliftoff, set the direction indicator to the magnetic compass. During hover turns, check the heading indicatorfor proper operation and ensure that it has not precessed significantly. The turn indicator should alsoindicate a turn in the correct direction. During takeoff,check the attitude indicator for proper indication andrecheck it during the first turn.MAGNETIC COMPASSIn some helicopters, the magnetic compass is the onlydirection seeking instrument. Although the compassappears to move, it is actually mounted in such a waythat the helicopter turns about the compass card as thecard maintains its alignment with magnetic north.COMPASS ERRORSThe magnetic compass can only give you reliabledirectional information if you understand its limitationsand inherent errors. These include magnetic variation,compass deviation, and magnetic dip.MAGNETIC VARIATIONWhen you fly under visual flight rules, you ordinarily navigate by referring to charts, which are orientedFigure 12-7. The gyros in both the turn-and-slip indicator andthe turn coordinator are mounted so that they rotate in a vertical plane. The gimbal in the turn coordinator is set at an angle,or canted, which means precession allows the gyro to senseboth rate of roll and rate of turn. The gimbal in the turn-and-slipindicator is horizontal. In this case, precession allows the gyroto sense only rate of turn. When the needle or miniature aircraftis aligned with the turn index, you are in a standard-rate turn.GyroRotationGimbalRotationTURN-AND-SLIPINDICATORGimbalGimbalRotationGyroRotationCanted GyroTURNCOORDINATORHorizontalGyroInclinometerFigure 12-8. In a coordinated turn (instrument 1), the ball iscentered. In a skid (instrument 2), the rate of turn is too greatfor the angle of bank, and the ball moves to the outside of theturn. Conversely, in a slip (instrument 3), the rate of turn istoo small for the angle of bank, and the ball moves to theinside of the turn.12-5to true north. Because the aircraft compass is orientedto magnetic north, you must make allowances for thedifference between these poles in order to navigateproperly. You do this by applying a correction calledvariation to convert a true direction to a magnet direction. Variation at a given point is the angular difference between the true and magnetic poles. The amountof variation depends on where you are located on theearth’s surface. Isogonic lines connect points wherethe variation is equal, while the agonic line defines thepoints where the variation is zero. COMPASS DEVIATION
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发表于 2009-3-21 00:04:41
Besides the magnetic fields generated by the earth, othermagnetic fields are produced by metal and electricalaccessories within the helicopter. These magnetic fieldsdistort the earth’s magnet force and cause the compassto swing away from the correct heading. Manufacturersoften install compensating magnets within the compasshousing to reduce the effects of deviation. These magnets are usually adjusted while the engine is running andall electrical equipment is operating. Deviation error,however, cannot be completely eliminated; therefore, acompass correction card is mounted near the compass.The compass correction card corrects for deviation thatoccurs from one heading to the next as the lines of forceinteract at different angles.MAGNETIC DIPMagnetic dip is the result of the vertical component ofthe earth’s magnetic field. This dip is virtually nonexistent at the magnetic equator, since the lines of forceare parallel to the earth’s surface and the vertical component is minimal. As you move a compass toward thepoles, the vertical component increases, and magneticdip becomes more apparent at these higher latitudes.Magnetic dip is responsible for compass errors duringacceleration, deceleration, and turns.Acceleration and deceleration errors are fluctuationsin the compass during changes in speed. In the northern hemisphere, the compass swings toward the northduring acceleration and toward the south during deceleration. When the speed stabilizes, the compassreturns to an accurate indication. This error is mostpronounced when you are flying on a heading of eastor west, and decreases gradually as you fly closer to anorth or south heading. The error does not occur whenyou are flying directly north or south. The memoryaid, ANDS (Accelerate North, Decelerate South) mayhelp you recall this error. In the southern hemisphere,this error occurs in the opposite direction.Turning errors are most apparent when you are turningto or from a heading of north or south. This errorincreases as you near the poles as magnetic dip becomesmore apparent. There is no turning error when flyingnear the magnetic equator. In the northern hemisphere,when you make a turn from a northerly heading, thecompass gives an initial indication of a turn in theopposite direction. It then begins to show the turn inthe proper direction, but lags behind the actual heading. The amount of lag decreases as the turn continues,then disappears as the helicopter reaches a heading ofeast or west. When you make a turn from a southerlyheading, the compass gives an indication of a turn inthe correct direction, but leads the actual heading. Thiserror also disappears as the helicopter approaches aneast or west heading.INSTRUMENT CHECK—Prior to flight, make sure thatthe compass is full of fluid. During hover turns, thecompass should swing freely and indicate known headings. Since that magnetic compass is required for allflight operations, the aircraft should never be flownwith a faulty compass.INSTRUMENT FLIGHTTo achieve smooth, positive control of the helicopterduring instrument flight, you need to develop threefundamental skills. They are instrument cross-check,instrument interpretation, and aircraft control.INSTRUMENT CROSS-CHECKCross-checking, sometimes referred to as scanning, isthe continuous and logical observation of instrumentsfor attitude and performance information. In attitudeinstrument flying, an attitude is maintained by referenceto the instruments, which produces the desired result inperformance. Due to human error, instrument error, andhelicopter performance differences in various atmospheric and loading conditions, it is difficult toestablish an attitude and have performance remainconstant for a long period of time. These variables makeATrueNorth PoleMagneticNorth PoleAgonicLine20°20°15°15°10° 5°5°0°Isogonic Lines17°10°Figure 12-9. Variation at point A in the western United Statesis 17°. Since the magnetic north pole is located to the east ofthe true north pole in relation to this point, the variation iseasterly. When the magnetic pole falls to the west of the truenorth pole, variation is westerly.12-6it necessary for you to constantly check the instrumentsand make appropriate changes in the helicopter’s attitude. The actual technique may vary depending on whatinstruments are installed and where they are installed,as well as your experience and proficiency level. Forthis discussion, we will concentrate on the six basicflight instruments discussed earlier. At first, you may have a tendency to cross-checkrapidly, looking directly at the instruments withoutknowing exactly what information you are seeking.However, with familiarity and practice, the instrumentcross-check reveals definite trends during specificflight conditions. These trends help you control thehelicopter as it makes a transition from one flightcondition to another.If you apply your full concentration to a single instrument,
