concerned, low-level operations(below about 5000 feet) probablywon't need you to get tooconcerned, apart from taking noticeof airspeed placards and powerlimitations, because some of thepower lost with altitude is regainedwith cooler temperatures. You willfind that at least 75% power isavailable to a fair height, but becareful (some pilots report no realdifference up to 8500 feet).Power available is reduced withheight (and temperature), and rotorsturn at the same speed, so, as youincrease altitude, higher pitch andpower settings will be required (insome helicopters, like the 500C, therotor blades will stall before youreach engine limits). The dynamicpressure applied to the ASI is alsoSpecial Use Of Aircraft 223reduced, so IAS will read less inrelation to TAS, so, if you maintain aparticular airspeed, yourgroundspeed will increaseaccordingly, and you will be goingfaster than you think. The ASI willalso be slower to react.Density Altitude is your real altituderesulting from height, temperatureand humidity. The more the densityof the air decreases for any of thosereasons, the higher your machinethinks it is. The effects are found atsea level, as well as in mountains,when temperatures are high – forexample, 90° (F) at sea level is really1900 feet as far as your aircraft isconcerned. In extreme situations,you may have to restrict youroperations to early morning or lateafternoon in some areas.Larger control movements will beneeded, with more lag, so controlsmust be moved smoothly andgradually, or the effects may wellcancel each other out – you may beon the ground well before that largehandful of collective pitch even takeseffect! Rotor RPM will rise veryquickly with the least excuse.Your maximum weight for a givenaltitude (and vice versa), as well ascruising speed in relation to themboth should be known, at leastapproximately, in advance. You alsoneed to know the Hover ceiling Inand Out of Ground Effect(HIGE/HOGE) for any weight, soyou know you can come to a lowhover properly, however briefly, andrecover from an unsuitable landingsite (hovering should actually beminimised, partly because you can’trely on ground effect being present,and you have less power anyway, butalso because you need to keep a littleup your sleeve if the wind shifts, oryou begin to lose tail rotor authority.Having said that, no-hover landingsare not recommended, because ofthe chances of snagging the skids onsomething). Check the performancecharts in the back of the FlightManual, and start practising hoversabout 1-2 feet off the ground,bearing in mind, of course, that thesaid charts were established by testpilots, in controlled situations.If you allow for these effects as partof your flight planning, fine, but it's
easy to get used to a particular placeand air density and a correspondingtake-off run, base leg, etc., and youmay get caught out one day whenthings change.IllusionsThere is a psychological aspect tomountain flying. In the initial stages,it requires a good deal of selfcontrol, as you overcome a certainamount of fear and tension, which isnot good when you really need to berelaxed on the controls. You willalso have to cope with some opticalillusions.Almost the first thing you will noticeis the lack of a natural horizon, andwill maybe want to use the mountaintops or sides as a substitute. This,however, will probably cause aclimb, or other exaggerated attitudes,and make it difficult to estimate theheight of distant ground, either froma cockpit or on the ground itself, soyou will find it best to superimpose ahorizon of your own below thepeaks. This is where using yourinstruments will help, both to keepattitude and give you a good idea ofyour height and speed (however,224 Operational Flyingyou’re not supposed to beinstrument flying!).Close to the ground, you will get animpression of increased speed,especially near to a ridge. Forexample, climbing along a longshallow slope is often coupled withan unconscious attempt to maintainheight without increasing power so,unless you keep an eye on the ASI,you will be in danger of graduallyreducing speed—if your airspeed isreducing, then either the nose hasbeen lifted or you're in adowndraught (downdraughts will beassociated with a loss of height orairspeed for the same power). Youcan also tell if you're in adowndraught by watching theposition of the nose—if it yaws intothe slope, the air is flowingdownwards and vice versa. A lack ofcloud above, i.e. descending air, isalso a possible indication.Downdraughts can frequentlyexceed your climbing capabilities.Strong updraughts can suspend youin mid-air with zero power – if theair subsides suddenly, you will begoing down faster than you canapply it. Do not fight it, but guidethe aircraft towards a lifting slope, ortry for a cleaner column of air. Youmight get help from the groundcushion, but the effect will be less ona slope or grass. When valley flying,upslopes or slopes exposed to thesun can produce updraughts, soplace yourself on a convergingcourse to the line of the ridge andpositioned to obtain a straight flightpath two thirds up the slope and oneacross, which is generally the area ofsmoothest flight. However, localconditions could vary this.You could climb on a lee slope (thatis, the other side from where thewind is coming from), takingadvantage of the updraught formedby stronger wind returning on itself(i.e. riding the backlash, which tendsto occur with abrupt surfaces).
Also, there is so little room tomanoeuvre if something goes wrong,or you meet someone coming theother way. If you have to do this,converging on the ridge line at 45ogives you the best chance of anescape route.Similarly, try and avoid flight alonglee slopes, but if you need to(because life's sometimes like that),smoothest flight will be obtained byflying as close as possible to theground, say about six inches, soyou’re in the boundary layer, whichis a steady movement of air close tothe surface, with a vertical element.This gives even less room for error,though. A good illustration of theboundary layer comes from your carafter it's just been washed—waterleft on the bodywork will not beSpecial Use Of Aircraft 225affected by the air flowing over it,because it's in a layer all to itself. Airnext to mountains behaves in thesame way and, when landing at least,will be mostly what you work with.WindsWinds can increase your operationalceiling, payload, rate of climb, rangeand cruise speed. They can also dothe opposite, and be very difficult topredict, with formidable up anddowndraughts associated with them.When cruising downwind, along alee slope or not, sudden windreversals could make you exceedVNE or even take away your airspeedcompletely.There are several types of wind,loosely be grouped into prevailing orlocal, with the latter subdivided intoother types, such as anabatic,katabatic, etc., and which areinfinitely variable (glacier windsdescend 24 hours a day). Theprevailing wind is steady and fairlyreliable, and starts to affect you fromabout 6000 feet upwards. Smokefrom local fires may be used todetect direction, as can water, butthis may only give half the story. Forinstance, it's not uncommon for thewindsocks at each end of Banffairstrip in the Rockies to be 180° outwith each other! Indeed, upperwinds can come in many directionsat different levels, and are usually theopposite of lower winds. Wheremountains are concerned, they alsoacquire a vertical element, which isactually where the boundary layercomes from.As a guide to speed, whitecaps onwater foam at 10 mph. Darkdepressed puddles on water arecalled bearpaws (or catpaws) and arecaused by downbursts. The mostimportant thing to watch out for isthe funnelling of wind as itprogresses down a valley, soalthough the mean windspeed maybe reported as 5 knots or so, youmay find it as high as 30 in someplaces, and not necessarily comingfrom the expected direction.In fact, understanding how airmoves around terrain is one of thekeys to good mountain flying,particularly the demarcation linesbetween smooth and turbulent air.In general, that moving up is
smooth, and that moving down isturbulent. You can visualise thedifference if you think of a waterfall,and the state of the water before andafter dropping over the edge. Closeto the ground, the air moves inlaminar fashion (the boundary layer),but the depth of the layer and thegust spread will vary considerably,depending on the nature of thesurface and its heating. The flow willbe broken if the ground becomesrough, or there are trees, and thewind is strong. Turbulence will occuron both sides, resulting in anupdraught close to the leeward sideand a downdraught close to thewindward side as air is made to curl.The movement of air over a crestline has a venturi effect, giving anincreased windspeed over thesummit and a correspondingreduction of pressure, which couldcause your altimeter to over-read.On passing over or round anobstacle, the air may becometurbulent or have formed into rollswith a vertical or horizontal axis.The general effect of a series ofridges is to form rolls between thecrest lines, possibly causing a226 Operational Flyingdangerous situation where adowndraught can exist on anupslope where an updraught wouldnormally be expected:As a result, on top of steep ridgesthere may be an area of nil or reversewinds which is difficult to locate onthe first recce. The vertical distanceto which a mountain chain willinfluence the movement of air isabout 3-5 times its height, changingwith the windspeed.Horizontally, the effect is variableand most noticeable in stableconditions with more than 20 knotsof wind, when standing waves willform. As you probably know, youcan recognise the existence of theseby lenticular clouds, but you will alsosee ragged cloud around the peak.These should be avoided at all costsdue to their turbulence, especially atthe wind speeds that lead to theircreation. As well as shockloading,momentary loss of control mayoccur, not to mention coffee all overthe place.A couple of thoughts for whenyou’re very high up; how much timeit takes to get down if you have aproblem, and meeting anyone else atthat height on an airway who doesn’texpect you. And oxygen.Landing SitesThose on peaks or crests usuallypresent you with more escape routesthan any on flanks or valley bottomsso, wherever possible, landingsshould be made on ground higherthan the immediate surroundings, soyou can vary the approach accordingto the wind and have a clearovershoot path. Customers, though,have this annoying habit of wantingto land on the most obscure sites!Use the windward sides of a slope;leeward sides should only be used inoperational necessity, because windflowing down the slope can increaseits apparent angle (you need more
lateral cyclic to hold the helicopter inplace, and you could run out whenyou reduce power to lower thedownwind skid). Don't forget youwill not have the full effects of aground cushion, if at all. Whereconditions allow, go as far to thewindward edge as possible, to avoidsuddenly finding yourself in dead orreversed airflow (as if on a leeslope)and make overshooting easier. Thewind coming over the peak will haveincreased in speed, due to Venturieffects, so a 15 knot wind can easilybecome double that, aside from youraltimeter misreading.Finding the wind direction can beinteresting if the site is bare andgives you no information, and itdoesn’t help that mountain flyingtends to take place in high pressureconditions, that is, where the windsare light and variable. We are nowtalking about local winds, caused byconvection, for instance, or katabaticeffects, combined with the prevailingwind influenced by the ground, oreven a mixture of them all. Even acloud shadow can increase the speedof a downflowing wind from a coldsurface. You could judge its effectson the machine itself, flying roundthe site with a constant speed andSpecial Use Of Aircraft 227power setting, or a constant altitude,which is otherwise known as a contourcrawl, because you use one contourall the way round.Look at your power settings,whether the air is turbulent, yourgroundspeed varies, whether youdrift or whether the nose yaws intoor out from the slope. How muchpedal you use to keep straight is agood help – a lot of right pedalmeans the wind is from the left, forexample, and a fair amount ofvibration means it is behind you, butit may be a good idea, if you can’thave it at the front, to get the windoff to the side that requires the useof the power pedal (the left, in a206), in case tail rotor authoritybecomes a problem. Aft cyclic wouldindicate a tailwind as well.So, with constant power andairspeed (say 40-50 kts), when yourise, you will be on the windwardside, and vice versa. On the otherhand, you would use less power onthe windward side if you kept aconstant height. However, useturbulence as a guide only in lighterwinds – any found in updraughts willbe from mechanical effects, such astrees. Smoke grenades are often usedif there’s nothing else.Aside from picking a speed slowenough to detect changes and yetgive enough for a margin of safety(and cope with any turbulence),when testing for wind, you shouldalso fly about 50-100 feet below thetop of the peak you want to land on,to keep yourself away from thedemarcation line and reduce thechances of getting the rotors in anupdraught on the leeward side. Also,keep tight in to the side, to stayinside the boundary layer.The demarcation line is the point atwhich smooth air is separated from
turbulent air around a peak, rathersimilar to that over an aerofoil.Above or to the side of the line, airis relatively smooth and upflowing –below, it is downflowing. It steepensas wind velocity increases (and theseverity of the slope), as does thearea of downflow, and movestoward the top of the hill. One tip:you don't have to keep the wholehelicopter one side or the other—many pilots work the line by keepingonly the blades on one side of it. Thefuselage by itself is not affectedmuch by turbulence.Having decided on wind direction ingeneral, you now need to look moreclosely at your proposed landing site.In strong wind conditions, you won'tneed the contour crawl at all,because it's obvious where the mainbody of wind is coming from, but itmay have very little influence overyour final approach anyway.The basic manoeuvre is a figure-ofeight type inspection, making allturns away from rising ground(returning towards the site) to giveyou a good view all the time. Youcould go round in a circle, but thelanding point would be out of sightmost of the time.228 Operational FlyingAs with any other potentially dodgylanding site, you need to check forSize, Shape, Surroundings, Slope,Surface and Sun (you don’t want it inyour eyes). The most important,however, in this case, is Slope, asthere's no point trying at all if youcan't land. You will get little idea ofground conditions if you overfly thesite, so what you must do is have alook at eye-level, which results in theaptly named Eye-Level Pass (if thesite isn't surrounded by trees).The most economical way is to startwith a downwind pass, turn roundand land, which is entirely possible ifyou know the wind direction beforeyou start. Sometimes, though, this isnot obvious at all, so just make aneducated guess and fly at about 40kts in the direction you think isdownwind very close to the site, levelwith your eyes. This point is crucial. Asyou do so, note the reading on thealtimeter (those people used to QFEmay want to set it to zero), andclimb up an extra hundred feet asyou increase speed to about 60knots, using the collective.At 100 feet, turn round for anotherapproach and repeat the process,taking note of the new groundspeedand deciding which way the wind iscoming now you are closer. If thereis no real difference in speed, checkfor vibration through the pedals, aftcyclic, etc, or anything that mightindicate the wind is from behind.The next step is an initial approachand overshoot, but if you have tomake a circuit, you may as well doanother eye-level pass and get asmuch information as you can.Turn in at around 50-60 knots (atthe 100 feet), taking particular noteof escape routes, up and downdraughts and turbulent areas.Maintain a constant angle, aimingdirectly for the point you wish toland on, controlling your speed with
collective and avoiding any last-minutecorrections. The idea is to keep thefuselage as level as possible, so don'tmove the cyclic at all, if you can helpit. One reason for using thecollective in this way is to minimiselarge control movements in the finalstages, as this is a shallow approach.ApproachesThere are several schools of thoughtabout these, but no real standard –as with many other activitiesinvolving helicopters, there is morethan one “right” answer to this one.A fairly flat, disc-loaded (shallow)one will (in theory, anyway)minimise collective for the hover,and give you the most control as youkeep translational lift as long aspossible, but there’s very little upyour sleeve at the end, and you needto be very aware of your winds, asforward speed will mask the effectsright to the last minute, although itdoes give you a good idea of thelevel of your site. This assumes youSpecial Use Of Aircraft 229remember your training and keepgoing forward and down, so thecyclic is ahead of the game andoperating in the cleaner air in frontof the machine that helps withtranslation. In other words, keep therotor disc forward, so the flow of airis from front to back, especiallywhere snow is concerned, but youshouldn’t use the shallow approachwith powdered snow anyway,because you will lose sight of yourlanding point at the critical momentin the resulting white cloud. Theother thing to bear in mind is thatyou are trying to land at probably theonly spot available, and if you have aproblem in a shallow approach, youaren't going to get there.You could, on the other hand, use asteeper angle, particularly if you'regoing into a clearing surrounded bytall trees, increasing with the windstrength, but this requires largehandfuls of power and attitudechanges in the final stages if youdon’t get ground effect, so youwouldn’t try this in an underpoweredpiston-engined machine that reallyshouldn’t be there in the firstplace—the engine may be able tocope with it, but can your tail rotor?(leading with the pedals will help).Anyway, since ground effect reducesyour torque requirement for thehover by up to 15%, if you approachin such a way that you need no morethan that amount to stop, youshould find your descent stoppingnicely in the right place, assumingthe surface is conducive to it, andwhether you have high skids or not.You also have some potential energyavailable for an escape.I guess you could use whateverworks—I generally turn in steeparound 60 kts with the disc loaded asmuch as possible, consistent withdescending at about 250 fpm – if theblades have some tension on them,they are less likely to be overstressed.Not only that, the controls are moreresponsive. The power used will giveyou a good idea of what you need inthe hover, so you have an early
chance to abort if you are using toomuch (you get to know withexperience). This works, because 250fpm reduces the thrust required totransition into the hover by about15%, i.e. much the same as forground effect. 250 fpm is about 20feet every 5 seconds, if you haven'tgot a VSI (altimeters usually have 20-foot segments).Whichever you choose, if themachine wants to weather cock, letit—there's no point in using poweror making a lot of effort to keepstraight if you're going in the rightdirection anyway. Keeping the wholeof the windward side in view overthe crest will keep you forward ofthe demarcation line and in theupflowing air. Coming at 45° willhelp with escape routes.When you make a final approach toland, remember that you may not beable to hover when you get there. Ifyou do manage it, make it low,somewhere between 1-2 feet, andbrief, one or two seconds. No-hoverlandings are not recommended.In a confined area, there will be apoint beyond which you'recommitted, so don't go beyond ituntil you’re sure. Pick a point to aimfor where you know your tail will beclear, not too far towards the end,and fly the machine in, in as smootha movement as possible, going overthe lip to the clearing at around a230 Operational Flyingwalking pace. As for power checks,you will know very early on if you'rerunning out (keep an eye on thetorque). The size of any surroundingtrees will give you a false illusion asto the size of the clearing, in that bigtrees will make it look smaller andvice versa. A typical clearing will havestumps and slash all over the surface– if you don’t have logs to land on(and these produce their ownproblems when they are slippery),take off a cleanly as possible, toavoid your skids getting caught insomething (also be aware that talltrees will sway from yourdownwash). When landing, if there'sroom, try to move forward slightly,as this will bring the tail up, awayfrom the garbage.So as not to use pedals too much,you can use the cyclic to turn themachine if need be. It is always agood idea to do a clearing turnbefore taking off, but often youcannot, so exercise extra caution ifyou think someone may be behindyou. In a Bell, as you go out of aclearing, a little aft cyclic willproduce a little extra lift, but don'texpect the same from an AS 350, oryou will clip the trees.Anyway, always be prepared to breakoff at any time, even if only secondsfrom success. Never commityourself till the very last moment.Short cuts don't exist withmountains—they've been around alot longer than you have!Landing sites on the bottoms ofvalleys often have difficult access,and frequently leave no escape routeonce an approach has started. In thiscase, it's important to have safe
power reserves before committingyourself. In any case, placing theaircraft downwind near to groundshould be avoided, but if you haveto, go low and slow whenapproaching downwind with a lastminute turn into wind.In snow, try landing with the sunbehind you, as the aircraft shadowwill give you a useful guide to theground slope and surface andprovide a focus for a sight pictureapproach. Some people use thelanding light. For takeoff, try not tohover too much. A jump takeoff isuseful if little power is available,where you get light on the skids,proceed to the edge with full RPMand tip yourself over the edge. Goodfun, but you should be able to hoverat least for a moment, just in caseyour C of G is out. In a confinedarea, for a JetRanger, at least, youneed about 15% torque in hand todo a proper vertical takeoff, soyou’re probably OK if you’rehovering at about 80%.Near the end of a cliff (or the deckof a ship), try and keep the tips ofthe rotors as far as possible over theedge, to avoid a vortex formed bywind movement over the edge (abacklash) mixing with yourdownwash, which will affect theairflow so much that you will needmore power in a low hover. It’s asimilar effect to the recirculationfound when hovering near abuilding, where the accelerated airgoing through the disc actually pullsyour machine towards the wall, andmore power is used to stop it.Log Pads and PlatformsLog pads are used when slopes aresteep, on rough ground. The quickand easy one is a single log acrossthe slope for your rear skid to a solidSpecial Use Of Aircraft 231mat of smaller ones. They can beslippery! Platforms are still madefrom logs, but are much morerefined. The problem with them allis, you can mostly only land one way,and there may be no room to turnonce you get there, so approachingwith the wind in totally the wrongdirection is often the only choice. Insuch cases, you need much moreanticipation than normal, and thewillingness to throw things awaymuch earlier. Of course, you don’tactually have to land, but it’s oftenworth a try. As with landing on rigsor ships, it may be possible toapproach to the hover next to it andmove sideways on.Here is a typical log arrangement(note the larger one at the back):Here is a typical forestry landing site(look at the slash on the ground):Summary·
Mountains take no sh*t fromnobody.·
Make turns away from risingground.·
Use the eye-level pass as muchas possible.·
Use controls for differentfunctions, collective for speed,etc.·
Take off as cleanly as possible
to avoid getting snaggedNight FlyingNight flying can be pleasant—there'sless traffic, you tend not to go in badweather and the air is denser, so theengine and flying controls are moreresponsive (if the controls becomeheavier than normal, your instructorhas his hands on as well!).Searching for an overdue aircraft inlow light conditions causes lots ofproblems, and route planning shouldtake account of this. Otherwise, it’smuch the same as for day, thoughthere are some aspects that demandsome thought. Plot your route onthe chart in the normal way, butnavigate with electronic aids orfeatures that are prominent at night,such as town lighting, lighted mastsor chimneys, large stretches of water(big black holes), aerodromes,highways, etc.Apart from reducing visibility, rainon th windscreen is a particularthreat when fixing your position by asingle light source. When little or nolight is on the surface and aprominent one comes into view, itmay seem that the light is above the232 Operational Flyinghorizon, which could lead you topitch into a steep attitude in keepingwith the resulting false horizon.Sometimes the effect is not muchmore than an uncomfortableclimbing sensation even when you'restraight and level, but an obscuredwindscreen could make objectsappear lower than they really are.This will be more apparent with highintensity runway lighting, which mayalso give you the same effect thatactors have on stage, where theycan't see the audience through thebright lighting. The lack of normalcontrast will also upset your altitudeperception, making you feel furtheraway and higher than you are. As aresult, on a final approach you couldfind yourself too low and fast. Thesolution is to use every piece ofsensory information you can,including landing lights andinstruments (look ahead and slightlyto the side of the light beam).Problems will arise if several of theabove factors affect you at once,especially if the landing point issloping—this is where frequentcross-checking of altimeters isimportant. The illusions you mightget with sloping ground include:Problem Illusion RiskDownslope Too low High approachUpslope Too high Low approachRain Closer Low approachNarrow Too high Low approachWide Too low High approach & flareBright lts Too low High approachThe trick with landing is to get to thepoint where you think the wheels aregoing to touch the ground – then godown another 30 feet.Helicopter landing sites must bechecked out in daylight on the sameday as they are to be used at night.Preflight checks should allow fornight flying—carry a torch, and 2landing lights are preferred.Permission to enter the rotor disc is
