EN
Natural disasters often cause that large share of acquisitions are performed under contingent conditions. High concentration of raw wood material and urgent need for its rapid removal from the forest demand use of harvesters. Material acquisition in such conditions is much harder and causes loss of performance for working machines. The objective of this paper was to determine the decrease in harvester performance during acquisition of fallen trees, wood scrap and suspended trees compared to acquisition of undamaged trees. Performance analysis was conducted on 10 harvesters from different power class working in the Czech Republic in years 2007−2012. The time structure was determined for the timber acquisition under contingent conditions in 22 forest stands. Later on it was compared with acquisition under normal conditions, i.e. scheduled acquisition on areas with good load carrying capacity and without obstacles and with an inclination up to 33%. There was significant difference in the mean operating time requirements between harvesters from the analysed power classes. Machines form 1st class differed significantly form devices from 2nd and 3rd classed. Results indicate that in spruce stands with wood scrap time consumption increases by 7% when average and high performance harvesters are used. The increase is by 33% during acquisition of fallen trees, while time consumption increases by as much as 79% during acquisition of suspended trees. During acquisition of subversion and wood scrap, performance of average and high performance harvesters ranged between 1.71 up to 17.37 m3/h depending on thickness and type of the damage to obtained trees. Important achievement of presented research is correction factors allowing to change harvester performance standards, while acquisition of fallen trees, wood scrap and suspended trees. Average value of correction factor is 1.19.