The rear doors need to be removedso that used harnesses can be placedinside quickly from either side—veryoften dropoff points are in placeswhere you can't land but only cometo a very low hover. Also, there is alittle less weight for the machine tocarry (50 lbs on a 206, I believe). Ifyou’re doing vertical reference, youwill need your door off as well soyou can stick your head out of theside. However, taking any door offwill mean checking weight andbalance and performance figures,and your VNE—going too fast maypressurise the cabin and blow thewindscreen out, which is particularlyserious in the AS 350. You will alsohave higher fuel consumption andslower flight times.You need a mirror so you can seethe behaviour of the hook and theload. The hook mechanism must bechecked for consistent electrical (andmechanical) operation, as must all180 Operational Flyingstandby release methods. All hooksmust be enclosed, that is, there mustbe no opportunity for the load tocome out of the mouth of the hookwhen flying.Check the manual and electricalreleases, and don’t accept the factthat the solenoid clicks as evidenceof it working. If there’s no-one elsearound, put a rope in and pull on itwhen you operate the mechanism.After you operate the manualrelease, check that the Bowden cablebetween the hook and the body ofthe helicopter doesn’t bind and stopthe hook from rearming. All witnessmarks should be aligned on theknurled knob or lever and the hookbody, make sure the hook movesover its full range of travel and thatthe bungee cord keeps it tight againstthe bottom of the fuselage, so youdon’t land on a vertically extendedhook, which may get snagged.Garbage on the landing site has beenknown to pull the manual releaseenough to allow the load to workitself free.Condition of sling equipmentBecause of the direct connection tothe aircraft and the potential fordamage, it should only be used onhelicopter operations, and any wornor frayed items should be discarded(you’re generally allowed up to 10randomly distributed frayed wires ona steel sling, or 5 in one strand).Nylon deteriorates when exposed topetroleum, and wire rope rusts anddoesn’t like being mistreated, soprotect them from moisture andheat, and inspect them regularly.Cables without their internal greasewill snap readily. The maximumlength for nylon or poly rope shouldbe 6 feet.There must be an eye in each end ofa sling, preferably reinforced withsteel, to protect the rope, and theshackle that goes inside the eye mustbe the right size for the hook,otherwise it may come out by itself,or not come out when it’s supposedto (as a guide, the shackle pin shouldbe the same thickness as the rope –DON’T use a bolt instead; it will
bend). Using a swivel will stop theload from spinning and unravellingthe line. That is, you put the swivelin the hook, then attach the load tothe swivel (which, by the way, has athrust bearing in it). When using ascrew-pin shackle (clevis):with a rope of whatever description,don’t have the shackle downwards(as shown above), because the ropewill slide from side to side and havea tendency to open the legs (if it rollsfore and aft, it will also undo thepin). Rather, place the screw-pin inthe helicopter hook, padding theempty space either side with washersor similar, and use it upside down.Chokers, technically, are lanyardsthat hold loads with a slipping noosewhich tightens under tension – theharder the pull, the tighter the grip.Use chokers at the end of a linerather than by themselves so that, ifthey break, they are kept away fromthe blades by the line (this also helpsSpecial Use Of Aircraft 181prevent collective bounce—seebelow). Be aware that a choker has25% less safe working load becauseof the bend in the rope at the noose.The sharper the bend is, the less theSWL will be.Long lines (see below) have anelectrically operated hook at the end,which really makes them anextended hook, so there will be anelectrical cable for the releasemechanism, which should becarefully looked after; that is, the lineand cable must be detached properlyby ground crew, otherwise you willstrip the cables, which won’t impressthe engineers. If the insulationcomes off, you may also get a shortcircuit. Lines made of Spectron orKevlar are strong, but light, so willtrail after you more than a steel line,hence the maximum external loadspeed. These lines also get longerwhen new, so, if you can’t prestretch them, allow a good length ofextra electrical cable.Use barrel hooks on a sling forlifting fuel drums, although youmight find varying designs to suitdifferent helicopters. For example, a206 might have one for 2 drumswhile a 205 might have one for 5.Whatever you use, use the bungeecord to keep them together whenthere is no tension. Again, don’tconnect the hooks directly to theaircraft, but to a sling.Minimum Sling Specifications0-1500lb1500-3000lb3000-5000lb5000-10000lbLanyards(steel core)7/16” ½” 5/8” ¾”Lanyards (poly– 6ft)5/8” 1-1/8”1-1/4”1-3/8”Cables &Chokers –7/16” ½” 5/8” ¾”0-1500lb1500-3000lb3000-5000lb5000-10000lbsingle pointCables &Chokers –multi point3/8” 7/16” ½” 5/8”Shackles,Clevises½” 5/8” ¾” 7/8”Nets (lb) 1500 4000 5000 10000Ext hooks 7/16” 5/8”Use heavier than normal shackles toaccommodate multiple lines.Loading and unloading areasNon-involved people should beabsent, and there should be no loosearticles to be blown around by thedownwash and cause damage (it ispossible for slash to snag andinadvertently operate the manualrelease). Approach and departurelanes should be into wind.
Performance planningCheck your Hover performance Outof Ground Effect (HOGE), in caseit places a ceiling on your Max All-Up Weight, although mosthelicopters have a higher GrossWeight anyway for external loads.Here are charts of suggestedpayloads for various types accordingto Pressure Altitude:Preparation of loadsThere are 4 basic types of load –rectangular, cylindrical, heavycompact and nets, and five ways oflifting them, starting with nets andending up with a four-point sling(through 1-, 2- and 3-point). Netsare used for loads consisting ofmany small pieces, and are veryuseful for killing lift with loads thatcan act like a bucket as you fly (likean Argo). On a 206, one about 10-12feet square with a 2” square mesh is182 Operational Flyingquite suitable. Items should becarefully and evenly stacked in thecentre, with the net stretched roundthe load on the ground beforepickup. Individual light loads, suchas jerrycans or containers, ought tobe lashed together, since the net maynot completely enclose them at thetop. If there are many small items,consider a tarpaulin as a liner, whichwill stop them falling out. Be awarethat lifting points already attached toanything may have been designed forcranes, which don’t, as a rule, flysideways or get caught inupdraughts, etc.The weight of each load should beknown, with sand and stuff kept dryand, if possible, weighedimmediately before loading, as agood soaking will increase the weightdramatically and give you a surprisewhen you lift it. What happens mostoften, however, is that the guys justbundle stuff into a net, and as longas you don’t overtorque the machineyou’re OK. Although convenient,and mostly used for nets, a singlepoint hookup is not always the bestplan, particularly for short loads, andshould only be used on loadsdesigned for it, with a swivel, to stopthe line unravelling. You also needto be particularly careful about theload’s C of G, or it may tip and startrocking, so keep the heavy enddown. The shorter the cable, the lessthe tendency to swing.For a single pole or log, wrap therope or chain twice around the endof it and carry it vertically (steel ropegrips best):A 2-point sling is a commonmethod for long loads, such as drillcollars, pipe stems, lumber, etc:The ideal angle would be 45°, sincethe weight is then taken evenly oneach strand with the least stress. Inthose circumstances, a 400 lb loadwould have a strain of 200 on eachside. If the central angle becomes120°, it doubles, and at 150°, it's 800.Double-wrap the cables, and maybeslip some wood between them andthe load to stop them slipping.Three-point hookups are notcommon, and are usually for loadsdesigned for them, but boats arelifted this way. 4-point slings, on
the other hand, are quite commonand used for box-like loads, attachedto each corner—where the sling maycatch or damage the load, useSpecial Use Of Aircraft 183spreader bars to keep the rope away,as well as for stability:Beware of loads with a high C of G,as they may tip over.Tag lines (short lines underneath aload) are for loaders to grab on thesetdown. They should have a safetylatch and be heavy enough to stopwaving about, but long enough tocatch if the load is swinging too fast.Argos and Quads can be movedwith strops underneath. Just wrap astrop around the Quad engine,ensuring it goes inside the pedals (orthey will get bent). Then choke itand attach it to the hook.Personnel briefingAll concerned should be aware of:·
The hook-up·
The setdown·
Hand signals·
Proper use of radios (i.e. don’thog the airwaves)·
Direction to move in case ofengine failure*·
Not standing under the load·
Number of trips betweenrefuelling stops·
Retrieving slings and nets·
Use of protective equipment·
Accident procedure*This depends on which way therotors spin. With North Americanrotation (anticlockwise as viewedfrom above), the helicopter has anatural tendency to drift to the leftwhen the engine stops, so groundcrews should always work to theright of the helicopter and beprepared to go that way if they thinkanything untoward is happening.Astars go to the right.Hooking UpGround crew are nice to have, butyou often have to do it yourself:Note: Always lay it out in front!If you have one, the marshallershould be at least 25m from the loadwith his back to the wind so you cansee him from your high position. Ifhe needs to change places, he shouldcease marshalling first, so he doesn'tmove backwards into unseenobstructions. Using standard184 Operational Flyingmarshalling signals, you will bepositioned over the load, where theloaders apply the static dischargeprobe to the hook and place the eyeof the net or sling inside it.As mentioned above, loaders shouldalways work to one side of thehelicopter, and should also keepclear of the exit and approach paths,just in case you have to drop a load.The loaders then give an affirmativesignal to the marshaller, who givesyou the "move upwards" signal untilthe slack has been taken up. You willincrease hover height slowly, untilthe strain is taken, with the loadersguiding the strops as necessary,taking care to be free to move awayquickly should the need arise. At alltimes in the event of engine failure,
the ground staff must move in theopposite direction that the helicopterwould go, e.g. JetRanger to the left,staff to the right, or the opposite fora Squirrel (it depends which way theblades rotate). They should not turntheir back on the load, or get directlyunderneath it, or the flight path.Neither should they wrap linesdirectly around their wrists orbodies. When finished, they shouldclear the area as soon as possible.As you take up the weight and therope stretches, the difference inperformance will immediatelybecome obvious—it will feel as ifyou're attached to a large rubberband. Once you're hovering, and themarshaller is sure that the load isclear of the ground (and you are sureyou can lift safely, flashing thelanding lamp once to indicate this),the marshaller should check behindyou for other aircraft and give theaffirmative signal, as you will find itdifficult to do a half-turn to checkfor yourself. Keep a close eye onyour Ts and Ps at this point – if youdon’t have a power reserve, the loadis too heavy and you will have lesscontrol at the destination. You willfind that once a Bell reaches its limit,that's as far as it will get, but an AS350, if you sit for a few seconds atfull power, will corkscrew up a littlemore (don't confuse this with a Belltaking a few extra seconds to gatherup its skirts, so to speak).For extra lift in the 204B, reduce theRPM to 98%. For a 500D, changethe lateral angle of the rotor disc.Another tip for using less powerwhen lifting (for N Americanrotation, anyway) is to place the nose45° to the left with the wind on theright, and move forward, tostraighten into wind with the rightpedal. This uses a little less torqueand allows you to take a little morefuel or be easier on the machine.A load should "spring" slightly intothe air, or at least come off cleanly(experience will tell you thedifference). If it just about makes it,or is a strain even to get it to move,don't do it. Once off, the machinewill feel quite sluggish, as if it's tiedto the ground.Move forward slowly, giving dueregard to the load's inertia, withoutalternately slowing down andspeeding up, or you will confuse it.Rather, move forward and keepgoing to allow the load to follow,which sometimes takes a bit ofcourage, to see how it flies. Make allcontrol movements smoothly andevenly, keeping the downwash insidethe rotor disk – if you have to, takeout your aggression on the collective(see also Longlining, below). Try notSpecial Use Of Aircraft 185to allow the load to sink, as, if it hitsthe ground or gets tied up in a tree itwill trip you (there is a naturaltendency to sink as you go forwardanyway). The torque used at thispoint will give you a good idea ofwhat is needed for landing, so becareful if you are going to a higheraltitude (it reduces at about 2-3% per1000 feet in a 206).
One technique used in NewZealand, when you are near all-upweight and lifting the load into thehover would be difficult, is to stretchthe longline out behind, attach it tothe helicopter and, keeping it taut,build up some speed in a parabolalike manoeuvre before lifting theload (it's called the slingshot and I’mtold it requires some practice).However, if a log lifted this waystarts spinning, it will undo the rope,so another way is to get a 300-footline, get into the hover as far back aspossible with a taut line and take offstraight ahead to get sometranslational lift. This tends to crackthe blades, though.Once in flight, remember that theload is the part that should be keptstraight and level, not the helicopter,and keep away from anythingunderneath that could be damaged(well, try anyway!). Only withimminent danger to the aircraftshould a load be jettisoned, usuallyfrom excessive swinging(commercially, dropping loads isregarded as a non-macho thing todo, but it's your backside thehelicopter is strapped to). As a pointof interest, 10 gallons of fuel from500 feet will go straight to thebasement of a 3-storey house. If youdrop anything obnoxious in water,expect your company to pay for theclean-up and testing.Unless in rough air, beeping theRPM down helps with the lift andthe fuel burn.If you get an engine failure in themountains with a load attached,consider not jettisoning it. This maysound daft, but it may stop you fromfalling down a crevice or something– just try to keep the line taut so youdon’t get a nasty jerk at the end.Load BehaviourEvery load has its own VNE,unfortunately usually only found byexperiment, which is why you shouldalways start off slowly and build upto a point where it starts to givetrouble, then back off, as low aspossible, so there is less height to getrid of in a hurry. Most helicopterswill carry loads at quite high speeds,but the load itself might not be ableto handle it—a sudden input of dragwhen something falls off couldbecome quite a problem. Althoughcustomers don’t like to pay forunnecessary flying, there’s no rush.Take it easy. Also, remember yourmachine’s VNE with the doors off!External loads increase the frontalarea of the whole aircraft, whichnaturally increases drag, so you willneed more power overall. A loadmay be easy to lift, but presentenough drag to cause severedifficulties, particularly where youreach power limits too quickly tomaintain forward flight, and the loadovertakes you and pulls you along. Along-line needs more anticipation, soyou need a high degree of coordination and patience. It's not thesort of thing that can be learnt in anyother way than with lots of practice.186 Operational FlyingUnevenly shaped loads will tend tospin and, if they're slung withoutreference to their centre of gravity,
could tip over. A drogue chute canstabilise them, but use a windsocktype rather than a pure parachute,which will bounce around trying tospill the air out (or punch holes init). Naturally, these must be kept wellaway from the tail rotor. Logs or cuttimber usually fly poorly unless a tailis installed, which can be made outof a bough or piece of plywood, so itsticks out of the back.Oscillation or excessive vibrationcan come from a number of places,usually a combination of the stabilitycharacteristics of the load andforward speed. Heavy or denseloads, such as bags of cement ordrums of kerosene, will not usuallypresent problems due to their mass,but large-volume loads of lowdensity can oscillate at a certaincritical speed, again usually onlyfound by trial and error.You can dampen oscillation byreducing your airspeed to at least10% below this critical one, goingslower if necessary while increasingpower. Turning could provideenough centrifugal force to stop it aswell, which is also the usual remedywhen the load starts to swing, butthis will increase its effective weight,possibly to more than your liftingcapabilities (a good reason for notbeing too tight on payload), soapplying centrifugal force in thesecases could make things worse.Load swing is proportional to speedand the length of the sling – thefaster you go, the more it occursbecause of the load’s own lift anddrag. Put simply, the load will alwaysmove further in a swing than theaircraft does – if you move 4 feet tothe right, the load will travel eight,after a short pause, as it moves fourfeet to the other side of you. As ittakes as long to swing through 15° asit does through 45, like anypendulum, if you move the oppositeway, as is natural, you just make theload swing faster. Going with theswing, that is, load to the left,helicopter to the left, will stop itquickest (do it on your side so youcan see what’s going on, but it worksboth ways). This has the effect ofmoving the fulcrum point fromunder the helicopter to over theload, which removes the directionalvector. If you want it to in aparticular direction, wait till it isgoing that way before correcting it,then you will be almost pushing itinto place. Put simply, don’t pull theload the way you don’t want it to go.You can do the same with a fore andaft swing, but watch out! Very often,a load can suddenly produce moredrag, if something breaks off, forinstance, which puts the nose down,which you correct with aft cyclic thatsimply puts the tail rotor nearer theline. If the load starts forward again(as it will), and pulls the nose furtherup for you, correcting with forwardcyclic may set you nicely up for mastbumping, or at the very least leaveyou hanging with the stick rightback. The correct thing to do here isto apply collective, which will alsoadd an upward force (assuming theclouds aren’t too low). Using alonger line will help. Tail rotorpedals can also be used – when theball is out to one side, use theopposite pedal when it starts tocome back (not before). It is alwaysa good practice not to rush theSpecial Use Of Aircraft 187takeoff and to lift without a swing inthe first place.Scaffolding and planking can swingviolently with only a few knotschange in airspeed, and aerofoilshapes could even generate theirown flying characteristics. Bulkyloads with a tendency to float, suchas empty containers, will benefitfrom leaving doors or panels open,which will reduce drag and keep theload facing in one direction.Setting DownApproach into wind as much aspossible, coming into the hover highenough not to drag the load, so youmight be slightly steeper thannormal. You’re best to undershootrather than overshoot, as it’s easierto creep up to a target than go roundagain if you miss it. To preclude anairflow change making the loadunstable, slow down beforedescending. If the load hits theground, stop moving forward.Because of inertia, all manoeuvresshould be anticipated well in advanceand made smoothly (not suddenly)with reference to the speed of the load overthe ground. In a confined area, theload will tend to pull you down asthe wind effect is lost, so a couple ofknots in hand under these
circumstances may be desirable.Keep a constant scan going, becauseyou need all the information you canget, especially when it comes todepth perception. If everything goespear-shaped, it’s because your scanhas stopped, as it might if you getfixated suddenly tense up on thecontrols. Relax and start lookingaround again, it will soon get better.Once in the hover, you again comeunder the guidance of a marshaller,who signals descent until the loadtouches the ground and the cablesbecome slack (if you haven’t got amarshaller, you can judge yourheight from whether the groundcrew are looking up or down).Release the cables after moving toone side so they do not foul the loador hit someone on the head. Don’tdrop cables from anything morethan normal hover height, andespecially not under too muchtension, or you’ll get somebody inthe eye (also, whatever is in the hookshoots downward at a fast rate, andthe hook itself will be openedviolently and may be damaged). Amanual release is provided if theelectrical one doesn't operate, and,once it has done so, you should see a"load released" signal from themarshaller, whereupon you hover bythe side of the load while thereplacement sling is placed inside thecabin, having moved away vertically first.Behave at all times as if the load hasnot been released.If you can’t hover, keep max powerin and let the aircraft settle, withoutovertorquing – you will only bepulled down as far as it takes for theload to reach the ground, so just tryand give it a gentle arrival. If you aredelivering the load by yourself, landbehind it, as far back as possible soyou don’t get the sling under theskid, and to provide a little tensionfor the hook mechanism to work.There is a phenomenon calledCollective Bounce that occurs when asudden vertical force is placed on thehelicopter, making you think theload has reached the ground.Although really relevant to largermachines, it can occur on smallerones as well, and arises when the188 Operational Flyingresonance of the blades matches thatof a vibrating rope. The collectivemovements to correct this get out ofphase, due to the response lag, andthe answer is simply to stop movingthe collective or go intoautorotation, as the machine willself-destruct about the fifth bounce.A little extra friction will help.If a load starts spinning in flight, andcontinues in the hover (or yourdownwash sets it going), gently put iton the ground and pick it up again.If you have to hold a load while it isbeing secured to something else, takecare to ensure that ground crewsdon’t get themselves into positionsthat could be dangerous if you haveto release it - be conservative withallowable side winds.It's a good idea, particularly with along line (see below) to unhook ifyou shutdown, as it is very easy to
take off and forget it is there.Vertical Reference (Longlining)Longlining is slinging with a lineover 75 feet long. To do thissuccessfully, you have to maybe lookdownwards out of the door or abubble window, and not through thefront with a mirror, which is themore traditional method, hence itsalternative name of Vertical Reference(having said that, in some machines,notably the Astar, it's quite difficultto look out of the door, and theweather often means you need thedoors on, so using mirrors issometimes the only way). The longline has a hook at the end, which isinside a metal brush cage, to bothprotect it and provide weight whenyou are flying around with only theline attached. As the extended hookneeds an electrical supply, there willbe an extra cable to control it, tapedto the longline or incorporateddirectly into it. It will have a coupleof spare feet at either end, whichshould be kept well away from themanual release. You may also find acouple of changes to the airframe, inthe shape of bubble windows, orinstruments being repeated to theside (don’t kick them on your wayin) so you can see them, althoughyou soon get used to the sound ofmachinery getting near to theovertorque stage (don’t ask!).A metal line is made of two strandsof wire rope, wound around eachother, to help prevent spinning.Kevlar (i.e. Spectra) ones areextremely light by comparison, butjust as strong, although they canstretch. The only real pain with themis ground handling, because theyhave a tendency to get tangled upinto knots if they don't have aprotective covering (one suggestionis to get a large bucket, place thehook in and loop the line around theinside). However, they can also pickup twigs, etc., and grease from drillsis apparently not good for them -they can also snag on trees, as Ifound out once. Being light, they canbe hard to punch off in thosecircumstances, and tricky to positionfor the crews in a wind when empty.You need to be particularly carefulnot to kink any line, which meansnot driving over it, landing on it,dragging it along the ground, ordropping it from great heights.Long-lining (or vertical reference)proficiency to some customersinvolves putting a small load at theend of a 100 ft line onto a 4 x 8 ftsheet of plywood three times fromSpecial Use Of Aircraft 189different approach angles, or evenputting a load into a barrel.The human eye is actually a verypoor optical instrument (you wouldget a better picture from a pinholecamera), and it is your brain that isresponsible for turning the image theright way up in the first place, andfor resolving the many colours theeye is capable of distinguishing.What this boils down to is thatabove about 40 feet, it is verydifficult to judge height properly,especially through your central vision(the weakest part), and handlingdepth perception needs somepractice. It is for this reason that youshould always look at the loadthrough the same medium(preferably none at all) so you giveyourself the best chance. Forexample, you are not helpingyourself by looking at the load withone eye directly and the otherthrough a panel. Having the Sun onthe opposite side of the machine toyou is a useful tactic as well, so youcan use the load's shadow to tell itsheight from the ground better.The Astar is particularly bad forlooking out of as the pilot's door isfurther away from the fuselage thanother machines. As a result, it needsa minimum line of 100 feet (somesay 130) just to see the load througha hole in the floor between the doorand the seat. However, even thenyou only really see it when lifting, oron delivery, which is why you need amirror as well (I once had to do ajob with a 75' line on a TwinStar,and had to deliberately swing theload so I could see it, at least once ina while! In this case, long tag lineshelped the ground crew to catch it).Some machines won’t allow you tolook out with shoulder straps on, orwhen wearing a helmet, so try it allout on the ground first. Note whereyour hook attachment is and see ifyou can make control movementswith reference to its position.Naturally, there is some skillattached to longlining, but it isn’t toohard to learn, although you will have
to watch for vortex ring whendelivering the load, as you haveminimum speed with power on anda high rate of descent if the load ispulling you in. Anticipation is thekey, but you can only learn this aftersome experience, wherein lies theCatch-22 of needing experience butnot being able to get it. Fire chasingoffers the best free training, as youare often out on your own, andnobody is using a stopwatch.Although it's not the completeanswer (for example, I can drop waymore water in a given time with ashort line), many pilots preferlonglining, if only because problemswith the load occur further awayfrom the aircraft, and thereforeproduce less hassle with the controlsand tail rotor (and downwashdoesn't artificially increase the load’sweight or throw up dust). You’realso that bit further away frommechanical turbulence, althoughalmost always out of ground effectand right in the avoid curve, whichmay cause a legal problem. One bigplus is that, if your engine fails, theground crew have more of a chanceto get out of the way. Another is thatthe delivery point doesn't necessarilyhave to take the helicopter as well(of course, the real reason why pilotslike slinging in the first place is thatthere are no passengers!).190 Operational FlyingHowever, with a crowning fire (thatis, with only a small area in thesmoke at the head that you can getyour helicopter into), longlining canbe dangerous, because all you willsee is a bucket flying around – youcertainly won't see the line, and thepilot won't see you, being in thesmoke. As a result, some authoritieshave banned its use, at least inconcert with short-lining.There are three variations on thelonglining theme:·
Operational longlining, which canbe done by just about anycompetent pilot and is fairlyundemanding, provided there isa reasonable margin of poweravailable, subject to a couple ofcaveats which are mentionedbelow - it's when you areoperating to the limits of theload and the machine that thereal expertise comes into play,but even this is nothing morethan good downwashmanagement and smoothnesson the controls coupled withanticipation. There is littleaccuracy or speed involved withoperational longlining, and itcan be regarded as just anextension to normal slinging—it's commonly used in firesupport, where you dump awater bucket's contents into arelay tank, or pick up the hosesand equipment after theexcitement's over.·
Production longlining is the fast andefficient movement of materialsfrom one place to another,typically used in seismic work,where you try and drop 30-40
