picked up on take-off could stop thelanding gear from operating,amongst other things.Zero degrees is actually when waterbecomes supercooled and capable offreezing. Airframe icing happenswhen supercooled water dropletsstrike an airframe below that. Someof the droplet freezes on impact,releasing latent heat and warming theremainder which then flows back,turning into clear ice, which cangather without noticeable vibration.On the ground this can meanground resonance in a helicopter,and bits of ice flying off rotor bladesor propellers. In flight, the extraweight and drag could cause descentand improper operation of flyingcontrols. So—it's a good idea toavoid icing conditions but, in anycase, you shouldn't go if you haven'tgot the equipment, which naturallymust be serviceable (see Certificationfor Flight in Icing Conditions, above).The trend now is towards a "cleanair concept" which, essentially,means that nothing should be on theoutside of an aircraft that should notbe there, except, of course fordeicing fluid.All ice should be removed fromcritical areas before take-off,including hoar frost on the fuselage,because even a bad paint job willincrease drag, which is relevant ifyou're heavy, and hoar frost willhave a similar effect. Deicing detailsshould be entered in the relevantpart of the Tech Log, includingstart/end times, etc. The criticalareas include control surfaces,rotors, stabilisers and the like.The ability of an object toaccumulate ice is known as its catchefficiency; a sharp-edged object isbetter at it than a blunt-edged one,due to its lesser deflection of air.Speed is also a factor. Due to thespeed and geometry of a helicopter'smain rotor blades, their catchefficiency is greater than that of thefuselage, so ice on the outside of thecabin doesn't relate to what youmight have on the blades. In fact,Canadian Armed Forces tests showthat you can pick up a lethal load ofice on a Kiowa (206) rotor bladeinside 1-6 minutes, although it’s trueto say that 206 blades, being fairlycrude, don’t catch as much as moresophisticated ones, such as those onthe 407. Mind you, tailplanes havesharper leading edges than wings,and will collect ice more efficiently,so you might see nothing on thewing yet have it on the horizontalstabiliser. Because of the ratio of icethickness to the chord length, theeffects will be more marked.It’s the rate of accretion that'simportant, not the characteristics ofthe icing, although clear ice isdefinitely worse than rime ice, sincethe latter contains air bubbles and ismuch lighter and slower to build. Italso builds forward from the leadingOperational Procedures 109edge as opposed to spreadingbackwards. Variations on clear iceare freezing rain and freezing drizzle,both of which have larger droplets
and are caused by rain, snow or icecrystals falling through a layer ofwarmer air at lower altitudes.However, the latter’s droplets have amuch higher water content.Although aircraft are different,expect icing to occur (in the engineintake area, anyway) whenever theOAT is below 4o C. Otherwise, it canform in clear air when humidity ishigh. Clear ice is found most oftenin cumulus clouds and unstableconditions between 0 and –10 degsC, and rime ice in stratiform cloudsbetween –10and –20.Pitot head, static vent and fuel ventheaters should be on whenever youencounter icing, together withanything else you feel is appropriate.Try not to use deicing boots until atleast ½ inch of solid (not slushy) icehas formed, otherwise they willmerely stretch the ice covering andoperate inside the resulting cocoon.Waiting a while at least gives you theability to crack the ice off. I knowthat some experts have determinedthat this is not the case, but, trustme, they’re wrong. If you operate theboots too early, the ice coating onthem will merely flake and stay stuckon. Boots on horizontal stabilizers,by the way, will be less effective dueto their geometry.You need warmer air to get rid of iceeffectively – just flying in clear aircan take hours, but I suppose youcould at least say you won’t get anymore. Climbing out is often notpossible, due to lack of performanceor ATC considerations, anddescending has problems, too – ifyou’re getting clear ice, it’s a fair betthat the air is warmer above you,since it may be freezing rain, whichmeans an inversion, probably within1000 feet or so, as you might getbefore a warm front. In thisposition, landing on your firstattempt becomes more important asyou are unlikely to survive a goaround without picking up more ofthe stuff. You basically have threechoices, go up, down or back theway you came. Going up is a goodfirst choice if you know the tops arenearby, if only because you won’thave a chance to do so later, but youdo present more of the airframe toicing risk, which is why there is oftena minimum speed for climbing inicing conditions.Before going, check the freezinglevel is well above minimumaltitudes, which will help get rid ofice in the descent. Try to make surethe cloud tops are within reach aswell, or you have plenty of holes.TurbulenceThis also exists high up, not so muchdue to convection, but the passageof fronts or mountain waves. Youcan’t see the evidence of its existenceas there is little moisture to formcloud, hence Clear Air Turbulence.If turbulence is likely, mention it tothe cabin crew and advise thepassengers to return to, and/orremain in their seats, ensuring theirseat belts/harnesses are securelyfastened. Catering and other looseequipment should be stowed andsecured until the risk has passed. Fly
at recommended turbulence speed.110 Operational FlyingWindshearThis concerns airspeed changes overabout 10 kts resulting from suddenhorizontal or vertical changes inwind velocity—more severeexamples will change not onlyairspeed, but vertical speed andaircraft attitude as well. Officially, itbecomes dangerous when thevariations cause enoughdisplacement from your flight pathfor substantial corrective action;severe windshear is considered tocause airspeed changes of greaterthan 15 kts, or vertical speedchanges greater than 500 feet perminute. Expect it to occur mostlyinside 1000 feet agl.Although mostly associated withthunderstorms (see above), whereyou have the unpredictability ofmicrobursts to contend with, it's alsopresent with wake vortices,temperature inversions, mountainwaves and the passage of fronts, andcan occur over any size of area. Youcan even get it where rain is fallingfrom a cumulus cloud, as the air isgetting dense from the cooling, andwill therefore fall quicker. It's notrestricted to aeroplanes, either—helicopters can suffer from it aboveand below tree top level in forestclearings, when a backlash effect canconvert headwind to tailwind.All fronts are zones of windshear—the greater the temperaturedifference across them, the greaterthe changes will be. Warm frontstend to have less windshear thancold ones, but as they're slowermoving, you catch it for longer. Ingeneral, the faster the front moves,the more vigorous the weatherassociated with it; if it goes slower,the visibility will be worse, but youcan still get windshear even then andalways for up to an hour after itspassage.One significant effect of windshearis, of course, loss of airspeed at acritical moment, similar to an effectin mountain flying, where a windreversal could result in none at all!You would typically get this from adownburst out of a convective typecloud, where initially you get anincrease in airspeed from the extraheadwind, but if you don’t anticipatethe reverse to happen as you get tothe other side of the downburst, youwill not be in a position to cope withthe resulting loss. This has led to theclassification of windshear as eitherperformance increasing or performancedecreasing. Windshear encounterednear the ground (say below 1000feet) is the most critical, mainlybecause you can't quickly build upairspeed—remember the old saying;altitude is money in the bank, butspeed is money in the pocket.The effects also depend on theaircraft and its situation, in thatpropeller driven types suffer lessthan jets, and light aircraft tend to beless vulnerable than heavy ones—those with a good power to weightratio will come off best. The take-offleaves you most vulnerable because
of the small scope for energyconversion, less amounts of excessengine power and the amount ofdrag from the gear and flaps, whichis not to say that landing is thatmuch better.In extremely simple terms, wherewindshear is expected, you shouldhave a little extra airspeed in hand;you can help with the following:Operational Procedures 111·
On take-off, use the longestrunway, less flap and moreairspeed up to about 1000 feetagl, but watch your gradient anduse about 10 kts more thanusual. If shear is indicated byrapidly fluctuating airspeedand/or rate of climb or descent,apply full power and aim toachieve maximum lift anddistance from the ground. Beprepared to make relativelyharsh control movements andpower changes, using fullthrottle if you have to—newengines are cheaper than newaircraft.·
In a jet, you can use higherangles of attack and still get asizeable amount of lift for amoderate increase in drag,because the wings are designedthat way. Various methods areused to inform you of the stall,and you want to keep the thingflying just above that point—something that may requiresome practice in a simulator.Similarly, if the shear isencountered during theapproach, positive applicationof power and flying controlsshould keep the speed and rateof descent within normal limits;if there is any doubt, abandonthe approach and take action asabove.·
Set the prop RPM to maximum(for flat pitch).Windshear should be reported toATC as soon as possible, for thebenefit of others. It can be detectedby radar, using Doppler Shift tocalculate how fast raindrops aremoving and subsequently the patternof air movement, specifically lookingfor headwind/tailwindcombinations. In theory, thistechnology could also be used todetect turbulence at higher levels,assuming raindrops are present.Your Company has to provide aformal windshear training program.If they don’t, there is an FAA videoavailable from the CAA to AOCholders only.JetstreamsThese occur at the tropopause, orthe boundary between lower air(troposphere) and the stratosophere,where it collects and channels airinto a high-speed stream due to astrong horizontal temperaturegradient. They lie to the North offrontal systems where thetemperature gradient is greatest, andare stronger in Winter. To qualify forthe name, the windspeed must be atleast 60 kts. A jetstream may only be
a few hundred miles wide, but bethousands of miles long. They haveextreme turbulence associated withthem, which can extend as much as15000 feet below the tropopause,usually on the polar side—also, headwind components will naturallyincrease your fuel consumption forthe trip. Bear in mind that thetropopause lowers in Winter, whichwill move the unstable air beneaththe jetstream further downwards aswell (it’s unstable because thejetstream is sucking it up like avacuum).Shallow fogIn shallow fog, you may be able tosee the whole of the approachand/or runway lights from aconsiderable distance, even though112 Operational Flyingreports from the aerodrome indicatefog. On descending into the foglayer, your visual reference is likelyto drop rapidly, in extreme casesfrom the full length of the runwayand approach lights to a very smallsegment. This may give theimpression that you're pitching noseup, making you more likely to hit theground after corrective movements.You should be prepared for a missedapproach whenever you have theslightest doubt about forwardvisibility. The minimum RVR forlanding from a visual circuit is 800m.WhiteoutDefined by the AmericanMeteorological Society as “Anatmospheric optical phenomenon ofthe polar regions in which theobserver appears to be engulfed in auniformly white glow”. That is, youcan only see dark nearby objects –no shadows, horizon or clouds, andyou lose your depth perception. Itoccurs over unbroken snow coverbeneath a uniformly overcast sky,when the light from both is aboutthe same. Blowing snow doesn’thelp. It’s particularly a problem if theground is rising. Once you suspectwhiteout, you should immediatelyclimb or level off towards an areawhere you can see things properly.Clear Air TurbulenceThis can sometimes be avoided bysimply changing the cruising level.Listen out for other aircraft reports.Rain, Snow and Other PrecipitationOn the ground, you may need slowertaxying speeds and higher powersettings to allow for reduction inbraking performance and theincrease in drag from snow, slush orstanding water, so watch your jetblast or propeller slipstream doesn'tblow anything into nearby aircraft.When taxyingTry not to collect snow andslush on the airframe, don't taxidirectly behind other aircraft,and take account of banks ofcleared snow and their proximityto wing- and propeller-tips orengine pods. Delay flap selectionto minimise the danger ofdamage, or getting slush on theirretraction mechanisms.On the runwayA contaminated runway hassignificant amounts of standing
water, ice, slush, snow or evenheavy frost along its surface.The most important factors areloss of friction whendecelerating, and displacementof (and impingement drag whenaccelerating through) whateveris on it, so it may be difficult tosteer, and take-off andaccelerate-stop distances may beincreased due to sloweracceleration, as will landingdistance because of poorbraking action and aquaplaning,which is a condition where thebuilt-up pressure of liquid underthe tyres at a certain speed willequal the weight of the aircraft.Higher speeds will lift the tyrescompletely, leaving them incontact with fluid alone, withthe consequent loss of traction,so there may be a period duringwhich, if one of your enginesstops on take-off, you will beunable to either continue or stopwithin the remaining runwaylength, and go water-skiingOperational Procedures 113merrily off the end (actually,you're more likely to go off theside, so choosing a longerrunway won't necessarily help).The duration of this risk periodis variable, but will varyaccording to your weight.Reverted Rubber Hydroplaninghappens when a locked tyregenerates enough heat fromfriction to cause the resultingsteam to lift the tyre off therunway. The heat causes therubber to revert to its basicchemical properties.A rough speed at whichaquaplaning can occur is about 9times the square root of yourtyre pressures, 100 pounds persquare inch therefore giving youabout 90 kts—if this is higherthan your expected take-offspeed you're naturally safer thanotherwise. The point to note isthat if you start aquaplaningabove the critical speed (forexample, when landing), you canexpect the process to continuebelow it, that is, you will slidearound to well below the speedyou would have expected it tostart if you were taking off.Most factors that will assist youunder these circumstances aredirectly under your control, andit's even more important toarrive for a "positive" landing atthe required 50 feet above thethreshold at the recommendedspeed on the recommendedglideslope than for normalsituations. Under-inflating tyresdoesn't help—each 2 or 3 lbsbelow proper pressure will lowerthe aquaplaning speed by 1 knot,so be careful if you've descendedrapidly from a colder altitude.Aquaplaning aside, it's obviouslya good idea to avoid using acontaminated runway, but if thisisn't possible, there aretechniques that may assist you toreach a speed at which you can
continue if an of engine fails, orstop in the shortest practicabledistance, which will include nottaking off with a tailwindcomponent or carryingunnecessary fuel. Therecommended maximum depthof slush or water for take-offshould not exceed 15mm, and ofdry snow 60mm. Wet snowshould be treated as slush.The airfield must have either apaved runway having anEmergency Distance Availableof not less than 1½ x TODR(say a PA23) or 2 x TODR (anAA5) or 1500 feet, whichever isthe greater; or a grass runwayhaving an Emergency DistanceAvailable of not less than 2 xTODR (PA23) or 2.66 x TODR(AA5) or 2000 feet, whichever isthe greater. The minimumcleared width should be 70 feet(see Performance for definitions).There should be provision foryou to identify the point on therunway which is 40% of EDAfrom the start of takeoff as acheck against acceleration. If .85V2 has been achieved by thismarker, continue the take-off,rotating at .9 V2. V2 should beachieved by 50 feet. If you can'tget that, then the take-off shouldbe abandoned, keeping thenosewheel in contact with therunway, the throttles closed and114 Operational Flyingmaximum (safe) braking applied.The maximum depth of slush orwater for landing should notexceed 3mm, with limitationsfor snow being the same as fortake-off.Touchdown should be madefirmly and at the beginning ofthe touchdown zone, thenosewheel lowered as early aspossible, and any retardingdevices such as spoilers, liftdump or reverse thrust usedbefore applying the brakes, togive the wheels time to spin up.Maximum anti-skid systemsshould be used immediately.Crosswind components shouldbe well below the normal dryrunway figure. However, releasethe brakes if you have difficultysteering, as anti-skid will reducecornering forces for directionalcontrol.Also, allow the engines to spooldown when changing fromreverse thrust to forward idle, orthey will transition to forwardthrust at a higher setting.Runway Braking ActionCritical fluid depths foraquaplaning can vary fromapprox 0.1 to 0.4 of an inch,depending on the surface. Theeffects of water or liquids on arunway that may affect brakingaction are:Condition DescriptionDamp Surface colour changed due tomoisture.Wet Surface soaked, no significantstanding water visible.
Water Patches Significant standing waterpatches visible.Flooded Extensive standing waterpatches visible.SandstormsTo be avoided. On the ground,aircraft should be under cover, or atleast have engine blanks and cockpitcovers fitted, as well as those forsystem and instrument intakes andprobes. These should be carefullyremoved before flight soaccumulations of dust are notdeposited in the places the coversare designed to protect. The stuffgets everywhere!Volcanic AshFlight through this can causeabrasion to all forward facing partsof aircraft, enough to impairvisibility through the windshieldsand severely damage aerofoil andcontrol surface leading edges.Airspeed indications may also becompletely unreliable throughblocking of pitot heads, and enginesmay become choked and shut down.Known areas of ash-producingvolcanic activity are found inNOTAMs, as deduced with the helpof a Cray computer. Flight into themshould be avoided, particularly atnight or in IMC when ash cloudswon't be seen—don't expect weatherradar to help. If you end up in one,the immediate action is to keep all orsome of the engines running andfind the shortest route out, whichmay be downwards.Mountain WavesWhere a high mountain range existswith an airflow greater than 20 knotsover it in stable conditions, standingwaves may exist downwind,noticeable by turbulence and strongpersistent up and down draughts.Waves form in the lee of mountainswhen a strong wind (over 20 kts) isOperational Procedures 115blowing broadside on (within about30°). They are usually standingwaves, with several miles betweenpeaks and troughs, extending 10 or20 000 feet above the range and upto 200 or 300 miles downwind,although the effects, such asturbulence and strong up & downdraughts reduce with height. Atnormal cruise altitudes, mountainwaves are usually free from clear-airturbulence, unless associated withjet-streams or thunderstorms.Watch out for long-term variationsin speed and pitch attitude in levelcruise (the variations may be large).Use the autopilot height-lock tomaintain altitude, but change poweras well. Bear in mind that at cruiseheight the margin between low andhigh speed limits can be relativelysmall. Near the ground in amountain wave area, severeturbulence and windshear may beencountered. This region is knownas a lee wave rotor, and is caused byflow separation behind the mountainrange (see also Mountain Flying).Take-off or landing should not beattempted. The quickest way out ofsevere turbulence is up, with thenext best directly away from the
range. Fly parallel to the range in anupdraught, avoiding peaks.Significant Temperature InversionsPerformance is affected by variationsin temperature, and inversions willdo so adversely. Large onesencountered shortly after take-offcan seriously degrade climbperformance, particularly whenheavy. Even a small inversion in theupper levels can prevent youreaching a preferred cruising altitude.At lower levels, expect deterioratingvisibility, as an inversion can preventfog clearance for prolonged periods.Another good reason for avoidingthe top of an inversion is that all theindustrial pollutants collect there,especially in the stubble burningseason which may includeincinerated pesticides.Wake TurbulenceA by-product of lift behind everyaircraft (including helicopters) inforward flight, particularly severefrom heavy ones. Wake vortices arehorizontally concentrated whirlwindsstreaming from the wingtips, fromthe separation point between highpressure below and low pressureabove the wing. The heavier andslower the aircraft, the more severethey will be, and flaps, etc. will onlyhave a small effect in breaking themup. The effects become undetectableafter a time, varying from a fewseconds to a few minutes after thedeparture or arrival, although theyhave been detected at 20 minutes.Vortices are most hazardous to otheraircraft during take-off, initial climb,final approach and landing.Although there is a danger ofshockloading, the biggest problem isloss of control near the ground. Youare safest if you keep above theapproach and take-off path of theother aircraft, but, for generalpurposes, allow at least 3 minutesbehind any greater than the Lightcategory for the effects to disappear(but see the table below).Wake generation begins when thenosewheel lifts off on take-off andcontinues until it touches downagain after landing. Vortices will driftdownwind, at about 400-500 fpm forlarger aircraft, levelling out at about900 feet below the altitude at whichthey were generated. Eventually they116 Operational Flyingexpand to occupy an oval area about1 wingspan high and 2 wide. Thosefrom large aircraft tend to moveaway from one another so, on a calmday, the runway itself will remainfree, depending on how near therunway edge the offending wingswere. They will also drift with wind,so your landings and take-offsshould occur upwind of movingheavy aircraft and before the pointof its take-off and after the point oflanding. Although ATC willnormally suggest an interval, usethese tables as a guide:Successive aircraft on finalsAlthough ATC will normally suggestan interval, the table below can beused as a guide, although there isnever a guarantee you will notencounter wake turbulence,
whatever separations are given:LeadingAircraftFollowingAircraftMin dist(miles)Heavy Heavy 4Medium 5Small 6Light 8Medium Medium* 3Small 4Light 6Small Med or Small 3Light 4Note: If the leading medium is a B757, increaseto 4 miles, as they are difficult to slow down andlose height with, and often fly steeperapproaches. BV234, Puma, Super Puma, EH101 and S61N helicopters are Small. Bell 212,Sikorsky S76 and smaller machines are Light.Departing aircraftApplies to IFR and VFR flights.Same or parallel runways less than760m apart (inc grass)Leading Follow Departing From Min spaceHeavy Med/Sm/Lt Same takeoffposn2 minsMedium/Small Light Same takeoffposn2 minsHeavy Med/Sm/ Lt Intermediateposn3 minsMedium/Small Light Intermediateposn3 minsRunways with displaced landingthresholds where flight paths crossLeadiing Following Min spaceHeavy Arrival Med/Small/Light Dep 2 minsHeavy Departure Med/Small/Light Arr 2 minsMedium Arrival Light/Small Dep 2 minsMedium Departure Light/Small Arr 2 minsCrossing and diverging or parallelrunways over 760m apartLead Crossing Behind Min Dist Time EquivHvy Hvy/Med/Sm/Lt 4/5/6/8 m 2/3/3/4 mMed Med/Sm/Lt 3/4/6 miles 2/2/3 minsSmall Med or Sm/Lt ¾ miles 2/2 minsOpposite direction runwaysThere should be at least 2 minutesbetween a light, small or mediumand a heavy, and between light and asmall or medium within 760 m (agrass strip is a runway).HelicoptersRotor downwash is waketurbulence from helicopters,which is easy to forget whenhovering near a runwaythreshold or parked aircraft withlittle wind. Downwash alsocreates dust storms and can lifteven heavy objects into the air,instantly presenting ForeignObject Damage (FOD) hazardsto engines, main and tail rotorblades (so don't bolt your FOD,it gives you ingestion!—old RAFjoke, on which I hope there's noOperational Procedures 117copyright). Plastic bags orpackaging sheets are FOD, too.Generally speaking, the largerthe helicopter, the greater thepotential danger (obvious,really). Still air conditions permit
