Summary Fellers are off-road vehicles designed for forestry work that serve as a motor vehicle for moving a feller-buncher or a multifunction harvester head. The use of fellers with a multifunction felling head has greatly increased in Québec in recent years for many reasons, which include: The reduction in the surface area of skidder trails; The elimination of delimbing areas along roads; The reduction in machine diversity, with just one machine bucking and shaping the trees into logs, while several were required with the old methods; The reduction in soil degradation because the shaping of trees is done on site and vegetation cover can be created as needed. The IRSST began a series of studies on mechanized felling at the same time (April 2004) as the Commission de la santé et de la sécurité du travail (CSST, Québec workers' compensation board) required the lockout of fellers during maintenance interventions. An initial study demonstrated, for example, that mechanized felling accidents are more serious than the average accident. A second study documented, on recent machines, the safety devices used to ensure the safety of people around the feller. This third and longer study had two main goals: 1) to document the operators' activity in order to better understand the hazardous phenomena and the hazardous situations that could result in accidents during interventions on the felling head, and 2) to do a risk analysis on the pressure adjustment operations and to propose means of risk reduction. Many field observations, in conjunction with a theoretical analysis, have enabled the research team to carry out its mandate. The variety and frequency of the interventions on the head were measured. The impact, on worker safety, of the sequence of operations necessary during an intervention on the felling head was quantified. The operations generating hazardous situations for the operators, when outside the cabin, were identified. Solutions for risk suppression at source as well as for the use of protective devices or safe procedures were proposed. For the pressure adjustment operation, the risk analysis identified six causes responsible for possible hazardous events. Material as well as organizational solution scenarios were proposed. The impact of each of these solution scenarios on the workers' safety and on the complexity of the implementation was quantified. This study resulted in the most reliable portrait possible of the different interventions on a felling head, thus allowing the causes of these interventions to be understood. The research team hopes that this study will improve the safety of forestry workers who use a feller equipped with a multifunction harvesting head.