Employed since the early 1950’s, helicopter yarding is incredibly versatile due to its ability to avoid many of the obstacles that encumber ground based and skyline systems, including site sensitivity, urgency to remove or deliver the product, lack of access, and slope of the terrain. The use of helicopters in forestry continues to expand.
Advantages of helicopter logging are:
- The number of miles of roads may be reduced.
- It can be used on any type of terrain.
- It can be used with any type of silvicultural prescription.
- It will reduce the impact to the environment because the logs are lifted vertically.
- It is able to produce large volumes of logs quickly.
Planning for helicopter logging is critical. Being such an expensive operation, it is critical that support is available. This includes having enough transportation available to remove the large volumes of timber from the landing. Helicopter logging typically requires two landings - a service landing for the aircraft to refuel as well as periodic maintenance - and a log landing for dropping off the extracted timber.
Helicopter operations have high operational costs as well as high fixed costs for move in and out. To overcome these costs the operations must maximize the amount of material removed per turn and minimize the time per turn.
The payload they are capable of lifting and identified as heavy lift, medium lift, or light helicopters typically categorizes helicopters. Although there is no formal classification system, heavy lift helicopters are often considered to be those capable of lifting an external load of 15,000 pounds or more. Similarly, medium lift helicopters have external-load capacities of 2,000-15,000 pounds, and light helicopters, which are generally used for utility purposes, have still lower lifting capacities. The size of helicopter appropriate for a particular application is determined by the size and value of the material being extracted.
Helicopter logging is accomplished by suspending below the aircraft a long line of wire rope to which chokers can be attached. In some operations, a grapple may be used instead of chokers. The long line is typically between 90 and 300 feet in length, depending upon topography and the height of trees above which the helicopter must hover. Long chokers are used, and these are pre-set. The choker ends are then brought together to make up loads that are estimated as being slightly less than the helicopter’s lifting capacity.
The chokers are attached to the long line by a hook. The hook commonly has two slots to accept chokers. One is used to accommodate an estimated reasonable payload of logs and the other is used as a ‘bonus’, to target the optimal payload. This system allows the pilot to release one side of the hook, if the payload is too heavy or if the logs become entangled in the residual stand. The helicopter pilot controls the hook remotely.
When a grapple is used instead of chokers, a spotter is required in the stand in place of choker setters. The spotter guides the helicopter to logs requiring removal. This results in some labor cost savings.
There is a fuel truck and service truck at the service landing. At the log landing, there will typically be two or more loaders, one moves material from the drop zone to the processing or decking area. The other loader will be involved in loading trucks for transport. If mechanical processing is occurring at the landing, there will be at least one processor present.
A smaller helicopter may be used to ferry choker setters into remote locations and to deliver chokers.
The number of personnel engaged in a helicopter operation will vary based on the size of the helicopter being used. A larger helicopter is capable of bringing more material to the landing faster. This requires more loaders to move the material through the landing, more chasers to unhook turns, and more in-woods choker setters and hookers to prepare and hook the turns. The jobs include:
- Choker setters - pre-set chokers on logs to be extracted, working ahead of the helicopter so that they are not endangered by dropping logs and so that the helicopter does not have to wait while chokers are being set.
- Hooker - hooks a turn of choked logs to a hook suspended below the helicopter on the end of a “drop line” or “tag line”. The hooker is responsible for building the required payload on the hook.
- Chaser - unhooks the chokers at the landing.
In addition to these jobs there are the maintenance personnel responsible for general upkeep and maintenance of the helicopter during the operation. There are also the loader and processor operators to handle the logs delivered to the landing. If processors are not used, chainsaw operators are usually required at the landing to trim excess branches and buck logs. It is often more cost effective to do the final bucking at the landing in order to maximize the payload of the helicopter.
A typical flight cycle for a logging helicopter can be described as follows. Beginning at the service landing, the helicopter will fly to the harvest area and begin yarding logs. During the hooking element there will often be a person, the hooker, on the ground with pre-choked logs. The logs are ready to be connected to the hook at the end of the helicopter’s long line. The pilot locates the hooker and maneuvers the hook near the hooker. Then the hooker slides the chokers into the hook. The helicopter then climbs vertically to lift the logs off of the ground and clear of the forest canopy.
The inhaul element involves flying the load of logs from the hooking point to the landing. At the landing the pilot sets the logs on the ground in the drop zone and releases the chokers from the hook. With the load released, the pilot clears the log landing and enters the outhaul element to return to the woods for another load of logs. The entire process, outhaul, hook, inhaul, and unhook, is commonly referred to as a turn. The hooking element begins again and the cycle is repeated. If no problems occur, this continues for 60 to 90 minutes, until the helicopter must be refueled. The pilot must then return to the service landing for fuel. When the helicopter is in the outhaul, hooking, inhaul, or unhooking elements, this is called the flight cycle. The service cycle refers to when the helicopter is flying to the service landing or is in the process of refueling or undergoing maintenance.
There are some variations to this process. When chokers are removed from logs at the landing, they are typically coiled into bundles that can be periodically delivered to the choker setters by helicopter. Coiled chokers are placed in the hook immediately following the release of the logs at the landing. The chokers are usually delivered to the hooker prior to the last turn in the service cycle. The lead choker setter will often place flagging where the chokers should be dropped. The designated locations are intended to best accommodate choking the next available logs.
After releasing the chokers, the pilot maneuvers the hook to yard the next turn. In some instances, multiple choker drops are made to multiple choker setting crews prior to yarding the next turn. This is accomplished by using both slots in the hook to deliver chokers and having the hooker re-attach surplus coiled chokers for the remaining choker drop.
External Yarding Distance
There is a direct relationship between yarding distance and cycle time. Yarding distance is measured along the flight path of the aircraft and not necessarily along a straight line from the unit to the landing. The effective yarding distance for helicopter yarding is influenced by the elevation difference between the landing and the log pickup point, wind direction in relation to the approach to the log landing area, atmospheric conditions at the logging site, and obstacles such as hills and power lines that may require deviation from a straight-line flight path.
Large differences in elevation between the landing and the log pickup point require increased flight distance to dissipate altitude safely on descent or to climb safely when flying to an uphill landing. When the direct flight grade exceeds approximately 28%, an adjustment is appropriate to reflect a more accurate yarding distance. The following equation can be used to estimate the adjusted average yarding distance as measured on a map (i.e., horizontal distance):
Average Yarding Distance (adjusted) = (Elevation Difference )/0.28;
Where, Elevation Difference is the difference in elevation between the log pickup point and the landing.
Felling and Bucking
Some of the concerns are the same in felling timber for a helicopter timber sale as for any other type of logging system. The timber should be felled in a manner that will minimize breakage. In partial cutting operations, protection of the residual stand is an important consideration in selection of the desired felling pattern. However, in a helicopter sale, a special effort must be made to bunch felled trees so that the logs can be bundled together by the hooker to meet the target payload.
Logs are generally bucked to the lengths and grades desired by the processing mill. In addition, the log bucker must consider the weight of the manufactured log. Logs must be bucked so individual logs do not exceed the net lifting capability of the helicopter and all cuts must be complete so that each log is free of the adjacent one. In large timber, logs may have to be split. In smaller timber, enough logs must be available to achieve a reasonable payload. In general, even when the largest commercial helicopter is used for yarding, trees over 60" DBH will have butt logs shorter than 32'. In sugar pine and other heavy species, butt logs from trees over 60" DBH may have to be split or cut to very short lengths.
Local weather conditions must be considered when determining the logging season for the sale. Winds above 35 mph (56 kilometers per hour) will probably curtail operations. Gusty crosswinds are more adverse to the operation, and fog that restricts visibility will stop yarding operations. Deep snow may cause problems for the crew setting chokers and finding logs.
The hook circle defines a physical area on the ground from which choked logs can be packaged together to form a turn. The length of chokers, capacity of the helicopter, and the lengths and weights of the logs being removed will determine this. In areas of small timber, a larger circle may be required to build a turn. In areas of large timber, a smaller circle may suffice to maximize the payload. If the hook circle exceeds a certain radius, it may be impossible to build full turns, decreasing the efficiency of the operation. For planning purposes, the hook circle is fixed at a reasonable size for the helicopter being used. Wood availability is then measured for the given circle.
Helicopter extraction is an expensive operation that requires either high volume per turns or fast turn times, preferably both. It is the hooker’s job to ensure that the hook is loaded with a full payload each turn. He does this by estimating the weight on each choker and combining the chokers in a way to maximize the load. Wood availability is a measure of the available payload within the hook circle. It is estimated prior to felling the timber.
For a given hook circle, the total weight of cut material in the circle is measured and compared against the working payload of the helicopter. This is expressed as a percent of the working payload. A wood availability of less than 80% is generally regarded as an indication that helicopter productivity will be low.
High crown closure negatively impacts the productivity of the operation by increasing turn times. High crown closure usually means that less material is being removed per unit area. This requires stringing long chokers to collect logs over a greater distance. High crown closure results in longer search times by the helicopter to locate the hooker. It also reduces the rate at which turns can be lifted through the canopy because of the need to prevent damage to the residual stand. Stringing long chokers to build a turn also results in slower lift times and increased damage to residual trees.
Helicopters can be used effectively for any silvicultural prescription that is financially viable, and that does not require scarification of the ground during yarding to prepare a seed bed. Helicopter logging considerations for even-age silvicultural prescriptions include the following:
Helicopters have the advantage that they can operate in small, oddly shaped clearcuts at nearly the same cost as large contiguous clearcuts.
Group selection harvesting may be a good alternative.
- In an uneven aged or mixed species stand, a clearcutting prescription can increase the logging cost and decrease the average product value to an unacceptable or inoperable level.
- Slash disposal requirements can render an entire sale financially infeasible.
- Site preparation costs may be more expensive or financially infeasible due to the inaccessibility of the unit and due to the lack of scarification.
- Pre-commercial and commercial thinning may not be financially feasible due to inaccessibility and/or market conditions.
Seed tree and shelterwood
Helicopters can reduce almost all damage to the understory and therefore can operate with little impact in most 2-3 age stands. There have been many operations where helicopters have removed up to 30 MBF per acre in one entry and left an adequately stocked understory.
Commercial thinning with helicopters is physically possible, but the product value may not be high enough to make it financially feasible.
Patch cuts, strip cuts, or clearcuts are most efficient for helicopter yarding. A longer tag line is normally used when operating near standing timber at unit boundaries. In partial cuts, the stand must be opened up enough to allow the pilot to see the hooker, permit lowering the load line, and permit logs to be lifted above the canopy without hanging up in leave trees. Less than 60% canopy cover is often desirable.
Wood availability has a large influence on the ability to reach optimum turn weight. The planner must look at the average DBH of the cut trees and the preferred log lengths to determine if there are sufficient logs available to make up economically feasible turns. Gathering lines with sliding bells or chokers with a nubbin and bell on each end are used for assembling turns. If more than 10 logs are needed to reach the optimum turn weight, then it may be more economic to look at other cutting prescriptions or a different size helicopter.
Partial cuts in mature timber require long load or tag lines. There must be a minimum clearance between the helicopter and the tree crowns of 50 feet (about 15 meters). Cycle time increases when the load line gets longer than 200 feet (about 60 meters). A 250 foot (76 meters) loadline may be required.
In general, light partial cuts in big timber provide the greatest potential for accidents. Widow makers left after the felling operation pose hazards for cutters and choker setters. Snags and rotten topped trees are usually prevalent in this type of stand and are also hazardous to the ground crews. Flexibility should be allowed to open the stand in such a way that danger trees are left in groups and the hook circle has a relatively open canopy.
Canopy closure is a potentially serious problem when small timber is being thinned or when an overstory removal treatment is being carried out with residual trees that are 60 feet or more in height. The limbs of young trees are relatively small and supple, but threading the hook through the canopy may take more time. In taller timber, the limbs are larger and the canopy should be opened to at least 50 or 60 percent or. As an alternative, small group selection harvests might be prescribed.
Operating Near Power Lines
Landing and log pick-up should generally be avoided within 200 ft of power lines. The possibility of falling debris or logs may preclude flying loaded over power lines. As a minimum, the controlling authority of the power line should be consulted prior to such operations.
Yarding Across Roads and Trails
Yarding operations may be accomplished across roads when flaggers are posted to control vehicle traffic while flight operations are in progress. Operation across county and State highways will require written approval from the controlling authority. If hiking trails pass through or near an active helicopter logging area, the trails may need to be closed or flaggers posted during yarding. This may require special public involvement efforts.
The considerations for manual felling with helicopter extraction are the same as with cable extraction. Manual felling is often used on helicopter logging operations because steep slopes preclude the use of mechanical felling equipment. Manual felling places material on the ground in a more scattered pattern. To facilitate yarding, cutters may need to fell trees in a fan pattern around a central hook point.
Mechanical felling is possible in helicopter operations when ground conditions permit. As compared to ground-skidding equipment, felling machines only need to travel over the ground once. The damage caused by ground extraction equipment is usually a cumulative effect of many passes over the same ground dragging material. Felling machines generally pass only once through the stand. If processing is being done at the stump by the equipment, it may use the slash as a mat to travel over, further decreasing soil damage. When operating on steep slopes, greater than 35%, it may be desirable to manually fell the trees and then process them with mechanical equipment. This improves the productivity of the felling crew while taking advantage of the capabilities of processing equipment.
One advantage of mechanical felling and processing is that the machines can build turns as they work. Logs can be bunched closely together, preferably where there is a hole in the canopy to facilitate extraction. The operator can build turns that conform to the optimum payload of the helicopter. This decreases the length of chokers required since they do not need to be strung to reach scattered trees. This in turn limits residual stand damage and improves the safety of the workers on the ground.
The following is a selection of representative research studies and reports done on harvesting operations that included helicopters. These reports may be used to get an idea of productivity and impacts of different systems and uses of helicopters as well as some of their limitations. When reading these reports, keep in mind that they describe specific systems and stand treatments. Trying to apply the lessons learned from these reports to systems and treatments outside of the studies’ scope may have unintended or unforeseen consequences.
This is not a complete listing of research on the use of cable systems. Additional information can be found at the USDA Forest Service Treesearch website. This site provides reports on research performed by Forest Service Research and Development scientists and their collaborators.
- Title: Helicopter logging productivity on harvesting operations in southeast Alaska, using ecologically based silvicultural prescriptions.
Author: Christian, L.; Brackley, A.
Source: Western Journal of Applied Forestry. 22(2): 142-147
Description: This study examines production rates and costs for felling and helicopter yarding on eight units harvested in accordance with ecologically based silvicultural prescriptions. The units represent five levels of basal area retention. The levels of retention had irregular spatial arrangements caused by gaps and clumps that ranged from 0 percent retention (clearcut) to 75 percent of basal area retained. Turn time, as adjusted to a standardized distance, and turn weight were used as measures of productivity. There were statistically significant differences in adjusted turn time, depending on the treatment. Areas with higher levels of removal tended to have lower adjusted turn times. Average weighted cost per thousand board feet harvested was $322. Regardless of differences in turn time or lifted weight, helicopter logging is an expensive method of harvesting timber and should only be applied to areas that support stands with significant volumes of high-quality timber.
- Title: Vegetation responses to helicopter and ground based logging in blackwater floodplain forests
Author: Jones, R.H.; Stokes, S.L.; Lockaby, B.G.; Stanturf, John.A.
Source: Forest Ecology and Management 139: 215-225
Description: Logging in floodplains of low order, blackwater streams may damage cxisting seedlings and rootstocks, and create soil conditions that inhibit establishment and growth of regeneration after harvest. Removal of logs via helicopters has been advocated to minimize soil damage and facilitate rapid revegetation. We tested impacts of helicopter versus conventional skidder harvest systems on regeneration, woody plant community structure and biomass growth in three blackwater stream floodplains in southern Alabama. The helicopter treatment resulted in significantly greater woody plant density (19,900 versus 14,300 stems/ha by year 8), but both treatments were well-stocked with commercially valuable species. By Year 8, treatment effects on density of individual species were generally not significant; however, density of Cliftonia monophylla was lower on skidder plots (p=0.001) and density of Nyssu sylvancu var. bijloru was lower on helicopter plots (p=0.092). In both treatments, species richness within 0.004 had regeneration plots declined slightly between pre- and post harvest, but the Shannon diversity and evenness indices remained essentially unchanged through 8 years after treatment. Post-harvest survival of Acer rubrum, Cyrilla racemiflora and C. monophylla rootstocks was significantly lower on the skidder plots. In both treatments, species dominant before harvest remained so aftenvards. Species with the tallest sprouts in Year 8 were Liriodendmn tulipiferu, Magnolia virginiana, and A. rubrum. During the first 2 years after logging, aboveground biomass was greater in the helicopter treatment, but the difference was only significant in Year 1. We conclude that both harvesting methods had little effect on species composition. Skidding may result in a stand structure more favorable for commercial timber production; however, impacts of skidding on long-term productivity are not yet known.
- Akay, A., J. Sessions, P. Bettinger, R. Toupin, and A. Eklund. 2006. Evaluating the salvage value of fire-killed timber by helicopter - effects of time since fire and yarding distance. Western Journal of Applied Forestry 21(2) 102-107.
- Dykstra, D. P. 1980. Density altitude as a performance standard for helicopter research in forestry. Transactions of the American Society of Agricultural Engineers 23:541-545.
- Dykstra, D. P., D. E. Aulerich, and J. R. Henshaw. 1978. Prebunching to reduce helicopter logging costs. Journal of Forestry 76:362-364.
- Dykstra, D. P. 1976. Cable, balloon, and helicopter yarding compared for partial cutting and clearcutting in old-growth Douglas-fir. Forest Research Laboratory, Oregon State University, Corvallis, Oregon. Research Bulletin 22, 44 p.