First production trial of a helicopter slash grapple

Erosion and debris flows from forestry operations is currently a live topic. Severe weather events in the Nelson and Gisborne regions last year caused localised debris flows of mud and ‘slash’ (woody debris from windthrown trees, broken tops and branches, and residues from harvesting operations) beyond the forest boundary on to neighbouring properties. Concerns have been raised about industry practice and the efficacy of management controls over plantation forestry.

At an industry workshop sponsored by Forest Growers Research Ltd on harvest residue management on erosion prone land in Auckland in August 2018, forest industry stakeholders supported a project to address improved extraction of slash from the cutover and adjacent waterways. One idea to reduce the risk of build-up of ‘slash’ on the cutover in steep forested areas was to design, build and trial a helicopter slash grapple.

A prototype multi-tine slash grapple was designed by Helihawk Ltd of Taupo and built by Colchester Engineering Ltd of Matamata in February 2019 (Figure 1).

Figure 1: Helihawk Slash Grapple

The site for the first production trial of the Helihawk Slash Grapple was a steep radiata pine harvest area in Kanuka Forest, Gisborne, managed by PF Olsen Ltd. The helicopter contractor undertaking this trial was Wairarapa Helicopters Ltd based in Masterton, and the pilot was Tim Williams a very experienced helicopter pilot.

Results of analysis of time study data linked to net helicopter payload as measured by an on-board load cell, showed average net extraction productivity of 18.5 tonnes per productive flying hour when extracting slash only. It was recommended that as much log material is extracted first before using the slash grapple.

In these conditions the cost of slash extraction averaged $135 per tonne of slash removed, or approximately $112 per metre of stream length. When the cost of cleaning the stream using the helicopter slash grapple was averaged over the volume of wood extracted from the harvest area, the net cost was only $1.05 per cubic metre of wood produced.

Using manual workers to remove harvest residues from steep or incised gullies, can readily be recognised as a dangerous task. Sending machinery down into gullies close to waterways to remove harvest residues, while effective, often results in soil disturbance and sedimentation of waterways.

This initial trial showed that the slash grapple designed, built and trialled by Helihawk Ltd was well matched to the task required and the helicopter used for the trial. The implementation of safe and efficient helicopter slash extraction substantially reduces or eliminates the unsafe nature of manual stream cleaning, and minimises the risk of negative environmental outcomes. While the environmental impact is low, high operating costs limit the use of helicopter slash extraction to sensitive or high risk areas only. 

Acknowledgement - Keith Raymond , Forest Growers Research Ltd