Tasks required for repairing mining equipment differ significantly from those required for most other mining work, especially in technical complexity and variety. For example, operation of mining equipment tends to be repetitive and routine, but maintaining that equipment is varied, complex and often urgent. Maintenance may be necessary in the field, forcing repair crews into awkward, cold, wet, dark, cramped, or dusty conditions. The nonroutine nature of mine-maintenance work means that each repair task presents unique characteristics and complexities. The challenges of mine maintenance can be characterized as human-factors difficulties. Improvement of both the safety and efficiency of maintenance represents a large, potential cost-saving. Maintenance typically accounts for a major percentage of mining costs; estimates vary from 30 to 50 percent of total operating costs. The U.S. Bureau of Mines has conducted safety research on human-factors difficulties related to surface-mine maintenance since 1985. This research indicates that more than one-third of all surface-mining accidents occur during equipment maintenance, and that overexertion and slip-and-fall injuries are responsible for 64 percent of all the injury lost time due to maintenance. Estimates of maintenance accident costs range from $12,000 to $14,000 per accident.